Merge #8717

8717: Update match checking algorithm r=iDawer a=iDawer

I've recently got interest in the match checking to extend the current algo to support reporting witnesses of non-exhaustiveness.
It appears the algo is outdated from rustc's implementation. I decided to rewrite it based on the latest rustc's version. It is a diff-based port to ra codebase. That means you can diff-compare these files to rustc.
I'm striving to keep minimal ra-related changes in the algo to make it easier to backport future changes from the upstream.

Based on upstream algorithm of version rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22)
https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

The goal of this PR is to cover the current `missing-match-arm` diagnostic.

What is remaining to do:
- [x] Error handling. The errors that are unrelated to match checking will be handled before the check. Just like how it made in rustc.
  - [x] Lowering `hir_def::expr::Pat` to `hir_ty::diagnostics::match_check::Pat`. rustc's match checking works on top of `rustc_mir_build::thir::Pat`, which is lowered from `hir::Pat` and carries some extra semantics used by the check. All unrelated checks are done there. RA could use this to rule out running the check on unimplemented cases (`Pat::ConstBlock`, etc).
  - [x] ~~Proper~~Loose typecheck of match arm patterns (https://github.com/rust-analyzer/rust-analyzer/pull/8840, https://github.com/rust-analyzer/rust-analyzer/pull/8875).
- [x] Tests from `hir_ty::diagnostics::match_check::tests`.
- [x] Clean up `todo`s
- [x] Test run on real repos https://github.com/rust-analyzer/rust-analyzer/pull/8717#issuecomment-847120265.

Co-authored-by: Dawer <[email protected]>

5 files changed, 2767 insertions, 805 deletions

diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 86f82e3fa..3efbce773 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -2,9 +2,11 @@
 //! through the body using inference results: mismatched arg counts, missing
 //! fields, etc.
 
-use std::sync::Arc;
+use std::{cell::RefCell, sync::Arc};
 
-use hir_def::{expr::Statement, path::path, resolver::HasResolver, AssocItemId, DefWithBodyId};
+use hir_def::{
+    expr::Statement, path::path, resolver::HasResolver, AssocItemId, DefWithBodyId, HasModule,
+};
 use hir_expand::name;
 use rustc_hash::FxHashSet;
 use syntax::{ast, AstPtr};
@@ -12,7 +14,10 @@ use syntax::{ast, AstPtr};
 use crate::{
     db::HirDatabase,
     diagnostics::{
-        match_check::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
+        match_check::{
+            self,
+            usefulness::{compute_match_usefulness, expand_pattern, MatchCheckCtx, PatternArena},
+        },
         MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkOrSomeInTailExpr,
         MissingPatFields, RemoveThisSemicolon,
     },
@@ -294,12 +299,12 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             &infer.type_of_expr[match_expr]
         };
 
-        let cx = MatchCheckCtx { match_expr, body, infer: infer.clone(), db };
-        let pats = arms.iter().map(|arm| arm.pat);
+        let pattern_arena = RefCell::new(PatternArena::new());
 
-        let mut seen = Matrix::empty();
-        for pat in pats {
-            if let Some(pat_ty) = infer.type_of_pat.get(pat) {
+        let mut m_arms = Vec::new();
+        let mut has_lowering_errors = false;
+        for arm in arms {
+            if let Some(pat_ty) = infer.type_of_pat.get(arm.pat) {
                 // We only include patterns whose type matches the type
                 // of the match expression. If we had a InvalidMatchArmPattern
                 // diagnostic or similar we could raise that in an else
@@ -315,14 +320,25 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                         .as_reference()
                         .map(|(match_expr_ty, ..)| match_expr_ty == pat_ty)
                         .unwrap_or(false))
-                    && types_of_subpatterns_do_match(pat, &cx.body, &infer)
+                    && types_of_subpatterns_do_match(arm.pat, &body, &infer)
                 {
                     // If we had a NotUsefulMatchArm diagnostic, we could
                     // check the usefulness of each pattern as we added it
                     // to the matrix here.
-                    let v = PatStack::from_pattern(pat);
-                    seen.push(&cx, v);
-                    continue;
+                    let m_arm = match_check::MatchArm {
+                        pat: self.lower_pattern(
+                            arm.pat,
+                            &mut pattern_arena.borrow_mut(),
+                            db,
+                            &body,
+                            &mut has_lowering_errors,
+                        ),
+                        has_guard: arm.guard.is_some(),
+                    };
+                    m_arms.push(m_arm);
+                    if !has_lowering_errors {
+                        continue;
+                    }
                 }
             }
 
@@ -330,34 +346,73 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             // fit the match expression, we skip this diagnostic. Skipping the entire
             // diagnostic rather than just not including this match arm is preferred
             // to avoid the chance of false positives.
+            #[cfg(test)]
+            match_check::tests::report_bail_out(db, self.owner, arm.pat, self.sink);
             return;
         }
 
-        match is_useful(&cx, &seen, &PatStack::from_wild()) {
-            Ok(Usefulness::Useful) => (),
-            // if a wildcard pattern is not useful, then all patterns are covered
-            Ok(Usefulness::NotUseful) => return,
-            // this path is for unimplemented checks, so we err on the side of not
-            // reporting any errors
-            _ => return,
-        }
+        let cx = MatchCheckCtx {
+            module: self.owner.module(db.upcast()),
+            match_expr,
+            infer: &infer,
+            db,
+            pattern_arena: &pattern_arena,
+            eprint_panic_context: &|| {
+                use syntax::AstNode;
+                if let Ok(scrutinee_sptr) = source_map.expr_syntax(match_expr) {
+                    let root = scrutinee_sptr.file_syntax(db.upcast());
+                    if let Some(match_ast) = scrutinee_sptr.value.to_node(&root).syntax().parent() {
+                        eprintln!(
+                            "Match checking is about to panic on this expression:\n{}",
+                            match_ast.to_string(),
+                        );
+                    }
+                }
+            },
+        };
+        let report = compute_match_usefulness(&cx, &m_arms);
 
-        if let Ok(source_ptr) = source_map.expr_syntax(id) {
-            let root = source_ptr.file_syntax(db.upcast());
-            if let ast::Expr::MatchExpr(match_expr) = &source_ptr.value.to_node(&root) {
-                if let (Some(match_expr), Some(arms)) =
-                    (match_expr.expr(), match_expr.match_arm_list())
-                {
-                    self.sink.push(MissingMatchArms {
-                        file: source_ptr.file_id,
-                        match_expr: AstPtr::new(&match_expr),
-                        arms: AstPtr::new(&arms),
-                    })
+        // FIXME Report unreacheble arms
+        // https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/check_match.rs#L200-L201
+
+        let witnesses = report.non_exhaustiveness_witnesses;
+        // FIXME Report witnesses
+        // eprintln!("compute_match_usefulness(..) -> {:?}", &witnesses);
+        if !witnesses.is_empty() {
+            if let Ok(source_ptr) = source_map.expr_syntax(id) {
+                let root = source_ptr.file_syntax(db.upcast());
+                if let ast::Expr::MatchExpr(match_expr) = &source_ptr.value.to_node(&root) {
+                    if let (Some(match_expr), Some(arms)) =
+                        (match_expr.expr(), match_expr.match_arm_list())
+                    {
+                        self.sink.push(MissingMatchArms {
+                            file: source_ptr.file_id,
+                            match_expr: AstPtr::new(&match_expr),
+                            arms: AstPtr::new(&arms),
+                        })
+                    }
                 }
             }
         }
     }
 
+    fn lower_pattern(
+        &self,
+        pat: PatId,
+        pattern_arena: &mut PatternArena,
+        db: &dyn HirDatabase,
+        body: &Body,
+        have_errors: &mut bool,
+    ) -> match_check::PatId {
+        let mut patcx = match_check::PatCtxt::new(db, &self.infer, body);
+        let pattern = patcx.lower_pattern(pat);
+        let pattern = pattern_arena.alloc(expand_pattern(pattern));
+        if !patcx.errors.is_empty() {
+            *have_errors = true;
+        }
+        pattern
+    }
+
     fn validate_results_in_tail_expr(&mut self, body_id: ExprId, id: ExprId, db: &dyn HirDatabase) {
         // the mismatch will be on the whole block currently
         let mismatch = match self.infer.type_mismatch_for_expr(body_id) {
diff --git a/crates/hir_ty/src/diagnostics/match_check.rs b/crates/hir_ty/src/diagnostics/match_check.rs
index e8dd669bf..a9a99f57a 100644
--- a/crates/hir_ty/src/diagnostics/match_check.rs
+++ b/crates/hir_ty/src/diagnostics/match_check.rs
@@ -1,871 +1,416 @@
-//! This module implements match statement exhaustiveness checking and usefulness checking
-//! for match arms.
+//! Validation of matches.
 //!
-//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which
-//! contains very detailed documentation about the algorithms used here. I've duplicated
-//! most of that documentation below.
+//! This module provides lowering from [hir_def::expr::Pat] to [self::Pat] and match
+//! checking algorithm.
 //!
-//! This file includes the logic for exhaustiveness and usefulness checking for
-//! pattern-matching. Specifically, given a list of patterns for a type, we can
-//! tell whether:
-//! - (a) the patterns cover every possible constructor for the type (exhaustiveness).
-//! - (b) each pattern is necessary (usefulness).
-//!
-//! The algorithm implemented here is a modified version of the one described in
-//! <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
-//! However, to save future implementors from reading the original paper, we
-//! summarize the algorithm here to hopefully save time and be a little clearer
-//! (without being so rigorous).
-//!
-//! The core of the algorithm revolves about a "usefulness" check. In particular, we
-//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
-//! a matrix). `U(P, p)` represents whether, given an existing list of patterns
-//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
-//! uncovered values of the type).
-//!
-//! If we have this predicate, then we can easily compute both exhaustiveness of an
-//! entire set of patterns and the individual usefulness of each one.
-//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
-//! match doesn't increase the number of values we're matching)
-//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
-//! pattern to those that have come before it doesn't increase the number of values
-//! we're matching).
-//!
-//! During the course of the algorithm, the rows of the matrix won't just be individual patterns,
-//! but rather partially-deconstructed patterns in the form of a list of patterns. The paper
-//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
-//! new pattern `p`.
-//!
-//! For example, say we have the following:
-//!
-//! ```ignore
-//! // x: (Option<bool>, Result<()>)
-//! match x {
-//!     (Some(true), _) => (),
-//!     (None, Err(())) => (),
-//!     (None, Err(_)) => (),
-//! }
-//! ```
-//!
-//! Here, the matrix `P` starts as:
-//!
-//! ```text
-//! [
-//!     [(Some(true), _)],
-//!     [(None, Err(()))],
-//!     [(None, Err(_))],
-//! ]
-//! ```
-//!
-//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
-//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
-//! all the values it covers are already covered by row 2.
-//!
-//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of
-//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
-//! To match the paper, the top of the stack is at the beginning / on the left.
-//!
-//! There are two important operations on pattern-stacks necessary to understand the algorithm:
-//!
-//! 1. We can pop a given constructor off the top of a stack. This operation is called
-//!    `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
-//!    `None`) and `p` a pattern-stack.
-//!    If the pattern on top of the stack can cover `c`, this removes the constructor and
-//!    pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
-//!    Otherwise the pattern-stack is discarded.
-//!    This essentially filters those pattern-stacks whose top covers the constructor `c` and
-//!    discards the others.
-//!
-//!    For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
-//!    pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
-//!    `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
-//!    nothing back.
-//!
-//!    This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
-//!    on top of the stack, and we have four cases:
-//!
-//!    * 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We push onto
-//!           the stack the arguments of this constructor, and return the result:
-//!
-//!          r_1, .., r_a, p_2, .., p_n
-//!
-//!    * 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and return
-//!           nothing.
-//!    * 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
-//!           arguments (its arity), and return the resulting stack:
-//!
-//!          _, .., _, p_2, .., p_n
-//!
-//!    * 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
-//!
-//!          S(c, (r_1, p_2, .., p_n))
-//!          S(c, (r_2, p_2, .., p_n))
-//!
-//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
-//!    a pattern-stack.
-//!    This is used when we know there are missing constructor cases, but there might be
-//!    existing wildcard patterns, so to check the usefulness of the matrix, we have to check
-//!    all its *other* components.
-//!
-//!    It is computed as follows. We look at the pattern `p_1` on top of the stack,
-//!    and we have three cases:
-//!    * 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
-//!    * 1.2. `p_1 = _`. We return the rest of the stack:
-//!
-//!          p_2, .., p_n
-//!
-//!    * 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
-//!
-//!          D((r_1, p_2, .., p_n))
-//!          D((r_2, p_2, .., p_n))
-//!
-//!    Note that the OR-patterns are not always used directly in Rust, but are used to derive the
-//!    exhaustive integer matching rules, so they're written here for posterity.
-//!
-//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
-//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
-//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
-//!
-//!
-//! The algorithm for computing `U`
-//! -------------------------------
-//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
-//! That means we're going to check the components from left-to-right, so the algorithm
-//! operates principally on the first component of the matrix and new pattern-stack `p`.
-//! This algorithm is realized in the `is_useful` function.
-//!
-//! Base case (`n = 0`, i.e., an empty tuple pattern):
-//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), then
-//!   `U(P, p)` is false.
-//! - Otherwise, `P` must be empty, so `U(P, p)` is true.
-//!
-//! Inductive step (`n > 0`, i.e., whether there's at least one column [which may then be expanded
-//! into further columns later]). We're going to match on the top of the new pattern-stack, `p_1`:
-//!
-//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
-//!   Then, the usefulness of `p_1` can be reduced to whether it is useful when
-//!   we ignore all the patterns in the first column of `P` that involve other constructors.
-//!   This is where `S(c, P)` comes in:
-//!
-//!   ```text
-//!   U(P, p) := U(S(c, P), S(c, p))
-//!   ```
-//!
-//!   This special case is handled in `is_useful_specialized`.
-//!
-//!   For example, if `P` is:
-//!
-//!   ```text
-//!   [
-//!       [Some(true), _],
-//!       [None, 0],
-//!   ]
-//!   ```
-//!
-//!   and `p` is `[Some(false), 0]`, then we don't care about row 2 since we know `p` only
-//!   matches values that row 2 doesn't. For row 1 however, we need to dig into the
-//!   arguments of `Some` to know whether some new value is covered. So we compute
-//!   `U([[true, _]], [false, 0])`.
-//!
-//! - If `p_1 == _`, then we look at the list of constructors that appear in the first component of
-//!   the rows of `P`:
-//!     - If there are some constructors that aren't present, then we might think that the
-//!       wildcard `_` is useful, since it covers those constructors that weren't covered
-//!       before.
-//!       That's almost correct, but only works if there were no wildcards in those first
-//!       components. So we need to check that `p` is useful with respect to the rows that
-//!       start with a wildcard, if there are any. This is where `D` comes in:
-//!       `U(P, p) := U(D(P), D(p))`
-//!
-//!       For example, if `P` is:
-//!       ```text
-//!       [
-//!           [_, true, _],
-//!           [None, false, 1],
-//!       ]
-//!       ```
-//!       and `p` is `[_, false, _]`, the `Some` constructor doesn't appear in `P`. So if we
-//!       only had row 2, we'd know that `p` is useful. However row 1 starts with a
-//!       wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
-//!
-//!     - Otherwise, all possible constructors (for the relevant type) are present. In this
-//!       case we must check whether the wildcard pattern covers any unmatched value. For
-//!       that, we can think of the `_` pattern as a big OR-pattern that covers all
-//!       possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
-//!       example. The wildcard pattern is useful in this case if it is useful when
-//!       specialized to one of the possible constructors. So we compute:
-//!       `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
-//!
-//!       For example, if `P` is:
-//!       ```text
-//!       [
-//!           [Some(true), _],
-//!           [None, false],
-//!       ]
-//!       ```
-//!       and `p` is `[_, false]`, both `None` and `Some` constructors appear in the first
-//!       components of `P`. We will therefore try popping both constructors in turn: we
-//!       compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]],
-//!       [false])` for the `None` constructor. The first case returns true, so we know that
-//!       `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
-//!       before.
-//!
-//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
-//!
-//!   ```text
-//!   U(P, p) := U(P, (r_1, p_2, .., p_n))
-//!            || U(P, (r_2, p_2, .., p_n))
-//!   ```
-use std::{iter, sync::Arc};
-
-use hir_def::{
-    adt::VariantData,
-    body::Body,
-    expr::{Expr, Literal, Pat, PatId},
-    EnumVariantId, StructId, VariantId,
-};
+//! It is modeled on the rustc module `rustc_mir_build::thir::pattern`.
+
+mod deconstruct_pat;
+mod pat_util;
+pub(crate) mod usefulness;
+
+use hir_def::{body::Body, EnumVariantId, LocalFieldId, VariantId};
 use la_arena::Idx;
-use smallvec::{smallvec, SmallVec};
-
-use crate::{db::HirDatabase, AdtId, InferenceResult, Interner, TyExt, TyKind};
-
-#[derive(Debug, Clone, Copy)]
-/// Either a pattern from the source code being analyzed, represented as
-/// as `PatId`, or a `Wild` pattern which is created as an intermediate
-/// step in the match checking algorithm and thus is not backed by a
-/// real `PatId`.
-///
-/// Note that it is totally valid for the `PatId` variant to contain
-/// a `PatId` which resolves to a `Wild` pattern, if that wild pattern
-/// exists in the source code being analyzed.
-enum PatIdOrWild {
-    PatId(PatId),
-    Wild,
-}
 
-impl PatIdOrWild {
-    fn as_pat(self, cx: &MatchCheckCtx) -> Pat {
-        match self {
-            PatIdOrWild::PatId(id) => cx.body.pats[id].clone(),
-            PatIdOrWild::Wild => Pat::Wild,
-        }
-    }
+use crate::{db::HirDatabase, InferenceResult, Interner, Substitution, Ty, TyKind};
 
-    fn as_id(self) -> Option<PatId> {
-        match self {
-            PatIdOrWild::PatId(id) => Some(id),
-            PatIdOrWild::Wild => None,
-        }
-    }
-}
+use self::pat_util::EnumerateAndAdjustIterator;
 
-impl From<PatId> for PatIdOrWild {
-    fn from(pat_id: PatId) -> Self {
-        Self::PatId(pat_id)
-    }
-}
+pub(crate) use self::usefulness::MatchArm;
 
-impl From<&PatId> for PatIdOrWild {
-    fn from(pat_id: &PatId) -> Self {
-        Self::PatId(*pat_id)
-    }
-}
+pub(crate) type PatId = Idx<Pat>;
 
-#[derive(Debug, Clone, Copy, PartialEq)]
-pub(super) enum MatchCheckErr {
-    NotImplemented,
-    MalformedMatchArm,
-    /// Used when type inference cannot resolve the type of
-    /// a pattern or expression.
-    Unknown,
+#[derive(Clone, Debug)]
+pub(crate) enum PatternError {
+    Unimplemented,
+    UnresolvedVariant,
+    MissingField,
+    ExtraFields,
 }
 
-/// The return type of `is_useful` is either an indication of usefulness
-/// of the match arm, or an error in the case the match statement
-/// is made up of types for which exhaustiveness checking is currently
-/// not completely implemented.
-///
-/// The `std::result::Result` type is used here rather than a custom enum
-/// to allow the use of `?`.
-pub(super) type MatchCheckResult<T> = Result<T, MatchCheckErr>;
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) struct FieldPat {
+    pub(crate) field: LocalFieldId,
+    pub(crate) pattern: Pat,
+}
 
-#[derive(Debug)]
-/// A row in a Matrix.
-///
-/// This type is modeled from the struct of the same name in `rustc`.
-pub(super) struct PatStack(PatStackInner);
-type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) struct Pat {
+    pub(crate) ty: Ty,
+    pub(crate) kind: Box<PatKind>,
+}
 
-impl PatStack {
-    pub(super) fn from_pattern(pat_id: PatId) -> PatStack {
-        Self(smallvec!(pat_id.into()))
+impl Pat {
+    pub(crate) fn wildcard_from_ty(ty: Ty) -> Self {
+        Pat { ty, kind: Box::new(PatKind::Wild) }
     }
+}
 
-    pub(super) fn from_wild() -> PatStack {
-        Self(smallvec!(PatIdOrWild::Wild))
-    }
+/// Close relative to `rustc_mir_build::thir::pattern::PatKind`
+#[derive(Clone, Debug, PartialEq)]
+pub(crate) enum PatKind {
+    Wild,
 
-    fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
-        Self(SmallVec::from_slice(slice))
-    }
+    /// `x`, `ref x`, `x @ P`, etc.
+    Binding {
+        subpattern: Option<Pat>,
+    },
+
+    /// `Foo(...)` or `Foo{...}` or `Foo`, where `Foo` is a variant name from an ADT with
+    /// multiple variants.
+    Variant {
+        substs: Substitution,
+        enum_variant: EnumVariantId,
+        subpatterns: Vec<FieldPat>,
+    },
+
+    /// `(...)`, `Foo(...)`, `Foo{...}`, or `Foo`, where `Foo` is a variant name from an ADT with
+    /// a single variant.
+    Leaf {
+        subpatterns: Vec<FieldPat>,
+    },
+
+    /// `box P`, `&P`, `&mut P`, etc.
+    Deref {
+        subpattern: Pat,
+    },
+
+    // FIXME: for now, only bool literals are implemented
+    LiteralBool {
+        value: bool,
+    },
+
+    /// An or-pattern, e.g. `p | q`.
+    /// Invariant: `pats.len() >= 2`.
+    Or {
+        pats: Vec<Pat>,
+    },
+}
 
-    fn from_vec(v: PatStackInner) -> PatStack {
-        Self(v)
-    }
+pub(crate) struct PatCtxt<'a> {
+    db: &'a dyn HirDatabase,
+    infer: &'a InferenceResult,
+    body: &'a Body,
+    pub(crate) errors: Vec<PatternError>,
+}
 
-    fn get_head(&self) -> Option<PatIdOrWild> {
-        self.0.first().copied()
+impl<'a> PatCtxt<'a> {
+    pub(crate) fn new(db: &'a dyn HirDatabase, infer: &'a InferenceResult, body: &'a Body) -> Self {
+        Self { db, infer, body, errors: Vec::new() }
     }
 
-    fn tail(&self) -> &[PatIdOrWild] {
-        self.0.get(1..).unwrap_or(&[])
+    pub(crate) fn lower_pattern(&mut self, pat: hir_def::expr::PatId) -> Pat {
+        // FIXME: implement pattern adjustments (implicit pattern dereference; "RFC 2005-match-ergonomics")
+        // More info https://github.com/rust-lang/rust/issues/42640#issuecomment-313535089
+        let unadjusted_pat = self.lower_pattern_unadjusted(pat);
+        unadjusted_pat
     }
 
-    fn to_tail(&self) -> PatStack {
-        Self::from_slice(self.tail())
-    }
+    fn lower_pattern_unadjusted(&mut self, pat: hir_def::expr::PatId) -> Pat {
+        let mut ty = &self.infer[pat];
+        let variant = self.infer.variant_resolution_for_pat(pat);
 
-    fn replace_head_with<I, T>(&self, pats: I) -> PatStack
-    where
-        I: Iterator<Item = T>,
-        T: Into<PatIdOrWild>,
-    {
-        let mut patterns: PatStackInner = smallvec![];
-        for pat in pats {
-            patterns.push(pat.into());
-        }
-        for pat in &self.0[1..] {
-            patterns.push(*pat);
-        }
-        PatStack::from_vec(patterns)
-    }
+        let kind = match self.body[pat] {
+            hir_def::expr::Pat::Wild => PatKind::Wild,
 
-    /// Computes `D(self)`.
-    ///
-    /// See the module docs and the associated documentation in rustc for details.
-    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
-        if matches!(self.get_head()?.as_pat(cx), Pat::Wild) {
-            Some(self.to_tail())
-        } else {
-            None
-        }
-    }
+            hir_def::expr::Pat::Lit(expr) => self.lower_lit(expr),
 
-    /// Computes `S(constructor, self)`.
-    ///
-    /// See the module docs and the associated documentation in rustc for details.
-    fn specialize_constructor(
-        &self,
-        cx: &MatchCheckCtx,
-        constructor: &Constructor,
-    ) -> MatchCheckResult<Option<PatStack>> {
-        let head = match self.get_head() {
-            Some(head) => head,
-            None => return Ok(None),
-        };
+            hir_def::expr::Pat::Path(ref path) => {
+                return self.lower_path(pat, path);
+            }
 
-        let head_pat = head.as_pat(cx);
-        let result = match (head_pat, constructor) {
-            (Pat::Tuple { args: pat_ids, ellipsis }, &Constructor::Tuple { arity }) => {
-                if let Some(ellipsis) = ellipsis {
-                    let (pre, post) = pat_ids.split_at(ellipsis);
-                    let n_wild_pats = arity.saturating_sub(pat_ids.len());
-                    let pre_iter = pre.iter().map(Into::into);
-                    let wildcards = iter::repeat(PatIdOrWild::Wild).take(n_wild_pats);
-                    let post_iter = post.iter().map(Into::into);
-                    Some(self.replace_head_with(pre_iter.chain(wildcards).chain(post_iter)))
-                } else {
-                    Some(self.replace_head_with(pat_ids.iter()))
-                }
+            hir_def::expr::Pat::Tuple { ref args, ellipsis } => {
+                let arity = match *ty.kind(&Interner) {
+                    TyKind::Tuple(arity, _) => arity,
+                    _ => panic!("unexpected type for tuple pattern: {:?}", ty),
+                };
+                let subpatterns = self.lower_tuple_subpats(args, arity, ellipsis);
+                PatKind::Leaf { subpatterns }
             }
-            (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => {
-                match cx.body.exprs[lit_expr] {
-                    Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => {
-                        Some(self.to_tail())
-                    }
-                    // it was a bool but the value doesn't match
-                    Expr::Literal(Literal::Bool(_)) => None,
-                    // perhaps this is actually unreachable given we have
-                    // already checked that these match arms have the appropriate type?
-                    _ => return Err(MatchCheckErr::NotImplemented),
+
+            hir_def::expr::Pat::Bind { subpat, .. } => {
+                if let TyKind::Ref(.., rty) = ty.kind(&Interner) {
+                    ty = rty;
                 }
+                PatKind::Binding { subpattern: self.lower_opt_pattern(subpat) }
             }
-            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?),
-            (Pat::Path(_), constructor) => {
-                // unit enum variants become `Pat::Path`
-                let pat_id = head.as_id().expect("we know this isn't a wild");
-                let variant_id: VariantId = match constructor {
-                    &Constructor::Enum(e) => e.into(),
-                    &Constructor::Struct(s) => s.into(),
-                    _ => return Err(MatchCheckErr::NotImplemented),
-                };
-                if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) {
-                    None
-                } else {
-                    Some(self.to_tail())
-                }
+
+            hir_def::expr::Pat::TupleStruct { ref args, ellipsis, .. } if variant.is_some() => {
+                let expected_len = variant.unwrap().variant_data(self.db.upcast()).fields().len();
+                let subpatterns = self.lower_tuple_subpats(args, expected_len, ellipsis);
+                self.lower_variant_or_leaf(pat, ty, subpatterns)
             }
-            (Pat::TupleStruct { args: ref pat_ids, ellipsis, .. }, constructor) => {
-                let pat_id = head.as_id().expect("we know this isn't a wild");
-                let variant_id: VariantId = match constructor {
-                    &Constructor::Enum(e) => e.into(),
-                    &Constructor::Struct(s) => s.into(),
-                    _ => return Err(MatchCheckErr::MalformedMatchArm),
-                };
-                if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) {
-                    None
-                } else {
-                    let constructor_arity = constructor.arity(cx)?;
-                    if let Some(ellipsis_position) = ellipsis {
-                        // If there are ellipsis in the pattern, the ellipsis must take the place
-                        // of at least one sub-pattern, so `pat_ids` should be smaller than the
-                        // constructor arity.
-                        if pat_ids.len() < constructor_arity {
-                            let mut new_patterns: Vec<PatIdOrWild> = vec![];
-
-                            for pat_id in &pat_ids[0..ellipsis_position] {
-                                new_patterns.push((*pat_id).into());
-                            }
-
-                            for _ in 0..(constructor_arity - pat_ids.len()) {
-                                new_patterns.push(PatIdOrWild::Wild);
-                            }
-
-                            for pat_id in &pat_ids[ellipsis_position..pat_ids.len()] {
-                                new_patterns.push((*pat_id).into());
-                            }
-
-                            Some(self.replace_head_with(new_patterns.into_iter()))
-                        } else {
-                            return Err(MatchCheckErr::MalformedMatchArm);
-                        }
-                    } else {
-                        // If there is no ellipsis in the tuple pattern, the number
-                        // of patterns must equal the constructor arity.
-                        if pat_ids.len() == constructor_arity {
-                            Some(self.replace_head_with(pat_ids.into_iter()))
-                        } else {
-                            return Err(MatchCheckErr::MalformedMatchArm);
-                        }
+
+            hir_def::expr::Pat::Record { ref args, .. } if variant.is_some() => {
+                let variant_data = variant.unwrap().variant_data(self.db.upcast());
+                let subpatterns = args
+                    .iter()
+                    .map(|field| {
+                        // XXX(iDawer): field lookup is inefficient
+                        variant_data.field(&field.name).map(|lfield_id| FieldPat {
+                            field: lfield_id,
+                            pattern: self.lower_pattern(field.pat),
+                        })
+                    })
+                    .collect();
+                match subpatterns {
+                    Some(subpatterns) => self.lower_variant_or_leaf(pat, ty, subpatterns),
+                    None => {
+                        self.errors.push(PatternError::MissingField);
+                        PatKind::Wild
                     }
                 }
             }
-            (Pat::Record { args: ref arg_patterns, .. }, constructor) => {
-                let pat_id = head.as_id().expect("we know this isn't a wild");
-                let (variant_id, variant_data) = match constructor {
-                    &Constructor::Enum(e) => (
-                        e.into(),
-                        cx.db.enum_data(e.parent).variants[e.local_id].variant_data.clone(),
-                    ),
-                    &Constructor::Struct(s) => {
-                        (s.into(), cx.db.struct_data(s).variant_data.clone())
-                    }
-                    _ => return Err(MatchCheckErr::MalformedMatchArm),
-                };
-                if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) {
-                    None
-                } else {
-                    match variant_data.as_ref() {
-                        VariantData::Record(struct_field_arena) => {
-                            // Here we treat any missing fields in the record as the wild pattern, as
-                            // if the record has ellipsis. We want to do this here even if the
-                            // record does not contain ellipsis, because it allows us to continue
-                            // enforcing exhaustiveness for the rest of the match statement.
-                            //
-                            // Creating the diagnostic for the missing field in the pattern
-                            // should be done in a different diagnostic.
-                            let patterns = struct_field_arena.iter().map(|(_, struct_field)| {
-                                arg_patterns
-                                    .iter()
-                                    .find(|pat| pat.name == struct_field.name)
-                                    .map(|pat| PatIdOrWild::from(pat.pat))
-                                    .unwrap_or(PatIdOrWild::Wild)
-                            });
-
-                            Some(self.replace_head_with(patterns))
-                        }
-                        _ => return Err(MatchCheckErr::Unknown),
-                    }
-                }
+            hir_def::expr::Pat::TupleStruct { .. } | hir_def::expr::Pat::Record { .. } => {
+                self.errors.push(PatternError::UnresolvedVariant);
+                PatKind::Wild
             }
-            (Pat::Or(_), _) => return Err(MatchCheckErr::NotImplemented),
-            (_, _) => return Err(MatchCheckErr::NotImplemented),
-        };
 
-        Ok(result)
-    }
-
-    /// A special case of `specialize_constructor` where the head of the pattern stack
-    /// is a Wild pattern.
-    ///
-    /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns
-    /// (N >= 0), where N is the arity of the given constructor.
-    fn expand_wildcard(
-        &self,
-        cx: &MatchCheckCtx,
-        constructor: &Constructor,
-    ) -> MatchCheckResult<PatStack> {
-        assert_eq!(
-            Pat::Wild,
-            self.get_head().expect("expand_wildcard called on empty PatStack").as_pat(cx),
-            "expand_wildcard must only be called on PatStack with wild at head",
-        );
+            hir_def::expr::Pat::Or(ref pats) => PatKind::Or { pats: self.lower_patterns(pats) },
 
-        let mut patterns: PatStackInner = smallvec![];
+            _ => {
+                self.errors.push(PatternError::Unimplemented);
+                PatKind::Wild
+            }
+        };
 
-        for _ in 0..constructor.arity(cx)? {
-            patterns.push(PatIdOrWild::Wild);
-        }
+        Pat { ty: ty.clone(), kind: Box::new(kind) }
+    }
 
-        for pat in &self.0[1..] {
-            patterns.push(*pat);
+    fn lower_tuple_subpats(
+        &mut self,
+        pats: &[hir_def::expr::PatId],
+        expected_len: usize,
+        ellipsis: Option<usize>,
+    ) -> Vec<FieldPat> {
+        if pats.len() > expected_len {
+            self.errors.push(PatternError::ExtraFields);
+            return Vec::new();
         }
 
-        Ok(PatStack::from_vec(patterns))
+        pats.iter()
+            .enumerate_and_adjust(expected_len, ellipsis)
+            .map(|(i, &subpattern)| FieldPat {
+                field: LocalFieldId::from_raw((i as u32).into()),
+                pattern: self.lower_pattern(subpattern),
+            })
+            .collect()
     }
-}
 
-/// A collection of PatStack.
-///
-/// This type is modeled from the struct of the same name in `rustc`.
-pub(super) struct Matrix(Vec<PatStack>);
+    fn lower_patterns(&mut self, pats: &[hir_def::expr::PatId]) -> Vec<Pat> {
+        pats.iter().map(|&p| self.lower_pattern(p)).collect()
+    }
 
-impl Matrix {
-    pub(super) fn empty() -> Self {
-        Self(vec![])
+    fn lower_opt_pattern(&mut self, pat: Option<hir_def::expr::PatId>) -> Option<Pat> {
+        pat.map(|p| self.lower_pattern(p))
     }
 
-    pub(super) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
-        if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
-            // Or patterns are expanded here
-            for pat_id in pat_ids {
-                self.0.push(row.replace_head_with([pat_id].iter()));
+    fn lower_variant_or_leaf(
+        &mut self,
+        pat: hir_def::expr::PatId,
+        ty: &Ty,
+        subpatterns: Vec<FieldPat>,
+    ) -> PatKind {
+        let kind = match self.infer.variant_resolution_for_pat(pat) {
+            Some(variant_id) => {
+                if let VariantId::EnumVariantId(enum_variant) = variant_id {
+                    let substs = match ty.kind(&Interner) {
+                        TyKind::Adt(_, substs) | TyKind::FnDef(_, substs) => substs.clone(),
+                        TyKind::Error => {
+                            return PatKind::Wild;
+                        }
+                        _ => panic!("inappropriate type for def: {:?}", ty),
+                    };
+                    PatKind::Variant { substs, enum_variant, subpatterns }
+                } else {
+                    PatKind::Leaf { subpatterns }
+                }
             }
-        } else {
-            self.0.push(row);
-        }
+            None => {
+                self.errors.push(PatternError::UnresolvedVariant);
+                PatKind::Wild
+            }
+        };
+        kind
     }
 
-    fn is_empty(&self) -> bool {
-        self.0.is_empty()
-    }
+    fn lower_path(&mut self, pat: hir_def::expr::PatId, _path: &hir_def::path::Path) -> Pat {
+        let ty = &self.infer[pat];
 
-    fn heads(&self) -> Vec<PatIdOrWild> {
-        self.0.iter().flat_map(|p| p.get_head()).collect()
-    }
+        let pat_from_kind = |kind| Pat { ty: ty.clone(), kind: Box::new(kind) };
 
-    /// Computes `D(self)` for each contained PatStack.
-    ///
-    /// See the module docs and the associated documentation in rustc for details.
-    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
-        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
+        match self.infer.variant_resolution_for_pat(pat) {
+            Some(_) => pat_from_kind(self.lower_variant_or_leaf(pat, ty, Vec::new())),
+            None => {
+                self.errors.push(PatternError::UnresolvedVariant);
+                pat_from_kind(PatKind::Wild)
+            }
+        }
     }
 
-    /// Computes `S(constructor, self)` for each contained PatStack.
-    ///
-    /// See the module docs and the associated documentation in rustc for details.
-    fn specialize_constructor(
-        &self,
-        cx: &MatchCheckCtx,
-        constructor: &Constructor,
-    ) -> MatchCheckResult<Self> {
-        let mut new_matrix = Matrix::empty();
-        for pat in &self.0 {
-            if let Some(pat) = pat.specialize_constructor(cx, constructor)? {
-                new_matrix.push(cx, pat);
+    fn lower_lit(&mut self, expr: hir_def::expr::ExprId) -> PatKind {
+        use hir_def::expr::{Expr, Literal::Bool};
+
+        match self.body[expr] {
+            Expr::Literal(Bool(value)) => PatKind::LiteralBool { value },
+            _ => {
+                self.errors.push(PatternError::Unimplemented);
+                PatKind::Wild
             }
         }
+    }
+}
 
-        Ok(new_matrix)
+pub(crate) trait PatternFoldable: Sized {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.super_fold_with(folder)
     }
 
-    fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
-        let mut matrix = Matrix::empty();
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self;
+}
 
-        for pat in iter {
-            // using push ensures we expand or-patterns
-            matrix.push(cx, pat);
-        }
+pub(crate) trait PatternFolder: Sized {
+    fn fold_pattern(&mut self, pattern: &Pat) -> Pat {
+        pattern.super_fold_with(self)
+    }
 
-        matrix
+    fn fold_pattern_kind(&mut self, kind: &PatKind) -> PatKind {
+        kind.super_fold_with(self)
     }
 }
 
-#[derive(Clone, Debug, PartialEq)]
-/// An indication of the usefulness of a given match arm, where
-/// usefulness is defined as matching some patterns which were
-/// not matched by an prior match arms.
-///
-/// We may eventually need an `Unknown` variant here.
-pub(super) enum Usefulness {
-    Useful,
-    NotUseful,
+impl<T: PatternFoldable> PatternFoldable for Box<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        let content: T = (**self).fold_with(folder);
+        Box::new(content)
+    }
 }
 
-pub(super) struct MatchCheckCtx<'a> {
-    pub(super) match_expr: Idx<Expr>,
-    pub(super) body: Arc<Body>,
-    pub(super) infer: Arc<InferenceResult>,
-    pub(super) db: &'a dyn HirDatabase,
+impl<T: PatternFoldable> PatternFoldable for Vec<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.iter().map(|t| t.fold_with(folder)).collect()
+    }
 }
 
-/// Given a set of patterns `matrix`, and pattern to consider `v`, determines
-/// whether `v` is useful. A pattern is useful if it covers cases which were
-/// not previously covered.
-///
-/// When calling this function externally (that is, not the recursive calls) it
-/// expected that you have already type checked the match arms. All patterns in
-/// matrix should be the same type as v, as well as they should all be the same
-/// type as the match expression.
-pub(super) fn is_useful(
-    cx: &MatchCheckCtx,
-    matrix: &Matrix,
-    v: &PatStack,
-) -> MatchCheckResult<Usefulness> {
-    // Handle two special cases:
-    // - enum with no variants
-    // - `!` type
-    // In those cases, no match arm is useful.
-    match cx.infer[cx.match_expr].strip_references().kind(&Interner) {
-        TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ..) => {
-            if cx.db.enum_data(*enum_id).variants.is_empty() {
-                return Ok(Usefulness::NotUseful);
-            }
-        }
-        TyKind::Never => return Ok(Usefulness::NotUseful),
-        _ => (),
+impl<T: PatternFoldable> PatternFoldable for Option<T> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        self.as_ref().map(|t| t.fold_with(folder))
     }
+}
 
-    let head = match v.get_head() {
-        Some(head) => head,
-        None => {
-            let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
-
-            return Ok(result);
-        }
-    };
-
-    if let Pat::Or(pat_ids) = head.as_pat(cx) {
-        let mut found_unimplemented = false;
-        let any_useful = pat_ids.iter().any(|&pat_id| {
-            let v = PatStack::from_pattern(pat_id);
-
-            match is_useful(cx, matrix, &v) {
-                Ok(Usefulness::Useful) => true,
-                Ok(Usefulness::NotUseful) => false,
-                _ => {
-                    found_unimplemented = true;
-                    false
+macro_rules! clone_impls {
+    ($($ty:ty),+) => {
+        $(
+            impl PatternFoldable for $ty {
+                fn super_fold_with<F: PatternFolder>(&self, _: &mut F) -> Self {
+                    Clone::clone(self)
                 }
             }
-        });
-
-        return if any_useful {
-            Ok(Usefulness::Useful)
-        } else if found_unimplemented {
-            Err(MatchCheckErr::NotImplemented)
-        } else {
-            Ok(Usefulness::NotUseful)
-        };
+        )+
     }
+}
 
-    if let Some(constructor) = pat_constructor(cx, head)? {
-        let matrix = matrix.specialize_constructor(&cx, &constructor)?;
-        let v = v
-            .specialize_constructor(&cx, &constructor)?
-            .expect("we know this can't fail because we get the constructor from `v.head()` above");
-
-        is_useful(&cx, &matrix, &v)
-    } else {
-        // expanding wildcard
-        let mut used_constructors: Vec<Constructor> = vec![];
-        for pat in matrix.heads() {
-            if let Some(constructor) = pat_constructor(cx, pat)? {
-                used_constructors.push(constructor);
-            }
-        }
-
-        // We assume here that the first constructor is the "correct" type. Since we
-        // only care about the "type" of the constructor (i.e. if it is a bool we
-        // don't care about the value), this assumption should be valid as long as
-        // the match statement is well formed. We currently uphold this invariant by
-        // filtering match arms before calling `is_useful`, only passing in match arms
-        // whose type matches the type of the match expression.
-        match &used_constructors.first() {
-            Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
-                // If all constructors are covered, then we need to consider whether
-                // any values are covered by this wildcard.
-                //
-                // For example, with matrix '[[Some(true)], [None]]', all
-                // constructors are covered (`Some`/`None`), so we need
-                // to perform specialization to see that our wildcard will cover
-                // the `Some(false)` case.
-                //
-                // Here we create a constructor for each variant and then check
-                // usefulness after specializing for that constructor.
-                let mut found_unimplemented = false;
-                for constructor in constructor.all_constructors(cx) {
-                    let matrix = matrix.specialize_constructor(&cx, &constructor)?;
-                    let v = v.expand_wildcard(&cx, &constructor)?;
-
-                    match is_useful(&cx, &matrix, &v) {
-                        Ok(Usefulness::Useful) => return Ok(Usefulness::Useful),
-                        Ok(Usefulness::NotUseful) => continue,
-                        _ => found_unimplemented = true,
-                    };
-                }
+clone_impls! { LocalFieldId, Ty, Substitution, EnumVariantId }
 
-                if found_unimplemented {
-                    Err(MatchCheckErr::NotImplemented)
-                } else {
-                    Ok(Usefulness::NotUseful)
-                }
-            }
-            _ => {
-                // Either not all constructors are covered, or the only other arms
-                // are wildcards. Either way, this pattern is useful if it is useful
-                // when compared to those arms with wildcards.
-                let matrix = matrix.specialize_wildcard(&cx);
-                let v = v.to_tail();
-
-                is_useful(&cx, &matrix, &v)
-            }
-        }
+impl PatternFoldable for FieldPat {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        FieldPat { field: self.field.fold_with(folder), pattern: self.pattern.fold_with(folder) }
     }
 }
 
-#[derive(Debug, Clone, Copy)]
-/// Similar to TypeCtor, but includes additional information about the specific
-/// value being instantiated. For example, TypeCtor::Bool doesn't contain the
-/// boolean value.
-enum Constructor {
-    Bool(bool),
-    Tuple { arity: usize },
-    Enum(EnumVariantId),
-    Struct(StructId),
-}
+impl PatternFoldable for Pat {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        folder.fold_pattern(self)
+    }
 
-impl Constructor {
-    fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> {
-        let arity = match self {
-            Constructor::Bool(_) => 0,
-            Constructor::Tuple { arity } => *arity,
-            Constructor::Enum(e) => {
-                match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
-                    VariantData::Tuple(struct_field_data) => struct_field_data.len(),
-                    VariantData::Record(struct_field_data) => struct_field_data.len(),
-                    VariantData::Unit => 0,
-                }
-            }
-            &Constructor::Struct(s) => match cx.db.struct_data(s).variant_data.as_ref() {
-                VariantData::Tuple(struct_field_data) => struct_field_data.len(),
-                VariantData::Record(struct_field_data) => struct_field_data.len(),
-                VariantData::Unit => 0,
-            },
-        };
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        Pat { ty: self.ty.fold_with(folder), kind: self.kind.fold_with(folder) }
+    }
+}
 
-        Ok(arity)
+impl PatternFoldable for PatKind {
+    fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
+        folder.fold_pattern_kind(self)
     }
 
-    fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> {
+    fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
         match self {
-            Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)],
-            Constructor::Tuple { .. } | Constructor::Struct(_) => vec![*self],
-            Constructor::Enum(e) => cx
-                .db
-                .enum_data(e.parent)
-                .variants
-                .iter()
-                .map(|(local_id, _)| {
-                    Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
-                })
-                .collect(),
+            PatKind::Wild => PatKind::Wild,
+            PatKind::Binding { subpattern } => {
+                PatKind::Binding { subpattern: subpattern.fold_with(folder) }
+            }
+            PatKind::Variant { substs, enum_variant, subpatterns } => PatKind::Variant {
+                substs: substs.fold_with(folder),
+                enum_variant: enum_variant.fold_with(folder),
+                subpatterns: subpatterns.fold_with(folder),
+            },
+            PatKind::Leaf { subpatterns } => {
+                PatKind::Leaf { subpatterns: subpatterns.fold_with(folder) }
+            }
+            PatKind::Deref { subpattern } => {
+                PatKind::Deref { subpattern: subpattern.fold_with(folder) }
+            }
+            &PatKind::LiteralBool { value } => PatKind::LiteralBool { value },
+            PatKind::Or { pats } => PatKind::Or { pats: pats.fold_with(folder) },
         }
     }
 }
 
-/// Returns the constructor for the given pattern. Should only return None
-/// in the case of a Wild pattern.
-fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> {
-    let res = match pat.as_pat(cx) {
-        Pat::Wild => None,
-        Pat::Tuple { .. } => {
-            let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
-            Some(Constructor::Tuple {
-                arity: cx.infer.type_of_pat[pat_id]
-                    .as_tuple()
-                    .ok_or(MatchCheckErr::Unknown)?
-                    .len(&Interner),
-            })
-        }
-        Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
-            Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
-            _ => return Err(MatchCheckErr::NotImplemented),
-        },
-        Pat::TupleStruct { .. } | Pat::Path(_) | Pat::Record { .. } => {
-            let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
-            let variant_id =
-                cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckErr::Unknown)?;
-            match variant_id {
-                VariantId::EnumVariantId(enum_variant_id) => {
-                    Some(Constructor::Enum(enum_variant_id))
-                }
-                VariantId::StructId(struct_id) => Some(Constructor::Struct(struct_id)),
-                _ => return Err(MatchCheckErr::NotImplemented),
-            }
-        }
-        _ => return Err(MatchCheckErr::NotImplemented),
-    };
+#[cfg(test)]
+pub(super) mod tests {
+    mod report {
+        use std::any::Any;
 
-    Ok(res)
-}
+        use hir_def::{expr::PatId, DefWithBodyId};
+        use hir_expand::{HirFileId, InFile};
+        use syntax::SyntaxNodePtr;
 
-fn all_constructors_covered(
-    cx: &MatchCheckCtx,
-    constructor: &Constructor,
-    used_constructors: &[Constructor],
-) -> bool {
-    match constructor {
-        Constructor::Tuple { arity } => {
-            used_constructors.iter().any(|constructor| match constructor {
-                Constructor::Tuple { arity: used_arity } => arity == used_arity,
-                _ => false,
-            })
-        }
-        Constructor::Bool(_) => {
-            if used_constructors.is_empty() {
-                return false;
-            }
+        use crate::{
+            db::HirDatabase,
+            diagnostics_sink::{Diagnostic, DiagnosticCode, DiagnosticSink},
+        };
 
-            let covers_true =
-                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true)));
-            let covers_false =
-                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false)));
+        /// In tests, match check bails out loudly.
+        /// This helps to catch incorrect tests that pass due to false negatives.
+        pub(crate) fn report_bail_out(
+            db: &dyn HirDatabase,
+            def: DefWithBodyId,
+            pat: PatId,
+            sink: &mut DiagnosticSink,
+        ) {
+            let (_, source_map) = db.body_with_source_map(def);
+            if let Ok(source_ptr) = source_map.pat_syntax(pat) {
+                let pat_syntax_ptr = source_ptr.value.either(Into::into, Into::into);
+                sink.push(BailedOut { file: source_ptr.file_id, pat_syntax_ptr });
+            }
+        }
 
-            covers_true && covers_false
+        #[derive(Debug)]
+        struct BailedOut {
+            file: HirFileId,
+            pat_syntax_ptr: SyntaxNodePtr,
         }
-        Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
-            for constructor in used_constructors {
-                if let Constructor::Enum(e) = constructor {
-                    if id == e.local_id {
-                        return true;
-                    }
-                }
-            }
 
-            false
-        }),
-        &Constructor::Struct(s) => used_constructors.iter().any(|constructor| match constructor {
-            &Constructor::Struct(sid) => sid == s,
-            _ => false,
-        }),
+        impl Diagnostic for BailedOut {
+            fn code(&self) -> DiagnosticCode {
+                DiagnosticCode("internal:match-check-bailed-out")
+            }
+            fn message(&self) -> String {
+                format!("Internal: match check bailed out")
+            }
+            fn display_source(&self) -> InFile<SyntaxNodePtr> {
+                InFile { file_id: self.file, value: self.pat_syntax_ptr.clone() }
+            }
+            fn as_any(&self) -> &(dyn Any + Send + 'static) {
+                self
+            }
+        }
     }
-}
 
-#[cfg(test)]
-mod tests {
     use crate::diagnostics::tests::check_diagnostics;
 
+    pub(crate) use self::report::report_bail_out;
+
     #[test]
     fn empty_tuple() {
         check_diagnostics(
@@ -1113,14 +658,18 @@ enum Either2 { C, D }
 fn main() {
     match Either::A {
         Either2::C => (),
+    //  ^^^^^^^^^^ Internal: match check bailed out
         Either2::D => (),
     }
     match (true, false) {
         (true, false, true) => (),
+    //  ^^^^^^^^^^^^^^^^^^^ Internal: match check bailed out
         (true) => (),
     }
     match (true, false) { (true,) => {} }
+    //                    ^^^^^^^ Internal: match check bailed out
     match (0) { () => () }
+            //  ^^ Internal: match check bailed out
     match Unresolved::Bar { Unresolved::Baz => () }
 }
         "#,
@@ -1133,7 +682,9 @@ fn main() {
             r#"
 fn main() {
     match false { true | () => {} }
+    //            ^^^^^^^^^ Internal: match check bailed out
     match (false,) { (true | (),) => {} }
+    //               ^^^^^^^^^^^^ Internal: match check bailed out
 }
 "#,
         );
@@ -1158,6 +709,25 @@ fn main() {
     }
 
     #[test]
+    fn malformed_match_arm_extra_fields() {
+        check_diagnostics(
+            r#"
+enum A { B(isize, isize), C }
+fn main() {
+    match A::B(1, 2) {
+        A::B(_, _, _) => (),
+    //  ^^^^^^^^^^^^^ Internal: match check bailed out
+    }
+    match A::B(1, 2) {
+        A::C(_) => (),
+    //  ^^^^^^^ Internal: match check bailed out
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
     fn expr_diverges() {
         check_diagnostics(
             r#"
@@ -1166,10 +736,12 @@ enum Either { A, B }
 fn main() {
     match loop {} {
         Either::A => (),
+    //  ^^^^^^^^^ Internal: match check bailed out
         Either::B => (),
     }
     match loop {} {
         Either::A => (),
+    //  ^^^^^^^^^ Internal: match check bailed out
     }
     match loop { break Foo::A } {
         //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
@@ -1357,6 +929,7 @@ fn enum_(never: Never) {
 }
 fn enum_ref(never: &Never) {
     match never {}
+        //^^^^^ Missing match arm
 }
 fn bang(never: !) {
     match never {}
@@ -1376,6 +949,11 @@ fn main() {
     match Option::<Never>::None {
         None => (),
         Some(never) => match never {},
+    //  ^^^^^^^^^^^ Internal: match check bailed out
+    }
+    match Option::<Never>::None {
+        //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
+        Option::Some(_never) => {},
     }
 }
 "#,
@@ -1513,6 +1091,151 @@ fn main() {
 "#,
         );
     }
+
+    #[test]
+    fn no_panic_at_unimplemented_subpattern_type() {
+        check_diagnostics(
+            r#"
+struct S { a: char}
+fn main(v: S) {
+    match v { S{ a }      => {} }
+    match v { S{ a: _x }  => {} }
+    match v { S{ a: 'a' } => {} }
+            //^^^^^^^^^^^ Internal: match check bailed out
+    match v { S{..}       => {} }
+    match v { _           => {} }
+    match v { }
+        //^ Missing match arm
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn binding() {
+        check_diagnostics(
+            r#"
+fn main() {
+    match true {
+        _x @ true => {}
+        false     => {}
+    }
+    match true { _x @ true => {} }
+        //^^^^ Missing match arm
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn binding_ref_has_correct_type() {
+        // Asserts `PatKind::Binding(ref _x): bool`, not &bool.
+        // If that's not true match checking will panic with "incompatible constructors"
+        // FIXME: make facilities to test this directly like `tests::check_infer(..)`
+        check_diagnostics(
+            r#"
+enum Foo { A }
+fn main() {
+    // FIXME: this should not bail out but current behavior is such as the old algorithm.
+    // ExprValidator::validate_match(..) checks types of top level patterns incorrecly.
+    match Foo::A {
+        ref _x => {}
+    //  ^^^^^^ Internal: match check bailed out
+        Foo::A => {}
+    }
+    match (true,) {
+        (ref _x,) => {}
+        (true,) => {}
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_non_exhaustive() {
+        check_diagnostics(
+            r#"
+//- /lib.rs crate:lib
+#[non_exhaustive]
+pub enum E { A, B }
+fn _local() {
+    match E::A { _ => {} }
+    match E::A {
+        E::A => {}
+        E::B => {}
+    }
+    match E::A {
+        E::A | E::B => {}
+    }
+}
+
+//- /main.rs crate:main deps:lib
+use lib::E;
+fn main() {
+    match E::A { _ => {} }
+    match E::A {
+        //^^^^ Missing match arm
+        E::A => {}
+        E::B => {}
+    }
+    match E::A {
+        //^^^^ Missing match arm
+        E::A | E::B => {}
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn match_guard() {
+        check_diagnostics(
+            r#"
+fn main() {
+    match true {
+        true if false => {}
+        true          => {}
+        false         => {}
+    }
+    match true {
+        //^^^^ Missing match arm
+        true if false => {}
+        false         => {}
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn pattern_type_is_of_substitution() {
+        cov_mark::check!(match_check_wildcard_expanded_to_substitutions);
+        check_diagnostics(
+            r#"
+struct Foo<T>(T);
+struct Bar;
+fn main() {
+    match Foo(Bar) {
+        _ | Foo(Bar) => {}
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn record_struct_no_such_field() {
+        check_diagnostics(
+            r#"
+struct Foo { }
+fn main(f: Foo) {
+    match f { Foo { bar } => () }
+    //        ^^^^^^^^^^^ Internal: match check bailed out
+}
+"#,
+        );
+    }
+
     mod false_negatives {
         //! The implementation of match checking here is a work in progress. As we roll this out, we
         //! prefer false negatives to false positives (ideally there would be no false positives). This
@@ -1533,11 +1256,44 @@ fn main() {
 fn main() {
     match 5 {
         10 => (),
+    //  ^^ Internal: match check bailed out
         11..20 => (),
     }
 }
 "#,
             );
         }
+
+        #[test]
+        fn reference_patterns_at_top_level() {
+            check_diagnostics(
+                r#"
+fn main() {
+    match &false {
+        &true => {}
+    //  ^^^^^ Internal: match check bailed out
+    }
+}
+            "#,
+            );
+        }
+
+        #[test]
+        fn reference_patterns_in_fields() {
+            check_diagnostics(
+                r#"
+fn main() {
+    match (&false,) {
+        (true,) => {}
+    //  ^^^^^^^ Internal: match check bailed out
+    }
+    match (&false,) {
+        (&true,) => {}
+    //  ^^^^^^^^ Internal: match check bailed out
+    }
+}
+            "#,
+            );
+        }
     }
 }
diff --git a/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
new file mode 100644
index 000000000..1f4219b42
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
@@ -0,0 +1,907 @@
+//! [`super::usefulness`] explains most of what is happening in this file. As explained there,
+//! values and patterns are made from constructors applied to fields. This file defines a
+//! `Constructor` enum, a `Fields` struct, and various operations to manipulate them and convert
+//! them from/to patterns.
+//!
+//! There's one idea that is not detailed in [`super::usefulness`] because the details are not
+//! needed there: _constructor splitting_.
+//!
+//! # Constructor splitting
+//!
+//! The idea is as follows: given a constructor `c` and a matrix, we want to specialize in turn
+//! with all the value constructors that are covered by `c`, and compute usefulness for each.
+//! Instead of listing all those constructors (which is intractable), we group those value
+//! constructors together as much as possible. Example:
+//!
+//! ```
+//! match (0, false) {
+//!     (0 ..=100, true) => {} // `p_1`
+//!     (50..=150, false) => {} // `p_2`
+//!     (0 ..=200, _) => {} // `q`
+//! }
+//! ```
+//!
+//! The naive approach would try all numbers in the range `0..=200`. But we can be a lot more
+//! clever: `0` and `1` for example will match the exact same rows, and return equivalent
+//! witnesses. In fact all of `0..50` would. We can thus restrict our exploration to 4
+//! constructors: `0..50`, `50..=100`, `101..=150` and `151..=200`. That is enough and infinitely
+//! more tractable.
+//!
+//! We capture this idea in a function `split(p_1 ... p_n, c)` which returns a list of constructors
+//! `c'` covered by `c`. Given such a `c'`, we require that all value ctors `c''` covered by `c'`
+//! return an equivalent set of witnesses after specializing and computing usefulness.
+//! In the example above, witnesses for specializing by `c''` covered by `0..50` will only differ
+//! in their first element.
+//!
+//! We usually also ask that the `c'` together cover all of the original `c`. However we allow
+//! skipping some constructors as long as it doesn't change whether the resulting list of witnesses
+//! is empty of not. We use this in the wildcard `_` case.
+//!
+//! Splitting is implemented in the [`Constructor::split`] function. We don't do splitting for
+//! or-patterns; instead we just try the alternatives one-by-one. For details on splitting
+//! wildcards, see [`SplitWildcard`]; for integer ranges, see [`SplitIntRange`]; for slices, see
+//! [`SplitVarLenSlice`].
+
+use std::{
+    cmp::{max, min},
+    iter::once,
+    ops::RangeInclusive,
+};
+
+use hir_def::{EnumVariantId, HasModule, LocalFieldId, VariantId};
+use smallvec::{smallvec, SmallVec};
+
+use crate::{AdtId, Interner, Scalar, Ty, TyExt, TyKind};
+
+use super::{
+    usefulness::{MatchCheckCtx, PatCtxt},
+    FieldPat, Pat, PatId, PatKind,
+};
+
+use self::Constructor::*;
+
+/// [Constructor] uses this in umimplemented variants.
+/// It allows porting match expressions from upstream algorithm without losing semantics.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub(super) enum Void {}
+
+/// An inclusive interval, used for precise integer exhaustiveness checking.
+/// `IntRange`s always store a contiguous range. This means that values are
+/// encoded such that `0` encodes the minimum value for the integer,
+/// regardless of the signedness.
+/// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
+/// This makes comparisons and arithmetic on interval endpoints much more
+/// straightforward. See `signed_bias` for details.
+///
+/// `IntRange` is never used to encode an empty range or a "range" that wraps
+/// around the (offset) space: i.e., `range.lo <= range.hi`.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(super) struct IntRange {
+    range: RangeInclusive<u128>,
+}
+
+impl IntRange {
+    #[inline]
+    fn is_integral(ty: &Ty) -> bool {
+        match ty.kind(&Interner) {
+            TyKind::Scalar(Scalar::Char)
+            | TyKind::Scalar(Scalar::Int(_))
+            | TyKind::Scalar(Scalar::Uint(_))
+            | TyKind::Scalar(Scalar::Bool) => true,
+            _ => false,
+        }
+    }
+
+    fn is_singleton(&self) -> bool {
+        self.range.start() == self.range.end()
+    }
+
+    fn boundaries(&self) -> (u128, u128) {
+        (*self.range.start(), *self.range.end())
+    }
+
+    #[inline]
+    fn from_bool(value: bool) -> IntRange {
+        let val = value as u128;
+        IntRange { range: val..=val }
+    }
+
+    #[inline]
+    fn from_range(lo: u128, hi: u128, scalar_ty: Scalar) -> IntRange {
+        if let Scalar::Bool = scalar_ty {
+            IntRange { range: lo..=hi }
+        } else {
+            unimplemented!()
+        }
+    }
+
+    fn is_subrange(&self, other: &Self) -> bool {
+        other.range.start() <= self.range.start() && self.range.end() <= other.range.end()
+    }
+
+    fn intersection(&self, other: &Self) -> Option<Self> {
+        let (lo, hi) = self.boundaries();
+        let (other_lo, other_hi) = other.boundaries();
+        if lo <= other_hi && other_lo <= hi {
+            Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi) })
+        } else {
+            None
+        }
+    }
+
+    /// See `Constructor::is_covered_by`
+    fn is_covered_by(&self, other: &Self) -> bool {
+        if self.intersection(other).is_some() {
+            // Constructor splitting should ensure that all intersections we encounter are actually
+            // inclusions.
+            assert!(self.is_subrange(other));
+            true
+        } else {
+            false
+        }
+    }
+}
+
+/// Represents a border between 2 integers. Because the intervals spanning borders must be able to
+/// cover every integer, we need to be able to represent 2^128 + 1 such borders.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+enum IntBorder {
+    JustBefore(u128),
+    AfterMax,
+}
+
+/// A range of integers that is partitioned into disjoint subranges. This does constructor
+/// splitting for integer ranges as explained at the top of the file.
+///
+/// This is fed multiple ranges, and returns an output that covers the input, but is split so that
+/// the only intersections between an output range and a seen range are inclusions. No output range
+/// straddles the boundary of one of the inputs.
+///
+/// The following input:
+/// ```
+///   |-------------------------| // `self`
+/// |------|  |----------|   |----|
+///    |-------| |-------|
+/// ```
+/// would be iterated over as follows:
+/// ```
+///   ||---|--||-|---|---|---|--|
+/// ```
+#[derive(Debug, Clone)]
+struct SplitIntRange {
+    /// The range we are splitting
+    range: IntRange,
+    /// The borders of ranges we have seen. They are all contained within `range`. This is kept
+    /// sorted.
+    borders: Vec<IntBorder>,
+}
+
+impl SplitIntRange {
+    fn new(range: IntRange) -> Self {
+        SplitIntRange { range, borders: Vec::new() }
+    }
+
+    /// Internal use
+    fn to_borders(r: IntRange) -> [IntBorder; 2] {
+        use IntBorder::*;
+        let (lo, hi) = r.boundaries();
+        let lo = JustBefore(lo);
+        let hi = match hi.checked_add(1) {
+            Some(m) => JustBefore(m),
+            None => AfterMax,
+        };
+        [lo, hi]
+    }
+
+    /// Add ranges relative to which we split.
+    fn split(&mut self, ranges: impl Iterator<Item = IntRange>) {
+        let this_range = &self.range;
+        let included_ranges = ranges.filter_map(|r| this_range.intersection(&r));
+        let included_borders = included_ranges.flat_map(|r| {
+            let borders = Self::to_borders(r);
+            once(borders[0]).chain(once(borders[1]))
+        });
+        self.borders.extend(included_borders);
+        self.borders.sort_unstable();
+    }
+
+    /// Iterate over the contained ranges.
+    fn iter(&self) -> impl Iterator<Item = IntRange> + '_ {
+        use IntBorder::*;
+
+        let self_range = Self::to_borders(self.range.clone());
+        // Start with the start of the range.
+        let mut prev_border = self_range[0];
+        self.borders
+            .iter()
+            .copied()
+            // End with the end of the range.
+            .chain(once(self_range[1]))
+            // List pairs of adjacent borders.
+            .map(move |border| {
+                let ret = (prev_border, border);
+                prev_border = border;
+                ret
+            })
+            // Skip duplicates.
+            .filter(|(prev_border, border)| prev_border != border)
+            // Finally, convert to ranges.
+            .map(|(prev_border, border)| {
+                let range = match (prev_border, border) {
+                    (JustBefore(n), JustBefore(m)) if n < m => n..=(m - 1),
+                    (JustBefore(n), AfterMax) => n..=u128::MAX,
+                    _ => unreachable!(), // Ruled out by the sorting and filtering we did
+                };
+                IntRange { range }
+            })
+    }
+}
+
+/// A constructor for array and slice patterns.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub(super) struct Slice {
+    _unimplemented: Void,
+}
+
+impl Slice {
+    /// See `Constructor::is_covered_by`
+    fn is_covered_by(self, _other: Self) -> bool {
+        unimplemented!() // never called as Slice contains Void
+    }
+}
+
+/// A value can be decomposed into a constructor applied to some fields. This struct represents
+/// the constructor. See also `Fields`.
+///
+/// `pat_constructor` retrieves the constructor corresponding to a pattern.
+/// `specialize_constructor` returns the list of fields corresponding to a pattern, given a
+/// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
+/// `Fields`.
+#[allow(dead_code)]
+#[derive(Clone, Debug, PartialEq)]
+pub(super) enum Constructor {
+    /// The constructor for patterns that have a single constructor, like tuples, struct patterns
+    /// and fixed-length arrays.
+    Single,
+    /// Enum variants.
+    Variant(EnumVariantId),
+    /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
+    IntRange(IntRange),
+    /// Ranges of floating-point literal values (`2.0..=5.2`).
+    FloatRange(Void),
+    /// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
+    Str(Void),
+    /// Array and slice patterns.
+    Slice(Slice),
+    /// Constants that must not be matched structurally. They are treated as black
+    /// boxes for the purposes of exhaustiveness: we must not inspect them, and they
+    /// don't count towards making a match exhaustive.
+    Opaque,
+    /// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
+    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
+    NonExhaustive,
+    /// Stands for constructors that are not seen in the matrix, as explained in the documentation
+    /// for [`SplitWildcard`].
+    Missing,
+    /// Wildcard pattern.
+    Wildcard,
+}
+
+impl Constructor {
+    pub(super) fn is_wildcard(&self) -> bool {
+        matches!(self, Wildcard)
+    }
+
+    fn as_int_range(&self) -> Option<&IntRange> {
+        match self {
+            IntRange(range) => Some(range),
+            _ => None,
+        }
+    }
+
+    fn as_slice(&self) -> Option<Slice> {
+        match self {
+            Slice(slice) => Some(*slice),
+            _ => None,
+        }
+    }
+
+    fn variant_id_for_adt(&self, adt: hir_def::AdtId) -> VariantId {
+        match *self {
+            Variant(id) => id.into(),
+            Single => {
+                assert!(!matches!(adt, hir_def::AdtId::EnumId(_)));
+                match adt {
+                    hir_def::AdtId::EnumId(_) => unreachable!(),
+                    hir_def::AdtId::StructId(id) => id.into(),
+                    hir_def::AdtId::UnionId(id) => id.into(),
+                }
+            }
+            _ => panic!("bad constructor {:?} for adt {:?}", self, adt),
+        }
+    }
+
+    /// Determines the constructor that the given pattern can be specialized to.
+    pub(super) fn from_pat(cx: &MatchCheckCtx<'_>, pat: PatId) -> Self {
+        match cx.pattern_arena.borrow()[pat].kind.as_ref() {
+            PatKind::Binding { .. } | PatKind::Wild => Wildcard,
+            PatKind::Leaf { .. } | PatKind::Deref { .. } => Single,
+            &PatKind::Variant { enum_variant, .. } => Variant(enum_variant),
+            &PatKind::LiteralBool { value } => IntRange(IntRange::from_bool(value)),
+            PatKind::Or { .. } => cx.bug("Or-pattern should have been expanded earlier on."),
+        }
+    }
+
+    /// Some constructors (namely `Wildcard`, `IntRange` and `Slice`) actually stand for a set of actual
+    /// constructors (like variants, integers or fixed-sized slices). When specializing for these
+    /// constructors, we want to be specialising for the actual underlying constructors.
+    /// Naively, we would simply return the list of constructors they correspond to. We instead are
+    /// more clever: if there are constructors that we know will behave the same wrt the current
+    /// matrix, we keep them grouped. For example, all slices of a sufficiently large length
+    /// will either be all useful or all non-useful with a given matrix.
+    ///
+    /// See the branches for details on how the splitting is done.
+    ///
+    /// This function may discard some irrelevant constructors if this preserves behavior and
+    /// diagnostics. Eg. for the `_` case, we ignore the constructors already present in the
+    /// matrix, unless all of them are.
+    pub(super) fn split<'a>(
+        &self,
+        pcx: PatCtxt<'_>,
+        ctors: impl Iterator<Item = &'a Constructor> + Clone,
+    ) -> SmallVec<[Self; 1]> {
+        match self {
+            Wildcard => {
+                let mut split_wildcard = SplitWildcard::new(pcx);
+                split_wildcard.split(pcx, ctors);
+                split_wildcard.into_ctors(pcx)
+            }
+            // Fast-track if the range is trivial. In particular, we don't do the overlapping
+            // ranges check.
+            IntRange(ctor_range) if !ctor_range.is_singleton() => {
+                let mut split_range = SplitIntRange::new(ctor_range.clone());
+                let int_ranges = ctors.filter_map(|ctor| ctor.as_int_range());
+                split_range.split(int_ranges.cloned());
+                split_range.iter().map(IntRange).collect()
+            }
+            Slice(_) => unimplemented!(),
+            // Any other constructor can be used unchanged.
+            _ => smallvec![self.clone()],
+        }
+    }
+
+    /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
+    /// For the simple cases, this is simply checking for equality. For the "grouped" constructors,
+    /// this checks for inclusion.
+    // We inline because this has a single call site in `Matrix::specialize_constructor`.
+    #[inline]
+    pub(super) fn is_covered_by(&self, pcx: PatCtxt<'_>, other: &Self) -> bool {
+        // This must be kept in sync with `is_covered_by_any`.
+        match (self, other) {
+            // Wildcards cover anything
+            (_, Wildcard) => true,
+            // The missing ctors are not covered by anything in the matrix except wildcards.
+            (Missing, _) | (Wildcard, _) => false,
+
+            (Single, Single) => true,
+            (Variant(self_id), Variant(other_id)) => self_id == other_id,
+
+            (IntRange(self_range), IntRange(other_range)) => self_range.is_covered_by(other_range),
+            (FloatRange(..), FloatRange(..)) => {
+                unimplemented!()
+            }
+            (Str(..), Str(..)) => {
+                unimplemented!()
+            }
+            (Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),
+
+            // We are trying to inspect an opaque constant. Thus we skip the row.
+            (Opaque, _) | (_, Opaque) => false,
+            // Only a wildcard pattern can match the special extra constructor.
+            (NonExhaustive, _) => false,
+
+            _ => pcx.cx.bug(&format!(
+                "trying to compare incompatible constructors {:?} and {:?}",
+                self, other
+            )),
+        }
+    }
+
+    /// Faster version of `is_covered_by` when applied to many constructors. `used_ctors` is
+    /// assumed to be built from `matrix.head_ctors()` with wildcards filtered out, and `self` is
+    /// assumed to have been split from a wildcard.
+    fn is_covered_by_any(&self, pcx: PatCtxt<'_>, used_ctors: &[Constructor]) -> bool {
+        if used_ctors.is_empty() {
+            return false;
+        }
+
+        // This must be kept in sync with `is_covered_by`.
+        match self {
+            // If `self` is `Single`, `used_ctors` cannot contain anything else than `Single`s.
+            Single => !used_ctors.is_empty(),
+            Variant(_) => used_ctors.iter().any(|c| c == self),
+            IntRange(range) => used_ctors
+                .iter()
+                .filter_map(|c| c.as_int_range())
+                .any(|other| range.is_covered_by(other)),
+            Slice(slice) => used_ctors
+                .iter()
+                .filter_map(|c| c.as_slice())
+                .any(|other| slice.is_covered_by(other)),
+            // This constructor is never covered by anything else
+            NonExhaustive => false,
+            Str(..) | FloatRange(..) | Opaque | Missing | Wildcard => {
+                pcx.cx.bug(&format!("found unexpected ctor in all_ctors: {:?}", self))
+            }
+        }
+    }
+}
+
+/// A wildcard constructor that we split relative to the constructors in the matrix, as explained
+/// at the top of the file.
+///
+/// A constructor that is not present in the matrix rows will only be covered by the rows that have
+/// wildcards. Thus we can group all of those constructors together; we call them "missing
+/// constructors". Splitting a wildcard would therefore list all present constructors individually
+/// (or grouped if they are integers or slices), and then all missing constructors together as a
+/// group.
+///
+/// However we can go further: since any constructor will match the wildcard rows, and having more
+/// rows can only reduce the amount of usefulness witnesses, we can skip the present constructors
+/// and only try the missing ones.
+/// This will not preserve the whole list of witnesses, but will preserve whether the list is empty
+/// or not. In fact this is quite natural from the point of view of diagnostics too. This is done
+/// in `to_ctors`: in some cases we only return `Missing`.
+#[derive(Debug)]
+pub(super) struct SplitWildcard {
+    /// Constructors seen in the matrix.
+    matrix_ctors: Vec<Constructor>,
+    /// All the constructors for this type
+    all_ctors: SmallVec<[Constructor; 1]>,
+}
+
+impl SplitWildcard {
+    pub(super) fn new(pcx: PatCtxt<'_>) -> Self {
+        let cx = pcx.cx;
+        let make_range = |start, end, scalar| IntRange(IntRange::from_range(start, end, scalar));
+
+        // Unhandled types are treated as non-exhaustive. Being explicit here instead of falling
+        // to catchall arm to ease further implementation.
+        let unhandled = || smallvec![NonExhaustive];
+
+        // This determines the set of all possible constructors for the type `pcx.ty`. For numbers,
+        // arrays and slices we use ranges and variable-length slices when appropriate.
+        //
+        // If the `exhaustive_patterns` feature is enabled, we make sure to omit constructors that
+        // are statically impossible. E.g., for `Option<!>`, we do not include `Some(_)` in the
+        // returned list of constructors.
+        // Invariant: this is empty if and only if the type is uninhabited (as determined by
+        // `cx.is_uninhabited()`).
+        let all_ctors = match pcx.ty.kind(&Interner) {
+            TyKind::Scalar(Scalar::Bool) => smallvec![make_range(0, 1, Scalar::Bool)],
+            // TyKind::Array(..) if ... => unhandled(),
+            TyKind::Array(..) | TyKind::Slice(..) => unhandled(),
+            &TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ref _substs) => {
+                let enum_data = cx.db.enum_data(enum_id);
+
+                // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
+                // additional "unknown" constructor.
+                // There is no point in enumerating all possible variants, because the user can't
+                // actually match against them all themselves. So we always return only the fictitious
+                // constructor.
+                // E.g., in an example like:
+                //
+                // ```
+                //     let err: io::ErrorKind = ...;
+                //     match err {
+                //         io::ErrorKind::NotFound => {},
+                //     }
+                // ```
+                //
+                // we don't want to show every possible IO error, but instead have only `_` as the
+                // witness.
+                let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(enum_id);
+
+                // If `exhaustive_patterns` is disabled and our scrutinee is an empty enum, we treat it
+                // as though it had an "unknown" constructor to avoid exposing its emptiness. The
+                // exception is if the pattern is at the top level, because we want empty matches to be
+                // considered exhaustive.
+                let is_secretly_empty = enum_data.variants.is_empty()
+                    && !cx.feature_exhaustive_patterns()
+                    && !pcx.is_top_level;
+
+                if is_secretly_empty || is_declared_nonexhaustive {
+                    smallvec![NonExhaustive]
+                } else if cx.feature_exhaustive_patterns() {
+                    unimplemented!() // see MatchCheckCtx.feature_exhaustive_patterns()
+                } else {
+                    enum_data
+                        .variants
+                        .iter()
+                        .map(|(local_id, ..)| Variant(EnumVariantId { parent: enum_id, local_id }))
+                        .collect()
+                }
+            }
+            TyKind::Scalar(Scalar::Char) => unhandled(),
+            TyKind::Scalar(Scalar::Int(..)) | TyKind::Scalar(Scalar::Uint(..)) => unhandled(),
+            TyKind::Never if !cx.feature_exhaustive_patterns() && !pcx.is_top_level => {
+                smallvec![NonExhaustive]
+            }
+            TyKind::Never => SmallVec::new(),
+            _ if cx.is_uninhabited(&pcx.ty) => SmallVec::new(),
+            TyKind::Adt(..) | TyKind::Tuple(..) | TyKind::Ref(..) => smallvec![Single],
+            // This type is one for which we cannot list constructors, like `str` or `f64`.
+            _ => smallvec![NonExhaustive],
+        };
+        SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
+    }
+
+    /// Pass a set of constructors relative to which to split this one. Don't call twice, it won't
+    /// do what you want.
+    pub(super) fn split<'a>(
+        &mut self,
+        pcx: PatCtxt<'_>,
+        ctors: impl Iterator<Item = &'a Constructor> + Clone,
+    ) {
+        // Since `all_ctors` never contains wildcards, this won't recurse further.
+        self.all_ctors =
+            self.all_ctors.iter().flat_map(|ctor| ctor.split(pcx, ctors.clone())).collect();
+        self.matrix_ctors = ctors.filter(|c| !c.is_wildcard()).cloned().collect();
+    }
+
+    /// Whether there are any value constructors for this type that are not present in the matrix.
+    fn any_missing(&self, pcx: PatCtxt<'_>) -> bool {
+        self.iter_missing(pcx).next().is_some()
+    }
+
+    /// Iterate over the constructors for this type that are not present in the matrix.
+    pub(super) fn iter_missing<'a>(
+        &'a self,
+        pcx: PatCtxt<'a>,
+    ) -> impl Iterator<Item = &'a Constructor> {
+        self.all_ctors.iter().filter(move |ctor| !ctor.is_covered_by_any(pcx, &self.matrix_ctors))
+    }
+
+    /// Return the set of constructors resulting from splitting the wildcard. As explained at the
+    /// top of the file, if any constructors are missing we can ignore the present ones.
+    fn into_ctors(self, pcx: PatCtxt<'_>) -> SmallVec<[Constructor; 1]> {
+        if self.any_missing(pcx) {
+            // Some constructors are missing, thus we can specialize with the special `Missing`
+            // constructor, which stands for those constructors that are not seen in the matrix,
+            // and matches the same rows as any of them (namely the wildcard rows). See the top of
+            // the file for details.
+            // However, when all constructors are missing we can also specialize with the full
+            // `Wildcard` constructor. The difference will depend on what we want in diagnostics.
+
+            // If some constructors are missing, we typically want to report those constructors,
+            // e.g.:
+            // ```
+            //     enum Direction { N, S, E, W }
+            //     let Direction::N = ...;
+            // ```
+            // we can report 3 witnesses: `S`, `E`, and `W`.
+            //
+            // However, if the user didn't actually specify a constructor
+            // in this arm, e.g., in
+            // ```
+            //     let x: (Direction, Direction, bool) = ...;
+            //     let (_, _, false) = x;
+            // ```
+            // we don't want to show all 16 possible witnesses `(<direction-1>, <direction-2>,
+            // true)` - we are satisfied with `(_, _, true)`. So if all constructors are missing we
+            // prefer to report just a wildcard `_`.
+            //
+            // The exception is: if we are at the top-level, for example in an empty match, we
+            // sometimes prefer reporting the list of constructors instead of just `_`.
+            let report_when_all_missing = pcx.is_top_level && !IntRange::is_integral(pcx.ty);
+            let ctor = if !self.matrix_ctors.is_empty() || report_when_all_missing {
+                Missing
+            } else {
+                Wildcard
+            };
+            return smallvec![ctor];
+        }
+
+        // All the constructors are present in the matrix, so we just go through them all.
+        self.all_ctors
+    }
+}
+
+/// A value can be decomposed into a constructor applied to some fields. This struct represents
+/// those fields, generalized to allow patterns in each field. See also `Constructor`.
+/// This is constructed from a constructor using [`Fields::wildcards()`].
+///
+/// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
+/// uninhabited. For that, we filter these fields out of the matrix. This is handled automatically
+/// in `Fields`. This filtering is uncommon in practice, because uninhabited fields are rarely used,
+/// so we avoid it when possible to preserve performance.
+#[derive(Debug, Clone)]
+pub(super) enum Fields {
+    /// Lists of patterns that don't contain any filtered fields.
+    /// `Slice` and `Vec` behave the same; the difference is only to avoid allocating and
+    /// triple-dereferences when possible. Frankly this is premature optimization, I (Nadrieril)
+    /// have not measured if it really made a difference.
+    Vec(SmallVec<[PatId; 2]>),
+}
+
+impl Fields {
+    /// Internal use. Use `Fields::wildcards()` instead.
+    /// Must not be used if the pattern is a field of a struct/tuple/variant.
+    fn from_single_pattern(pat: PatId) -> Self {
+        Fields::Vec(smallvec![pat])
+    }
+
+    /// Convenience; internal use.
+    fn wildcards_from_tys(cx: &MatchCheckCtx<'_>, tys: impl IntoIterator<Item = Ty>) -> Self {
+        let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
+        let pats = wilds.map(|pat| cx.alloc_pat(pat)).collect();
+        Fields::Vec(pats)
+    }
+
+    /// Creates a new list of wildcard fields for a given constructor.
+    pub(crate) fn wildcards(pcx: PatCtxt<'_>, constructor: &Constructor) -> Self {
+        let ty = pcx.ty;
+        let cx = pcx.cx;
+        let wildcard_from_ty = |ty: &Ty| cx.alloc_pat(Pat::wildcard_from_ty(ty.clone()));
+
+        let ret = match constructor {
+            Single | Variant(_) => match ty.kind(&Interner) {
+                TyKind::Tuple(_, substs) => {
+                    let tys = substs.iter(&Interner).map(|ty| ty.assert_ty_ref(&Interner));
+                    Fields::wildcards_from_tys(cx, tys.cloned())
+                }
+                TyKind::Ref(.., rty) => Fields::from_single_pattern(wildcard_from_ty(rty)),
+                &TyKind::Adt(AdtId(adt), ref substs) => {
+                    if adt_is_box(adt, cx) {
+                        // Use T as the sub pattern type of Box<T>.
+                        let subst_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner);
+                        Fields::from_single_pattern(wildcard_from_ty(subst_ty))
+                    } else {
+                        let variant_id = constructor.variant_id_for_adt(adt);
+                        let adt_is_local =
+                            variant_id.module(cx.db.upcast()).krate() == cx.module.krate();
+                        // Whether we must not match the fields of this variant exhaustively.
+                        let is_non_exhaustive =
+                            is_field_list_non_exhaustive(variant_id, cx) && !adt_is_local;
+
+                        cov_mark::hit!(match_check_wildcard_expanded_to_substitutions);
+                        let field_ty_data = cx.db.field_types(variant_id);
+                        let field_tys = || {
+                            field_ty_data
+                                .iter()
+                                .map(|(_, binders)| binders.clone().substitute(&Interner, substs))
+                        };
+
+                        // In the following cases, we don't need to filter out any fields. This is
+                        // the vast majority of real cases, since uninhabited fields are uncommon.
+                        let has_no_hidden_fields = (matches!(adt, hir_def::AdtId::EnumId(_))
+                            && !is_non_exhaustive)
+                            || !field_tys().any(|ty| cx.is_uninhabited(&ty));
+
+                        if has_no_hidden_fields {
+                            Fields::wildcards_from_tys(cx, field_tys())
+                        } else {
+                            //FIXME(iDawer): see MatchCheckCtx::is_uninhabited, has_no_hidden_fields is always true
+                            unimplemented!("exhaustive_patterns feature")
+                        }
+                    }
+                }
+                ty_kind => {
+                    cx.bug(&format!("Unexpected type for `Single` constructor: {:?}", ty_kind))
+                }
+            },
+            Slice(..) => {
+                unimplemented!()
+            }
+            Str(..) | FloatRange(..) | IntRange(..) | NonExhaustive | Opaque | Missing
+            | Wildcard => Fields::Vec(Default::default()),
+        };
+        ret
+    }
+
+    /// Apply a constructor to a list of patterns, yielding a new pattern. `self`
+    /// must have as many elements as this constructor's arity.
+    ///
+    /// This is roughly the inverse of `specialize_constructor`.
+    ///
+    /// Examples:
+    /// `ctor`: `Constructor::Single`
+    /// `ty`: `Foo(u32, u32, u32)`
+    /// `self`: `[10, 20, _]`
+    /// returns `Foo(10, 20, _)`
+    ///
+    /// `ctor`: `Constructor::Variant(Option::Some)`
+    /// `ty`: `Option<bool>`
+    /// `self`: `[false]`
+    /// returns `Some(false)`
+    pub(super) fn apply(self, pcx: PatCtxt<'_>, ctor: &Constructor) -> Pat {
+        let subpatterns_and_indices = self.patterns_and_indices();
+        let mut subpatterns =
+            subpatterns_and_indices.iter().map(|&(_, p)| pcx.cx.pattern_arena.borrow()[p].clone());
+        // FIXME(iDawer) witnesses are not yet used
+        const UNHANDLED: PatKind = PatKind::Wild;
+
+        let pat = match ctor {
+            Single | Variant(_) => match pcx.ty.kind(&Interner) {
+                TyKind::Adt(..) | TyKind::Tuple(..) => {
+                    // We want the real indices here.
+                    let subpatterns = subpatterns_and_indices
+                        .iter()
+                        .map(|&(field, pat)| FieldPat {
+                            field,
+                            pattern: pcx.cx.pattern_arena.borrow()[pat].clone(),
+                        })
+                        .collect();
+
+                    if let Some((adt, substs)) = pcx.ty.as_adt() {
+                        if let hir_def::AdtId::EnumId(_) = adt {
+                            let enum_variant = match ctor {
+                                &Variant(id) => id,
+                                _ => unreachable!(),
+                            };
+                            PatKind::Variant { substs: substs.clone(), enum_variant, subpatterns }
+                        } else {
+                            PatKind::Leaf { subpatterns }
+                        }
+                    } else {
+                        PatKind::Leaf { subpatterns }
+                    }
+                }
+                // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
+                // be careful to reconstruct the correct constant pattern here. However a string
+                // literal pattern will never be reported as a non-exhaustiveness witness, so we
+                // can ignore this issue.
+                TyKind::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
+                TyKind::Slice(..) | TyKind::Array(..) => {
+                    pcx.cx.bug(&format!("bad slice pattern {:?} {:?}", ctor, pcx.ty))
+                }
+                _ => PatKind::Wild,
+            },
+            Constructor::Slice(_) => UNHANDLED,
+            Str(_) => UNHANDLED,
+            FloatRange(..) => UNHANDLED,
+            Constructor::IntRange(_) => UNHANDLED,
+            NonExhaustive => PatKind::Wild,
+            Wildcard => return Pat::wildcard_from_ty(pcx.ty.clone()),
+            Opaque => pcx.cx.bug("we should not try to apply an opaque constructor"),
+            Missing => pcx.cx.bug(
+                "trying to apply the `Missing` constructor;\
+                this should have been done in `apply_constructors`",
+            ),
+        };
+
+        Pat { ty: pcx.ty.clone(), kind: Box::new(pat) }
+    }
+
+    /// Returns the number of patterns. This is the same as the arity of the constructor used to
+    /// construct `self`.
+    pub(super) fn len(&self) -> usize {
+        match self {
+            Fields::Vec(pats) => pats.len(),
+        }
+    }
+
+    /// Returns the list of patterns along with the corresponding field indices.
+    fn patterns_and_indices(&self) -> SmallVec<[(LocalFieldId, PatId); 2]> {
+        match self {
+            Fields::Vec(pats) => pats
+                .iter()
+                .copied()
+                .enumerate()
+                .map(|(i, p)| (LocalFieldId::from_raw((i as u32).into()), p))
+                .collect(),
+        }
+    }
+
+    pub(super) fn into_patterns(self) -> SmallVec<[PatId; 2]> {
+        match self {
+            Fields::Vec(pats) => pats,
+        }
+    }
+
+    /// Overrides some of the fields with the provided patterns. Exactly like
+    /// `replace_fields_indexed`, except that it takes `FieldPat`s as input.
+    fn replace_with_fieldpats(
+        &self,
+        new_pats: impl IntoIterator<Item = (LocalFieldId, PatId)>,
+    ) -> Self {
+        self.replace_fields_indexed(
+            new_pats.into_iter().map(|(field, pat)| (u32::from(field.into_raw()) as usize, pat)),
+        )
+    }
+
+    /// Overrides some of the fields with the provided patterns. This is used when a pattern
+    /// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start
+    /// with a `Fields` that is just one wildcard per field of the `Foo` struct, and override the
+    /// entry corresponding to `field1` with the pattern `Some(_)`. This is also used for slice
+    /// patterns for the same reason.
+    fn replace_fields_indexed(&self, new_pats: impl IntoIterator<Item = (usize, PatId)>) -> Self {
+        let mut fields = self.clone();
+
+        match &mut fields {
+            Fields::Vec(pats) => {
+                for (i, pat) in new_pats {
+                    if let Some(p) = pats.get_mut(i) {
+                        *p = pat;
+                    }
+                }
+            }
+        }
+        fields
+    }
+
+    /// Replaces contained fields with the given list of patterns. There must be `len()` patterns
+    /// in `pats`.
+    pub(super) fn replace_fields(
+        &self,
+        cx: &MatchCheckCtx<'_>,
+        pats: impl IntoIterator<Item = Pat>,
+    ) -> Self {
+        let pats = pats.into_iter().map(|pat| cx.alloc_pat(pat)).collect();
+
+        match self {
+            Fields::Vec(_) => Fields::Vec(pats),
+        }
+    }
+
+    /// Replaces contained fields with the arguments of the given pattern. Only use on a pattern
+    /// that is compatible with the constructor used to build `self`.
+    /// This is meant to be used on the result of `Fields::wildcards()`. The idea is that
+    /// `wildcards` constructs a list of fields where all entries are wildcards, and the pattern
+    /// provided to this function fills some of the fields with non-wildcards.
+    /// In the following example `Fields::wildcards` would return `[_, _, _, _]`. If we call
+    /// `replace_with_pattern_arguments` on it with the pattern, the result will be `[Some(0), _,
+    /// _, _]`.
+    /// ```rust
+    /// let x: [Option<u8>; 4] = foo();
+    /// match x {
+    ///     [Some(0), ..] => {}
+    /// }
+    /// ```
+    /// This is guaranteed to preserve the number of patterns in `self`.
+    pub(super) fn replace_with_pattern_arguments(
+        &self,
+        pat: PatId,
+        cx: &MatchCheckCtx<'_>,
+    ) -> Self {
+        // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
+        // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
+        let mut arena = cx.pattern_arena.borrow_mut();
+        match arena[pat].kind.as_ref() {
+            PatKind::Deref { subpattern } => {
+                assert_eq!(self.len(), 1);
+                let subpattern = subpattern.clone();
+                Fields::from_single_pattern(arena.alloc(subpattern))
+            }
+            PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
+                let subpatterns = subpatterns.clone();
+                let subpatterns = subpatterns
+                    .iter()
+                    .map(|field_pat| (field_pat.field, arena.alloc(field_pat.pattern.clone())));
+                self.replace_with_fieldpats(subpatterns)
+            }
+
+            PatKind::Wild
+            | PatKind::Binding { .. }
+            | PatKind::LiteralBool { .. }
+            | PatKind::Or { .. } => self.clone(),
+        }
+    }
+}
+
+fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_>) -> bool {
+    let attr_def_id = match variant_id {
+        VariantId::EnumVariantId(id) => id.into(),
+        VariantId::StructId(id) => id.into(),
+        VariantId::UnionId(id) => id.into(),
+    };
+    cx.db.attrs(attr_def_id).by_key("non_exhaustive").exists()
+}
+
+fn adt_is_box(adt: hir_def::AdtId, cx: &MatchCheckCtx<'_>) -> bool {
+    use hir_def::lang_item::LangItemTarget;
+    match cx.db.lang_item(cx.module.krate(), "owned_box".into()) {
+        Some(LangItemTarget::StructId(box_id)) => adt == box_id.into(),
+        _ => false,
+    }
+}
diff --git a/crates/hir_ty/src/diagnostics/match_check/pat_util.rs b/crates/hir_ty/src/diagnostics/match_check/pat_util.rs
new file mode 100644
index 000000000..b89b4f2bf
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/pat_util.rs
@@ -0,0 +1,56 @@
+//! Pattern untilities.
+//!
+//! Originates from `rustc_hir::pat_util`
+
+use std::iter::{Enumerate, ExactSizeIterator};
+
+pub(crate) struct EnumerateAndAdjust<I> {
+    enumerate: Enumerate<I>,
+    gap_pos: usize,
+    gap_len: usize,
+}
+
+impl<I> Iterator for EnumerateAndAdjust<I>
+where
+    I: Iterator,
+{
+    type Item = (usize, <I as Iterator>::Item);
+
+    fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
+        self.enumerate
+            .next()
+            .map(|(i, elem)| (if i < self.gap_pos { i } else { i + self.gap_len }, elem))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.enumerate.size_hint()
+    }
+}
+
+pub(crate) trait EnumerateAndAdjustIterator {
+    fn enumerate_and_adjust(
+        self,
+        expected_len: usize,
+        gap_pos: Option<usize>,
+    ) -> EnumerateAndAdjust<Self>
+    where
+        Self: Sized;
+}
+
+impl<T: ExactSizeIterator> EnumerateAndAdjustIterator for T {
+    fn enumerate_and_adjust(
+        self,
+        expected_len: usize,
+        gap_pos: Option<usize>,
+    ) -> EnumerateAndAdjust<Self>
+    where
+        Self: Sized,
+    {
+        let actual_len = self.len();
+        EnumerateAndAdjust {
+            enumerate: self.enumerate(),
+            gap_pos: gap_pos.unwrap_or(expected_len),
+            gap_len: expected_len - actual_len,
+        }
+    }
+}
diff --git a/crates/hir_ty/src/diagnostics/match_check/usefulness.rs b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
new file mode 100644
index 000000000..83b094a89
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
@@ -0,0 +1,1188 @@
+//! Based on rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22)
+//! https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
+//!
+//! -----
+//!
+//! This file includes the logic for exhaustiveness and reachability checking for pattern-matching.
+//! Specifically, given a list of patterns for a type, we can tell whether:
+//! (a) each pattern is reachable (reachability)
+//! (b) the patterns cover every possible value for the type (exhaustiveness)
+//!
+//! The algorithm implemented here is a modified version of the one described in [this
+//! paper](http://moscova.inria.fr/~maranget/papers/warn/index.html). We have however generalized
+//! it to accommodate the variety of patterns that Rust supports. We thus explain our version here,
+//! without being as rigorous.
+//!
+//!
+//! # Summary
+//!
+//! The core of the algorithm is the notion of "usefulness". A pattern `q` is said to be *useful*
+//! relative to another pattern `p` of the same type if there is a value that is matched by `q` and
+//! not matched by `p`. This generalizes to many `p`s: `q` is useful w.r.t. a list of patterns
+//! `p_1 .. p_n` if there is a value that is matched by `q` and by none of the `p_i`. We write
+//! `usefulness(p_1 .. p_n, q)` for a function that returns a list of such values. The aim of this
+//! file is to compute it efficiently.
+//!
+//! This is enough to compute reachability: a pattern in a `match` expression is reachable iff it
+//! is useful w.r.t. the patterns above it:
+//! ```rust
+//! match x {
+//!     Some(_) => ...,
+//!     None => ..., // reachable: `None` is matched by this but not the branch above
+//!     Some(0) => ..., // unreachable: all the values this matches are already matched by
+//!                     // `Some(_)` above
+//! }
+//! ```
+//!
+//! This is also enough to compute exhaustiveness: a match is exhaustive iff the wildcard `_`
+//! pattern is _not_ useful w.r.t. the patterns in the match. The values returned by `usefulness`
+//! are used to tell the user which values are missing.
+//! ```rust
+//! match x {
+//!     Some(0) => ...,
+//!     None => ...,
+//!     // not exhaustive: `_` is useful because it matches `Some(1)`
+//! }
+//! ```
+//!
+//! The entrypoint of this file is the [`compute_match_usefulness`] function, which computes
+//! reachability for each match branch and exhaustiveness for the whole match.
+//!
+//!
+//! # Constructors and fields
+//!
+//! Note: we will often abbreviate "constructor" as "ctor".
+//!
+//! The idea that powers everything that is done in this file is the following: a (matcheable)
+//! value is made from a constructor applied to a number of subvalues. Examples of constructors are
+//! `Some`, `None`, `(,)` (the 2-tuple constructor), `Foo {..}` (the constructor for a struct
+//! `Foo`), and `2` (the constructor for the number `2`). This is natural when we think of
+//! pattern-matching, and this is the basis for what follows.
+//!
+//! Some of the ctors listed above might feel weird: `None` and `2` don't take any arguments.
+//! That's ok: those are ctors that take a list of 0 arguments; they are the simplest case of
+//! ctors. We treat `2` as a ctor because `u64` and other number types behave exactly like a huge
+//! `enum`, with one variant for each number. This allows us to see any matcheable value as made up
+//! from a tree of ctors, each having a set number of children. For example: `Foo { bar: None,
+//! baz: Ok(0) }` is made from 4 different ctors, namely `Foo{..}`, `None`, `Ok` and `0`.
+//!
+//! This idea can be extended to patterns: they are also made from constructors applied to fields.
+//! A pattern for a given type is allowed to use all the ctors for values of that type (which we
+//! call "value constructors"), but there are also pattern-only ctors. The most important one is
+//! the wildcard (`_`), and the others are integer ranges (`0..=10`), variable-length slices (`[x,
+//! ..]`), and or-patterns (`Ok(0) | Err(_)`). Examples of valid patterns are `42`, `Some(_)`, `Foo
+//! { bar: Some(0) | None, baz: _ }`. Note that a binder in a pattern (e.g. `Some(x)`) matches the
+//! same values as a wildcard (e.g. `Some(_)`), so we treat both as wildcards.
+//!
+//! From this deconstruction we can compute whether a given value matches a given pattern; we
+//! simply look at ctors one at a time. Given a pattern `p` and a value `v`, we want to compute
+//! `matches!(v, p)`. It's mostly straightforward: we compare the head ctors and when they match
+//! we compare their fields recursively. A few representative examples:
+//!
+//! - `matches!(v, _) := true`
+//! - `matches!((v0,  v1), (p0,  p1)) := matches!(v0, p0) && matches!(v1, p1)`
+//! - `matches!(Foo { bar: v0, baz: v1 }, Foo { bar: p0, baz: p1 }) := matches!(v0, p0) && matches!(v1, p1)`
+//! - `matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)`
+//! - `matches!(Ok(v0), Err(p0)) := false` (incompatible variants)
+//! - `matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)`
+//! - `matches!([v0], [p0, .., p1]) := false` (incompatible lengths)
+//! - `matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)`
+//! - `matches!(v, p0 | p1) := matches!(v, p0) || matches!(v, p1)`
+//!
+//! Constructors, fields and relevant operations are defined in the [`super::deconstruct_pat`] module.
+//!
+//! Note: this constructors/fields distinction may not straightforwardly apply to every Rust type.
+//! For example a value of type `Rc<u64>` can't be deconstructed that way, and `&str` has an
+//! infinitude of constructors. There are also subtleties with visibility of fields and
+//! uninhabitedness and various other things. The constructors idea can be extended to handle most
+//! of these subtleties though; caveats are documented where relevant throughout the code.
+//!
+//! Whether constructors cover each other is computed by [`Constructor::is_covered_by`].
+//!
+//!
+//! # Specialization
+//!
+//! Recall that we wish to compute `usefulness(p_1 .. p_n, q)`: given a list of patterns `p_1 ..
+//! p_n` and a pattern `q`, all of the same type, we want to find a list of values (called
+//! "witnesses") that are matched by `q` and by none of the `p_i`. We obviously don't just
+//! enumerate all possible values. From the discussion above we see that we can proceed
+//! ctor-by-ctor: for each value ctor of the given type, we ask "is there a value that starts with
+//! this constructor and matches `q` and none of the `p_i`?". As we saw above, there's a lot we can
+//! say from knowing only the first constructor of our candidate value.
+//!
+//! Let's take the following example:
+//! ```
+//! match x {
+//!     Enum::Variant1(_) => {} // `p1`
+//!     Enum::Variant2(None, 0) => {} // `p2`
+//!     Enum::Variant2(Some(_), 0) => {} // `q`
+//! }
+//! ```
+//!
+//! We can easily see that if our candidate value `v` starts with `Variant1` it will not match `q`.
+//! If `v = Variant2(v0, v1)` however, whether or not it matches `p2` and `q` will depend on `v0`
+//! and `v1`. In fact, such a `v` will be a witness of usefulness of `q` exactly when the tuple
+//! `(v0, v1)` is a witness of usefulness of `q'` in the following reduced match:
+//!
+//! ```
+//! match x {
+//!     (None, 0) => {} // `p2'`
+//!     (Some(_), 0) => {} // `q'`
+//! }
+//! ```
+//!
+//! This motivates a new step in computing usefulness, that we call _specialization_.
+//! Specialization consist of filtering a list of patterns for those that match a constructor, and
+//! then looking into the constructor's fields. This enables usefulness to be computed recursively.
+//!
+//! Instead of acting on a single pattern in each row, we will consider a list of patterns for each
+//! row, and we call such a list a _pattern-stack_. The idea is that we will specialize the
+//! leftmost pattern, which amounts to popping the constructor and pushing its fields, which feels
+//! like a stack. We note a pattern-stack simply with `[p_1 ... p_n]`.
+//! Here's a sequence of specializations of a list of pattern-stacks, to illustrate what's
+//! happening:
+//! ```
+//! [Enum::Variant1(_)]
+//! [Enum::Variant2(None, 0)]
+//! [Enum::Variant2(Some(_), 0)]
+//! //==>> specialize with `Variant2`
+//! [None, 0]
+//! [Some(_), 0]
+//! //==>> specialize with `Some`
+//! [_, 0]
+//! //==>> specialize with `true` (say the type was `bool`)
+//! [0]
+//! //==>> specialize with `0`
+//! []
+//! ```
+//!
+//! The function `specialize(c, p)` takes a value constructor `c` and a pattern `p`, and returns 0
+//! or more pattern-stacks. If `c` does not match the head constructor of `p`, it returns nothing;
+//! otherwise if returns the fields of the constructor. This only returns more than one
+//! pattern-stack if `p` has a pattern-only constructor.
+//!
+//! - Specializing for the wrong constructor returns nothing
+//!
+//!   `specialize(None, Some(p0)) := []`
+//!
+//! - Specializing for the correct constructor returns a single row with the fields
+//!
+//!   `specialize(Variant1, Variant1(p0, p1, p2)) := [[p0, p1, p2]]`
+//!
+//!   `specialize(Foo{..}, Foo { bar: p0, baz: p1 }) := [[p0, p1]]`
+//!
+//! - For or-patterns, we specialize each branch and concatenate the results
+//!
+//!   `specialize(c, p0 | p1) := specialize(c, p0) ++ specialize(c, p1)`
+//!
+//! - We treat the other pattern constructors as if they were a large or-pattern of all the
+//!   possibilities:
+//!
+//!   `specialize(c, _) := specialize(c, Variant1(_) | Variant2(_, _) | ...)`
+//!
+//!   `specialize(c, 1..=100) := specialize(c, 1 | ... | 100)`
+//!
+//!   `specialize(c, [p0, .., p1]) := specialize(c, [p0, p1] | [p0, _, p1] | [p0, _, _, p1] | ...)`
+//!
+//! - If `c` is a pattern-only constructor, `specialize` is defined on a case-by-case basis. See
+//!   the discussion about constructor splitting in [`super::deconstruct_pat`].
+//!
+//!
+//! We then extend this function to work with pattern-stacks as input, by acting on the first
+//! column and keeping the other columns untouched.
+//!
+//! Specialization for the whole matrix is done in [`Matrix::specialize_constructor`]. Note that
+//! or-patterns in the first column are expanded before being stored in the matrix. Specialization
+//! for a single patstack is done from a combination of [`Constructor::is_covered_by`] and
+//! [`PatStack::pop_head_constructor`]. The internals of how it's done mostly live in the
+//! [`Fields`] struct.
+//!
+//!
+//! # Computing usefulness
+//!
+//! We now have all we need to compute usefulness. The inputs to usefulness are a list of
+//! pattern-stacks `p_1 ... p_n` (one per row), and a new pattern_stack `q`. The paper and this
+//! file calls the list of patstacks a _matrix_. They must all have the same number of columns and
+//! the patterns in a given column must all have the same type. `usefulness` returns a (possibly
+//! empty) list of witnesses of usefulness. These witnesses will also be pattern-stacks.
+//!
+//! - base case: `n_columns == 0`.
+//!     Since a pattern-stack functions like a tuple of patterns, an empty one functions like the
+//!     unit type. Thus `q` is useful iff there are no rows above it, i.e. if `n == 0`.
+//!
+//! - inductive case: `n_columns > 0`.
+//!     We need a way to list the constructors we want to try. We will be more clever in the next
+//!     section but for now assume we list all value constructors for the type of the first column.
+//!
+//!     - for each such ctor `c`:
+//!
+//!         - for each `q'` returned by `specialize(c, q)`:
+//!
+//!             - we compute `usefulness(specialize(c, p_1) ... specialize(c, p_n), q')`
+//!
+//!         - for each witness found, we revert specialization by pushing the constructor `c` on top.
+//!
+//!     - We return the concatenation of all the witnesses found, if any.
+//!
+//! Example:
+//! ```
+//! [Some(true)] // p_1
+//! [None] // p_2
+//! [Some(_)] // q
+//! //==>> try `None`: `specialize(None, q)` returns nothing
+//! //==>> try `Some`: `specialize(Some, q)` returns a single row
+//! [true] // p_1'
+//! [_] // q'
+//! //==>> try `true`: `specialize(true, q')` returns a single row
+//! [] // p_1''
+//! [] // q''
+//! //==>> base case; `n != 0` so `q''` is not useful.
+//! //==>> go back up a step
+//! [true] // p_1'
+//! [_] // q'
+//! //==>> try `false`: `specialize(false, q')` returns a single row
+//! [] // q''
+//! //==>> base case; `n == 0` so `q''` is useful. We return the single witness `[]`
+//! witnesses:
+//! []
+//! //==>> undo the specialization with `false`
+//! witnesses:
+//! [false]
+//! //==>> undo the specialization with `Some`
+//! witnesses:
+//! [Some(false)]
+//! //==>> we have tried all the constructors. The output is the single witness `[Some(false)]`.
+//! ```
+//!
+//! This computation is done in [`is_useful`]. In practice we don't care about the list of
+//! witnesses when computing reachability; we only need to know whether any exist. We do keep the
+//! witnesses when computing exhaustiveness to report them to the user.
+//!
+//!
+//! # Making usefulness tractable: constructor splitting
+//!
+//! We're missing one last detail: which constructors do we list? Naively listing all value
+//! constructors cannot work for types like `u64` or `&str`, so we need to be more clever. The
+//! first obvious insight is that we only want to list constructors that are covered by the head
+//! constructor of `q`. If it's a value constructor, we only try that one. If it's a pattern-only
+//! constructor, we use the final clever idea for this algorithm: _constructor splitting_, where we
+//! group together constructors that behave the same.
+//!
+//! The details are not necessary to understand this file, so we explain them in
+//! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function.
+
+use std::{cell::RefCell, iter::FromIterator};
+
+use hir_def::{expr::ExprId, HasModule, ModuleId};
+use la_arena::Arena;
+use once_cell::unsync::OnceCell;
+use rustc_hash::FxHashMap;
+use smallvec::{smallvec, SmallVec};
+
+use crate::{db::HirDatabase, InferenceResult, Interner, Ty};
+
+use super::{
+    deconstruct_pat::{Constructor, Fields, SplitWildcard},
+    Pat, PatId, PatKind, PatternFoldable, PatternFolder,
+};
+
+use self::{helper::PatIdExt, Usefulness::*, WitnessPreference::*};
+
+pub(crate) struct MatchCheckCtx<'a> {
+    pub(crate) module: ModuleId,
+    pub(crate) match_expr: ExprId,
+    pub(crate) infer: &'a InferenceResult,
+    pub(crate) db: &'a dyn HirDatabase,
+    /// Lowered patterns from arms plus generated by the check.
+    pub(crate) pattern_arena: &'a RefCell<PatternArena>,
+    pub(crate) eprint_panic_context: &'a dyn Fn(),
+}
+
+impl<'a> MatchCheckCtx<'a> {
+    pub(super) fn is_uninhabited(&self, _ty: &Ty) -> bool {
+        // FIXME(iDawer) implement exhaustive_patterns feature. More info in:
+        // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085
+        false
+    }
+
+    /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
+    pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool {
+        let has_non_exhaustive_attr =
+            self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists();
+        let is_local =
+            hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
+        has_non_exhaustive_attr && !is_local
+    }
+
+    // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
+    pub(super) fn feature_exhaustive_patterns(&self) -> bool {
+        // FIXME see MatchCheckCtx::is_uninhabited
+        false
+    }
+
+    pub(super) fn alloc_pat(&self, pat: Pat) -> PatId {
+        self.pattern_arena.borrow_mut().alloc(pat)
+    }
+
+    /// Get type of a pattern. Handles expanded patterns.
+    pub(super) fn type_of(&self, pat: PatId) -> Ty {
+        self.pattern_arena.borrow()[pat].ty.clone()
+    }
+
+    #[track_caller]
+    pub(super) fn bug(&self, info: &str) -> ! {
+        (self.eprint_panic_context)();
+        panic!("bug: {}", info);
+    }
+}
+
+#[derive(Copy, Clone)]
+pub(super) struct PatCtxt<'a> {
+    pub(super) cx: &'a MatchCheckCtx<'a>,
+    /// Type of the current column under investigation.
+    pub(super) ty: &'a Ty,
+    /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
+    /// subpattern.
+    pub(super) is_top_level: bool,
+}
+
+pub(crate) fn expand_pattern(pat: Pat) -> Pat {
+    LiteralExpander.fold_pattern(&pat)
+}
+
+struct LiteralExpander;
+
+impl PatternFolder for LiteralExpander {
+    fn fold_pattern(&mut self, pat: &Pat) -> Pat {
+        match (pat.ty.kind(&Interner), pat.kind.as_ref()) {
+            (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self),
+            _ => pat.super_fold_with(self),
+        }
+    }
+}
+
+impl Pat {
+    fn _is_wildcard(&self) -> bool {
+        matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild)
+    }
+}
+
+impl PatIdExt for PatId {
+    fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool {
+        matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. })
+    }
+
+    /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
+    fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self> {
+        fn expand(pat: PatId, vec: &mut Vec<PatId>, pat_arena: &mut PatternArena) {
+            if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() {
+                // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
+                // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
+                let pats = pats.clone();
+                for pat in pats {
+                    let pat = pat_arena.alloc(pat.clone());
+                    expand(pat, vec, pat_arena);
+                }
+            } else {
+                vec.push(pat)
+            }
+        }
+
+        let mut pat_arena = cx.pattern_arena.borrow_mut();
+        let mut pats = Vec::new();
+        expand(self, &mut pats, &mut pat_arena);
+        pats
+    }
+}
+
+/// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
+/// works well.
+#[derive(Clone)]
+pub(super) struct PatStack {
+    pats: SmallVec<[PatId; 2]>,
+    /// Cache for the constructor of the head
+    head_ctor: OnceCell<Constructor>,
+}
+
+impl PatStack {
+    fn from_pattern(pat: PatId) -> Self {
+        Self::from_vec(smallvec![pat])
+    }
+
+    fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self {
+        PatStack { pats: vec, head_ctor: OnceCell::new() }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.pats.is_empty()
+    }
+
+    fn len(&self) -> usize {
+        self.pats.len()
+    }
+
+    fn head(&self) -> PatId {
+        self.pats[0]
+    }
+
+    #[inline]
+    fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor {
+        self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head()))
+    }
+
+    // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an
+    // or-pattern. Panics if `self` is empty.
+    fn expand_or_pat(&self, cx: &MatchCheckCtx<'_>) -> impl Iterator<Item = PatStack> + '_ {
+        self.head().expand_or_pat(cx).into_iter().map(move |pat| {
+            let mut new_patstack = PatStack::from_pattern(pat);
+            new_patstack.pats.extend_from_slice(&self.pats[1..]);
+            new_patstack
+        })
+    }
+
+    /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations.
+    ///
+    /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
+    /// fields filled with wild patterns.
+    ///
+    /// This is roughly the inverse of `Constructor::apply`.
+    fn pop_head_constructor(
+        &self,
+        ctor_wild_subpatterns: &Fields,
+        cx: &MatchCheckCtx<'_>,
+    ) -> PatStack {
+        // We pop the head pattern and push the new fields extracted from the arguments of
+        // `self.head()`.
+        let mut new_fields =
+            ctor_wild_subpatterns.replace_with_pattern_arguments(self.head(), cx).into_patterns();
+        new_fields.extend_from_slice(&self.pats[1..]);
+        PatStack::from_vec(new_fields)
+    }
+}
+
+impl Default for PatStack {
+    fn default() -> Self {
+        Self::from_vec(smallvec![])
+    }
+}
+
+impl PartialEq for PatStack {
+    fn eq(&self, other: &Self) -> bool {
+        self.pats == other.pats
+    }
+}
+
+impl FromIterator<PatId> for PatStack {
+    fn from_iter<T>(iter: T) -> Self
+    where
+        T: IntoIterator<Item = PatId>,
+    {
+        Self::from_vec(iter.into_iter().collect())
+    }
+}
+
+/// A 2D matrix.
+#[derive(Clone)]
+pub(super) struct Matrix {
+    patterns: Vec<PatStack>,
+}
+
+impl Matrix {
+    fn empty() -> Self {
+        Matrix { patterns: vec![] }
+    }
+
+    /// Number of columns of this matrix. `None` is the matrix is empty.
+    pub(super) fn _column_count(&self) -> Option<usize> {
+        self.patterns.get(0).map(|r| r.len())
+    }
+
+    /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
+    /// expands it.
+    fn push(&mut self, row: PatStack, cx: &MatchCheckCtx<'_>) {
+        if !row.is_empty() && row.head().is_or_pat(cx) {
+            for row in row.expand_or_pat(cx) {
+                self.patterns.push(row);
+            }
+        } else {
+            self.patterns.push(row);
+        }
+    }
+
+    /// Iterate over the first component of each row
+    fn heads(&self) -> impl Iterator<Item = PatId> + '_ {
+        self.patterns.iter().map(|r| r.head())
+    }
+
+    /// Iterate over the first constructor of each row.
+    fn head_ctors<'a>(
+        &'a self,
+        cx: &'a MatchCheckCtx<'_>,
+    ) -> impl Iterator<Item = &'a Constructor> + Clone {
+        self.patterns.iter().map(move |r| r.head_ctor(cx))
+    }
+
+    /// This computes `S(constructor, self)`. See top of the file for explanations.
+    fn specialize_constructor(
+        &self,
+        pcx: PatCtxt<'_>,
+        ctor: &Constructor,
+        ctor_wild_subpatterns: &Fields,
+    ) -> Matrix {
+        let rows = self
+            .patterns
+            .iter()
+            .filter(|r| ctor.is_covered_by(pcx, r.head_ctor(pcx.cx)))
+            .map(|r| r.pop_head_constructor(ctor_wild_subpatterns, pcx.cx));
+        Matrix::from_iter(rows, pcx.cx)
+    }
+
+    fn from_iter(rows: impl IntoIterator<Item = PatStack>, cx: &MatchCheckCtx<'_>) -> Matrix {
+        let mut matrix = Matrix::empty();
+        for x in rows {
+            // Using `push` ensures we correctly expand or-patterns.
+            matrix.push(x, cx);
+        }
+        matrix
+    }
+}
+
+/// Given a pattern or a pattern-stack, this struct captures a set of its subpatterns. We use that
+/// to track reachable sub-patterns arising from or-patterns. In the absence of or-patterns this
+/// will always be either `Empty` (the whole pattern is unreachable) or `Full` (the whole pattern
+/// is reachable). When there are or-patterns, some subpatterns may be reachable while others
+/// aren't. In this case the whole pattern still counts as reachable, but we will lint the
+/// unreachable subpatterns.
+///
+/// This supports a limited set of operations, so not all possible sets of subpatterns can be
+/// represented. That's ok, we only want the ones that make sense for our usage.
+///
+/// What we're doing is illustrated by this:
+/// ```
+/// match (true, 0) {
+///     (true, 0) => {}
+///     (_, 1) => {}
+///     (true | false, 0 | 1) => {}
+/// }
+/// ```
+/// When we try the alternatives of the `true | false` or-pattern, the last `0` is reachable in the
+/// `false` alternative but not the `true`. So overall it is reachable. By contrast, the last `1`
+/// is not reachable in either alternative, so we want to signal this to the user.
+/// Therefore we take the union of sets of reachable patterns coming from different alternatives in
+/// order to figure out which subpatterns are overall reachable.
+///
+/// Invariant: we try to construct the smallest representation we can. In particular if
+/// `self.is_empty()` we ensure that `self` is `Empty`, and same with `Full`. This is not important
+/// for correctness currently.
+#[derive(Debug, Clone)]
+enum SubPatSet {
+    /// The empty set. This means the pattern is unreachable.
+    Empty,
+    /// The set containing the full pattern.
+    Full,
+    /// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each
+    /// of its subpatterns. Missing entries in the map are implicitly full, because that's the
+    /// common case.
+    Seq { subpats: FxHashMap<usize, SubPatSet> },
+    /// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing
+    /// entries in the map are implicitly empty. Note: we always flatten nested or-patterns.
+    Alt {
+        subpats: FxHashMap<usize, SubPatSet>,
+        /// Counts the total number of alternatives in the pattern
+        alt_count: usize,
+        /// We keep the pattern around to retrieve spans.
+        pat: PatId,
+    },
+}
+
+impl SubPatSet {
+    fn full() -> Self {
+        SubPatSet::Full
+    }
+
+    fn empty() -> Self {
+        SubPatSet::Empty
+    }
+
+    fn is_empty(&self) -> bool {
+        match self {
+            SubPatSet::Empty => true,
+            SubPatSet::Full => false,
+            // If any subpattern in a sequence is unreachable, the whole pattern is unreachable.
+            SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_empty()),
+            // An or-pattern is reachable if any of its alternatives is.
+            SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_empty()),
+        }
+    }
+
+    fn is_full(&self) -> bool {
+        match self {
+            SubPatSet::Empty => false,
+            SubPatSet::Full => true,
+            // The whole pattern is reachable only when all its alternatives are.
+            SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_full()),
+            // The whole or-pattern is reachable only when all its alternatives are.
+            SubPatSet::Alt { subpats, alt_count, .. } => {
+                subpats.len() == *alt_count && subpats.values().all(|set| set.is_full())
+            }
+        }
+    }
+
+    /// Union `self` with `other`, mutating `self`.
+    fn union(&mut self, other: Self) {
+        use SubPatSet::*;
+        // Union with full stays full; union with empty changes nothing.
+        if self.is_full() || other.is_empty() {
+            return;
+        } else if self.is_empty() {
+            *self = other;
+            return;
+        } else if other.is_full() {
+            *self = Full;
+            return;
+        }
+
+        match (&mut *self, other) {
+            (Seq { subpats: s_set }, Seq { subpats: mut o_set }) => {
+                s_set.retain(|i, s_sub_set| {
+                    // Missing entries count as full.
+                    let o_sub_set = o_set.remove(&i).unwrap_or(Full);
+                    s_sub_set.union(o_sub_set);
+                    // We drop full entries.
+                    !s_sub_set.is_full()
+                });
+                // Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since
+                // unioning with full returns full, we can drop those entries.
+            }
+            (Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => {
+                s_set.retain(|i, s_sub_set| {
+                    // Missing entries count as empty.
+                    let o_sub_set = o_set.remove(&i).unwrap_or(Empty);
+                    s_sub_set.union(o_sub_set);
+                    // We drop empty entries.
+                    !s_sub_set.is_empty()
+                });
+                // Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since
+                // unioning with empty changes nothing, we can take those entries as is.
+                s_set.extend(o_set);
+            }
+            _ => panic!("bug"),
+        }
+
+        if self.is_full() {
+            *self = Full;
+        }
+    }
+
+    /// Returns a list of the unreachable subpatterns. If `self` is empty (i.e. the
+    /// whole pattern is unreachable) we return `None`.
+    fn list_unreachable_subpatterns(&self, cx: &MatchCheckCtx<'_>) -> Option<Vec<PatId>> {
+        /// Panics if `set.is_empty()`.
+        fn fill_subpats(
+            set: &SubPatSet,
+            unreachable_pats: &mut Vec<PatId>,
+            cx: &MatchCheckCtx<'_>,
+        ) {
+            match set {
+                SubPatSet::Empty => panic!("bug"),
+                SubPatSet::Full => {}
+                SubPatSet::Seq { subpats } => {
+                    for (_, sub_set) in subpats {
+                        fill_subpats(sub_set, unreachable_pats, cx);
+                    }
+                }
+                SubPatSet::Alt { subpats, pat, alt_count, .. } => {
+                    let expanded = pat.expand_or_pat(cx);
+                    for i in 0..*alt_count {
+                        let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Empty);
+                        if sub_set.is_empty() {
+                            // Found a unreachable subpattern.
+                            unreachable_pats.push(expanded[i]);
+                        } else {
+                            fill_subpats(sub_set, unreachable_pats, cx);
+                        }
+                    }
+                }
+            }
+        }
+
+        if self.is_empty() {
+            return None;
+        }
+        if self.is_full() {
+            // No subpatterns are unreachable.
+            return Some(Vec::new());
+        }
+        let mut unreachable_pats = Vec::new();
+        fill_subpats(self, &mut unreachable_pats, cx);
+        Some(unreachable_pats)
+    }
+
+    /// When `self` refers to a patstack that was obtained from specialization, after running
+    /// `unspecialize` it will refer to the original patstack before specialization.
+    fn unspecialize(self, arity: usize) -> Self {
+        use SubPatSet::*;
+        match self {
+            Full => Full,
+            Empty => Empty,
+            Seq { subpats } => {
+                // We gather the first `arity` subpatterns together and shift the remaining ones.
+                let mut new_subpats = FxHashMap::default();
+                let mut new_subpats_first_col = FxHashMap::default();
+                for (i, sub_set) in subpats {
+                    if i < arity {
+                        // The first `arity` indices are now part of the pattern in the first
+                        // column.
+                        new_subpats_first_col.insert(i, sub_set);
+                    } else {
+                        // Indices after `arity` are simply shifted
+                        new_subpats.insert(i - arity + 1, sub_set);
+                    }
+                }
+                // If `new_subpats_first_col` has no entries it counts as full, so we can omit it.
+                if !new_subpats_first_col.is_empty() {
+                    new_subpats.insert(0, Seq { subpats: new_subpats_first_col });
+                }
+                Seq { subpats: new_subpats }
+            }
+            Alt { .. } => panic!("bug"), // `self` is a patstack
+        }
+    }
+
+    /// When `self` refers to a patstack that was obtained from splitting an or-pattern, after
+    /// running `unspecialize` it will refer to the original patstack before splitting.
+    ///
+    /// For example:
+    /// ```
+    /// match Some(true) {
+    ///     Some(true) => {}
+    ///     None | Some(true | false) => {}
+    /// }
+    /// ```
+    /// Here `None` would return the full set and `Some(true | false)` would return the set
+    /// containing `false`. After `unsplit_or_pat`, we want the set to contain `None` and `false`.
+    /// This is what this function does.
+    fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
+        use SubPatSet::*;
+        if self.is_empty() {
+            return Empty;
+        }
+
+        // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0
+        // | 1)`.
+        let set_first_col = match &mut self {
+            Full => Full,
+            Seq { subpats } => subpats.remove(&0).unwrap_or(Full),
+            Empty => unreachable!(),
+            Alt { .. } => panic!("bug"), // `self` is a patstack
+        };
+        let mut subpats_first_col = FxHashMap::default();
+        subpats_first_col.insert(alt_id, set_first_col);
+        let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count };
+
+        let mut subpats = match self {
+            Full => FxHashMap::default(),
+            Seq { subpats } => subpats,
+            Empty => unreachable!(),
+            Alt { .. } => panic!("bug"), // `self` is a patstack
+        };
+        subpats.insert(0, set_first_col);
+        Seq { subpats }
+    }
+}
+
+/// This carries the results of computing usefulness, as described at the top of the file. When
+/// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track
+/// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
+/// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
+/// witnesses of non-exhaustiveness when there are any.
+/// Which variant to use is dictated by `WitnessPreference`.
+#[derive(Clone, Debug)]
+enum Usefulness {
+    /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates
+    /// the whole pattern is unreachable. If not, this indicates that the pattern is reachable but
+    /// that some sub-patterns may be unreachable (due to or-patterns). In the absence of
+    /// or-patterns this will always be either `Empty` (the whole pattern is unreachable) or `Full`
+    /// (the whole pattern is reachable).
+    NoWitnesses(SubPatSet),
+    /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole
+    /// pattern is unreachable.
+    WithWitnesses(Vec<Witness>),
+}
+
+impl Usefulness {
+    fn new_useful(preference: WitnessPreference) -> Self {
+        match preference {
+            ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
+            LeaveOutWitness => NoWitnesses(SubPatSet::full()),
+        }
+    }
+    fn new_not_useful(preference: WitnessPreference) -> Self {
+        match preference {
+            ConstructWitness => WithWitnesses(vec![]),
+            LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
+        }
+    }
+
+    /// Combine usefulnesses from two branches. This is an associative operation.
+    fn extend(&mut self, other: Self) {
+        match (&mut *self, other) {
+            (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {}
+            (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o),
+            (WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
+            (NoWitnesses(s), NoWitnesses(o)) => s.union(o),
+            _ => unreachable!(),
+        }
+    }
+
+    /// When trying several branches and each returns a `Usefulness`, we need to combine the
+    /// results together.
+    fn merge(pref: WitnessPreference, usefulnesses: impl Iterator<Item = Self>) -> Self {
+        let mut ret = Self::new_not_useful(pref);
+        for u in usefulnesses {
+            ret.extend(u);
+            if let NoWitnesses(subpats) = &ret {
+                if subpats.is_full() {
+                    // Once we reach the full set, more unions won't change the result.
+                    return ret;
+                }
+            }
+        }
+        ret
+    }
+
+    /// After calculating the usefulness for a branch of an or-pattern, call this to make this
+    /// usefulness mergeable with those from the other branches.
+    fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
+        match self {
+            NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)),
+            WithWitnesses(_) => panic!("bug"),
+        }
+    }
+
+    /// After calculating usefulness after a specialization, call this to recontruct a usefulness
+    /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
+    /// with the results of specializing with the other constructors.
+    fn apply_constructor(
+        self,
+        pcx: PatCtxt<'_>,
+        matrix: &Matrix,
+        ctor: &Constructor,
+        ctor_wild_subpatterns: &Fields,
+    ) -> Self {
+        match self {
+            WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses),
+            WithWitnesses(witnesses) => {
+                let new_witnesses = if matches!(ctor, Constructor::Missing) {
+                    let mut split_wildcard = SplitWildcard::new(pcx);
+                    split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
+                    // Construct for each missing constructor a "wild" version of this
+                    // constructor, that matches everything that can be built with
+                    // it. For example, if `ctor` is a `Constructor::Variant` for
+                    // `Option::Some`, we get the pattern `Some(_)`.
+                    let new_patterns: Vec<_> = split_wildcard
+                        .iter_missing(pcx)
+                        .map(|missing_ctor| {
+                            Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
+                        })
+                        .collect();
+                    witnesses
+                        .into_iter()
+                        .flat_map(|witness| {
+                            new_patterns.iter().map(move |pat| {
+                                let mut witness = witness.clone();
+                                witness.0.push(pat.clone());
+                                witness
+                            })
+                        })
+                        .collect()
+                } else {
+                    witnesses
+                        .into_iter()
+                        .map(|witness| witness.apply_constructor(pcx, &ctor, ctor_wild_subpatterns))
+                        .collect()
+                };
+                WithWitnesses(new_witnesses)
+            }
+            NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+enum WitnessPreference {
+    ConstructWitness,
+    LeaveOutWitness,
+}
+
+/// A witness of non-exhaustiveness for error reporting, represented
+/// as a list of patterns (in reverse order of construction) with
+/// wildcards inside to represent elements that can take any inhabitant
+/// of the type as a value.
+///
+/// A witness against a list of patterns should have the same types
+/// and length as the pattern matched against. Because Rust `match`
+/// is always against a single pattern, at the end the witness will
+/// have length 1, but in the middle of the algorithm, it can contain
+/// multiple patterns.
+///
+/// For example, if we are constructing a witness for the match against
+///
+/// ```
+/// struct Pair(Option<(u32, u32)>, bool);
+///
+/// match (p: Pair) {
+///    Pair(None, _) => {}
+///    Pair(_, false) => {}
+/// }
+/// ```
+///
+/// We'll perform the following steps:
+/// 1. Start with an empty witness
+///     `Witness(vec![])`
+/// 2. Push a witness `true` against the `false`
+///     `Witness(vec![true])`
+/// 3. Push a witness `Some(_)` against the `None`
+///     `Witness(vec![true, Some(_)])`
+/// 4. Apply the `Pair` constructor to the witnesses
+///     `Witness(vec![Pair(Some(_), true)])`
+///
+/// The final `Pair(Some(_), true)` is then the resulting witness.
+#[derive(Clone, Debug)]
+pub(crate) struct Witness(Vec<Pat>);
+
+impl Witness {
+    /// Asserts that the witness contains a single pattern, and returns it.
+    fn single_pattern(self) -> Pat {
+        assert_eq!(self.0.len(), 1);
+        self.0.into_iter().next().unwrap()
+    }
+
+    /// Constructs a partial witness for a pattern given a list of
+    /// patterns expanded by the specialization step.
+    ///
+    /// When a pattern P is discovered to be useful, this function is used bottom-up
+    /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
+    /// of values, V, where each value in that set is not covered by any previously
+    /// used patterns and is covered by the pattern P'. Examples:
+    ///
+    /// left_ty: tuple of 3 elements
+    /// pats: [10, 20, _]           => (10, 20, _)
+    ///
+    /// left_ty: struct X { a: (bool, &'static str), b: usize}
+    /// pats: [(false, "foo"), 42]  => X { a: (false, "foo"), b: 42 }
+    fn apply_constructor(
+        mut self,
+        pcx: PatCtxt<'_>,
+        ctor: &Constructor,
+        ctor_wild_subpatterns: &Fields,
+    ) -> Self {
+        let pat = {
+            let len = self.0.len();
+            let arity = ctor_wild_subpatterns.len();
+            let pats = self.0.drain((len - arity)..).rev();
+            ctor_wild_subpatterns.replace_fields(pcx.cx, pats).apply(pcx, ctor)
+        };
+
+        self.0.push(pat);
+
+        self
+    }
+}
+
+/// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
+/// The algorithm from the paper has been modified to correctly handle empty
+/// types. The changes are:
+///   (0) We don't exit early if the pattern matrix has zero rows. We just
+///       continue to recurse over columns.
+///   (1) all_constructors will only return constructors that are statically
+///       possible. E.g., it will only return `Ok` for `Result<T, !>`.
+///
+/// This finds whether a (row) vector `v` of patterns is 'useful' in relation
+/// to a set of such vectors `m` - this is defined as there being a set of
+/// inputs that will match `v` but not any of the sets in `m`.
+///
+/// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
+///
+/// This is used both for reachability checking (if a pattern isn't useful in
+/// relation to preceding patterns, it is not reachable) and exhaustiveness
+/// checking (if a wildcard pattern is useful in relation to a matrix, the
+/// matrix isn't exhaustive).
+///
+/// `is_under_guard` is used to inform if the pattern has a guard. If it
+/// has one it must not be inserted into the matrix. This shouldn't be
+/// relied on for soundness.
+fn is_useful(
+    cx: &MatchCheckCtx<'_>,
+    matrix: &Matrix,
+    v: &PatStack,
+    witness_preference: WitnessPreference,
+    is_under_guard: bool,
+    is_top_level: bool,
+) -> Usefulness {
+    let Matrix { patterns: rows, .. } = matrix;
+
+    // The base case. We are pattern-matching on () and the return value is
+    // based on whether our matrix has a row or not.
+    // NOTE: This could potentially be optimized by checking rows.is_empty()
+    // first and then, if v is non-empty, the return value is based on whether
+    // the type of the tuple we're checking is inhabited or not.
+    if v.is_empty() {
+        let ret = if rows.is_empty() {
+            Usefulness::new_useful(witness_preference)
+        } else {
+            Usefulness::new_not_useful(witness_preference)
+        };
+        return ret;
+    }
+
+    assert!(rows.iter().all(|r| r.len() == v.len()));
+
+    // FIXME(Nadrieril): Hack to work around type normalization issues (see rust-lang/rust#72476).
+    let ty = matrix.heads().next().map_or(cx.type_of(v.head()), |r| cx.type_of(r));
+    let pcx = PatCtxt { cx, ty: &ty, is_top_level };
+
+    // If the first pattern is an or-pattern, expand it.
+    let ret = if v.head().is_or_pat(cx) {
+        //expanding or-pattern
+        let v_head = v.head();
+        let vs: Vec<_> = v.expand_or_pat(cx).collect();
+        let alt_count = vs.len();
+        // We try each or-pattern branch in turn.
+        let mut matrix = matrix.clone();
+        let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| {
+            let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false);
+            // If pattern has a guard don't add it to the matrix.
+            if !is_under_guard {
+                // We push the already-seen patterns into the matrix in order to detect redundant
+                // branches like `Some(_) | Some(0)`.
+                matrix.push(v, cx);
+            }
+            usefulness.unsplit_or_pat(i, alt_count, v_head)
+        });
+        Usefulness::merge(witness_preference, usefulnesses)
+    } else {
+        let v_ctor = v.head_ctor(cx);
+        // if let Constructor::IntRange(ctor_range) = v_ctor {
+        //     // Lint on likely incorrect range patterns (#63987)
+        //     ctor_range.lint_overlapping_range_endpoints(
+        //         pcx,
+        //         matrix.head_ctors_and_spans(cx),
+        //         matrix.column_count().unwrap_or(0),
+        //         hir_id,
+        //     )
+        // }
+
+        // We split the head constructor of `v`.
+        let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx));
+        // For each constructor, we compute whether there's a value that starts with it that would
+        // witness the usefulness of `v`.
+        let start_matrix = matrix;
+        let usefulnesses = split_ctors.into_iter().map(|ctor| {
+            // debug!("specialize({:?})", ctor);
+            // We cache the result of `Fields::wildcards` because it is used a lot.
+            let ctor_wild_subpatterns = Fields::wildcards(pcx, &ctor);
+            let spec_matrix =
+                start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns);
+            let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx);
+            let usefulness =
+                is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
+            usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
+        });
+        Usefulness::merge(witness_preference, usefulnesses)
+    };
+
+    ret
+}
+
+/// The arm of a match expression.
+#[derive(Clone, Copy)]
+pub(crate) struct MatchArm {
+    pub(crate) pat: PatId,
+    pub(crate) has_guard: bool,
+}
+
+/// Indicates whether or not a given arm is reachable.
+#[derive(Clone, Debug)]
+pub(crate) enum Reachability {
+    /// The arm is reachable. This additionally carries a set of or-pattern branches that have been
+    /// found to be unreachable despite the overall arm being reachable. Used only in the presence
+    /// of or-patterns, otherwise it stays empty.
+    Reachable(Vec<PatId>),
+    /// The arm is unreachable.
+    Unreachable,
+}
+
+/// The output of checking a match for exhaustiveness and arm reachability.
+pub(crate) struct UsefulnessReport {
+    /// For each arm of the input, whether that arm is reachable after the arms above it.
+    pub(crate) _arm_usefulness: Vec<(MatchArm, Reachability)>,
+    /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
+    /// exhaustiveness.
+    pub(crate) non_exhaustiveness_witnesses: Vec<Pat>,
+}
+
+/// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
+/// of its arms are reachable.
+///
+/// Note: the input patterns must have been lowered through
+/// `check_match::MatchVisitor::lower_pattern`.
+pub(crate) fn compute_match_usefulness(
+    cx: &MatchCheckCtx<'_>,
+    arms: &[MatchArm],
+) -> UsefulnessReport {
+    let mut matrix = Matrix::empty();
+    let arm_usefulness: Vec<_> = arms
+        .iter()
+        .copied()
+        .map(|arm| {
+            let v = PatStack::from_pattern(arm.pat);
+            let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true);
+            if !arm.has_guard {
+                matrix.push(v, cx);
+            }
+            let reachability = match usefulness {
+                NoWitnesses(subpats) if subpats.is_empty() => Reachability::Unreachable,
+                NoWitnesses(subpats) => {
+                    Reachability::Reachable(subpats.list_unreachable_subpatterns(cx).unwrap())
+                }
+                WithWitnesses(..) => panic!("bug"),
+            };
+            (arm, reachability)
+        })
+        .collect();
+
+    let wild_pattern =
+        cx.pattern_arena.borrow_mut().alloc(Pat::wildcard_from_ty(cx.infer[cx.match_expr].clone()));
+    let v = PatStack::from_pattern(wild_pattern);
+    let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, false, true);
+    let non_exhaustiveness_witnesses = match usefulness {
+        WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(),
+        NoWitnesses(_) => panic!("bug"),
+    };
+    UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
+}
+
+pub(crate) type PatternArena = Arena<Pat>;
+
+mod helper {
+    use super::MatchCheckCtx;
+
+    pub(super) trait PatIdExt: Sized {
+        // fn is_wildcard(self, cx: &MatchCheckCtx<'_>) -> bool;
+        fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool;
+        fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self>;
+    }
+
+    // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs
+    /// "Signaling" trait used in impl trait to tag lifetimes that you may
+    /// need to capture but don't really need for other reasons.
+    /// Basically a workaround; see [this comment] for details.
+    ///
+    /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
+    // FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed.
+    #[allow(unused_lifetimes)]
+    pub(crate) trait Captures<'a> {}
+
+    impl<'a, T: ?Sized> Captures<'a> for T {}
+}