Merge #3706

3706: missing match arms diagnostic r=flodiebold a=JoshMcguigan

Following up on https://github.com/rust-analyzer/rust-analyzer/pull/3689#issuecomment-602718222, this PR creates a missing match arms diagnostic.

At the moment this is a very early draft, but I wanted to open it just to get some initial feedback.

Initial questions:

* Have I roughly created the correct boilerplate? 
* Inside the new `validate_match` function:
  * Am I correct in thinking I want to do validation by comparing the match arms against `match_expr`? And when analyzing `match_expr` I should be looking at it as a `hir_def::expr::Expr`?
  * I mostly copied the chained if-let statements from the struct validation. Shouldn't there be a non-failable way to get an AstPtr from the hir data structures? 

Thanks for all the guidance.

Co-authored-by: Josh Mcguigan <[email protected]>

11 files changed, 1551 insertions, 11 deletions

diff --git a/crates/ra_hir/src/diagnostics.rs b/crates/ra_hir/src/diagnostics.rs
index a9040ea3d..c82883d0c 100644
--- a/crates/ra_hir/src/diagnostics.rs
+++ b/crates/ra_hir/src/diagnostics.rs
@@ -1,4 +1,4 @@
 //! FIXME: write short doc here
 pub use hir_def::diagnostics::UnresolvedModule;
 pub use hir_expand::diagnostics::{AstDiagnostic, Diagnostic, DiagnosticSink};
-pub use hir_ty::diagnostics::{MissingFields, MissingOkInTailExpr, NoSuchField};
+pub use hir_ty::diagnostics::{MissingFields, MissingMatchArms, MissingOkInTailExpr, NoSuchField};
diff --git a/crates/ra_hir_ty/Cargo.toml b/crates/ra_hir_ty/Cargo.toml
index 45be08430..9a4a7aa6f 100644
--- a/crates/ra_hir_ty/Cargo.toml
+++ b/crates/ra_hir_ty/Cargo.toml
@@ -9,6 +9,7 @@ doctest = false
 
 [dependencies]
 arrayvec = "0.5.1"
+smallvec = "1.2.0"
 ena = "0.13.1"
 log = "0.4.8"
 rustc-hash = "1.1.0"
diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
new file mode 100644
index 000000000..f29a25505
--- /dev/null
+++ b/crates/ra_hir_ty/src/_match.rs
@@ -0,0 +1,1411 @@
+//! This module implements match statement exhaustiveness checking and usefulness checking
+//! for match arms.
+//!
+//! 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 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
+//! summarise 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:
+//! ```
+//!     // x: (Option<bool>, Result<()>)
+//!     match x {
+//!         (Some(true), _) => {}
+//!         (None, Err(())) => {}
+//!         (None, Err(_)) => {}
+//!     }
+//! ```
+//! Here, the matrix `P` starts as:
+//! [
+//!     [(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 realised 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:
+//!           `U(P, p) := U(S(c, P), S(c, p))`
+//!           This special case is handled in `is_useful_specialized`.
+//!
+//!           For example, if `P` is:
+//!           [
+//!               [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:
+//!               [
+//!                   [_, 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:
+//!               [
+//!                   [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:
+//!           `U(P, p) := U(P, (r_1, p_2, .., p_n))
+//!                    || U(P, (r_2, p_2, .., p_n))`
+use std::sync::Arc;
+
+use smallvec::{smallvec, SmallVec};
+
+use crate::{
+    db::HirDatabase,
+    expr::{Body, Expr, Literal, Pat, PatId},
+    InferenceResult,
+};
+use hir_def::{adt::VariantData, EnumVariantId, VariantId};
+
+#[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,
+        }
+    }
+
+    fn as_id(self) -> Option<PatId> {
+        match self {
+            PatIdOrWild::PatId(id) => Some(id),
+            PatIdOrWild::Wild => None,
+        }
+    }
+}
+
+impl From<PatId> for PatIdOrWild {
+    fn from(pat_id: PatId) -> Self {
+        Self::PatId(pat_id)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct MatchCheckNotImplemented;
+
+/// 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 type MatchCheckResult<T> = Result<T, MatchCheckNotImplemented>;
+
+#[derive(Debug)]
+/// A row in a Matrix.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(crate) struct PatStack(PatStackInner);
+type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+
+impl PatStack {
+    pub(crate) fn from_pattern(pat_id: PatId) -> PatStack {
+        Self(smallvec!(pat_id.into()))
+    }
+
+    pub(crate) fn from_wild() -> PatStack {
+        Self(smallvec!(PatIdOrWild::Wild))
+    }
+
+    fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
+        Self(SmallVec::from_slice(slice))
+    }
+
+    fn from_vec(v: PatStackInner) -> PatStack {
+        Self(v)
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn head(&self) -> PatIdOrWild {
+        self.0[0]
+    }
+
+    fn get_head(&self) -> Option<PatIdOrWild> {
+        self.0.first().copied()
+    }
+
+    fn to_tail(&self) -> PatStack {
+        Self::from_slice(&self.0[1..])
+    }
+
+    fn replace_head_with(&self, pat_ids: &[PatId]) -> PatStack {
+        let mut patterns: PatStackInner = smallvec![];
+        for pat in pat_ids {
+            patterns.push((*pat).into());
+        }
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+        PatStack::from_vec(patterns)
+    }
+
+    /// 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.head().as_pat(cx), Pat::Wild) {
+            Some(self.to_tail())
+        } else {
+            None
+        }
+    }
+
+    /// 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 result = match (self.head().as_pat(cx), constructor) {
+            (Pat::Tuple(ref pat_ids), Constructor::Tuple { arity }) => {
+                debug_assert_eq!(
+                    pat_ids.len(),
+                    *arity,
+                    "we type check before calling this code, so we should never hit this case",
+                );
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (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(MatchCheckNotImplemented),
+                }
+            }
+            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?),
+            (Pat::Path(_), Constructor::Enum(constructor)) => {
+                // enums with no associated data become `Pat::Path`
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    None
+                } else {
+                    Some(self.to_tail())
+                }
+            }
+            (Pat::TupleStruct { args: ref pat_ids, .. }, Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    None
+                } else {
+                    Some(self.replace_head_with(pat_ids))
+                }
+            }
+            (Pat::Or(_), _) => return Err(MatchCheckNotImplemented),
+            (_, _) => return Err(MatchCheckNotImplemented),
+        };
+
+        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.head().as_pat(cx),
+            "expand_wildcard must only be called on PatStack with wild at head",
+        );
+
+        let mut patterns: PatStackInner = smallvec![];
+
+        for _ in 0..constructor.arity(cx)? {
+            patterns.push(PatIdOrWild::Wild);
+        }
+
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+
+        Ok(PatStack::from_vec(patterns))
+    }
+}
+
+#[derive(Debug)]
+/// A collection of PatStack.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(crate) struct Matrix(Vec<PatStack>);
+
+impl Matrix {
+    pub(crate) fn empty() -> Self {
+        Self(vec![])
+    }
+
+    pub(crate) 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(PatStack::from_pattern(pat_id));
+            }
+        } else {
+            self.0.push(row);
+        }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn heads(&self) -> Vec<PatIdOrWild> {
+        self.0.iter().map(|p| p.head()).collect()
+    }
+
+    /// 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)))
+    }
+
+    /// 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);
+            }
+        }
+
+        Ok(new_matrix)
+    }
+
+    fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
+        let mut matrix = Matrix::empty();
+
+        for pat in iter {
+            // using push ensures we expand or-patterns
+            matrix.push(cx, pat);
+        }
+
+        matrix
+    }
+}
+
+#[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 enum Usefulness {
+    Useful,
+    NotUseful,
+}
+
+pub struct MatchCheckCtx<'a> {
+    pub body: Arc<Body>,
+    pub infer: Arc<InferenceResult>,
+    pub db: &'a dyn HirDatabase,
+}
+
+/// 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(crate) fn is_useful(
+    cx: &MatchCheckCtx,
+    matrix: &Matrix,
+    v: &PatStack,
+) -> MatchCheckResult<Usefulness> {
+    if v.is_empty() {
+        let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
+
+        return Ok(result);
+    }
+
+    if let Pat::Or(pat_ids) = v.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
+                }
+            }
+        });
+
+        return if any_useful {
+            Ok(Usefulness::Useful)
+        } else if found_unimplemented {
+            Err(MatchCheckNotImplemented)
+        } else {
+            Ok(Usefulness::NotUseful)
+        };
+    }
+
+    if let Some(constructor) = pat_constructor(cx, v.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,
+                    };
+                }
+
+                if found_unimplemented {
+                    Err(MatchCheckNotImplemented)
+                } 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)
+            }
+        }
+    }
+}
+
+#[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),
+}
+
+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::Unit => 0,
+                    _ => return Err(MatchCheckNotImplemented),
+                }
+            }
+        };
+
+        Ok(arity)
+    }
+
+    fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> {
+        match self {
+            Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)],
+            Constructor::Tuple { .. } => 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(),
+        }
+    }
+}
+
+/// 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(pats) => Some(Constructor::Tuple { arity: pats.len() }),
+        Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
+            Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
+            _ => return Err(MatchCheckNotImplemented),
+        },
+        Pat::TupleStruct { .. } | Pat::Path(_) => {
+            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(MatchCheckNotImplemented)?;
+            match variant_id {
+                VariantId::EnumVariantId(enum_variant_id) => {
+                    Some(Constructor::Enum(enum_variant_id))
+                }
+                _ => return Err(MatchCheckNotImplemented),
+            }
+        }
+        _ => return Err(MatchCheckNotImplemented),
+    };
+
+    Ok(res)
+}
+
+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;
+            }
+
+            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)));
+
+            covers_true && covers_false
+        }
+        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
+        }),
+    }
+}
+
+fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool {
+    Some(enum_variant_id.into()) == cx.infer.variant_resolution_for_pat(pat_id)
+}
+
+#[cfg(test)]
+mod tests {
+    pub(super) use insta::assert_snapshot;
+    pub(super) use ra_db::fixture::WithFixture;
+
+    pub(super) use crate::test_db::TestDB;
+
+    pub(super) fn check_diagnostic_message(content: &str) -> String {
+        TestDB::with_single_file(content).0.diagnostics().0
+    }
+
+    pub(super) fn check_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(1, diagnostic_count, "no diagnostic reported");
+    }
+
+    pub(super) fn check_no_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(0, diagnostic_count, "expected no diagnostic, found one");
+    }
+
+    #[test]
+    fn empty_tuple_no_arms_diagnostic_message() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        assert_snapshot!(
+            check_diagnostic_message(content),
+            @"\"()\": Missing match arm\n"
+        );
+    }
+
+    #[test]
+    fn empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn empty_tuple_wild() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    _ => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    () => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                    (()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                    ((), ()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                    false => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (false, true) => {},
+                    (false, false) => {},
+                    (true, false) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_with_wilds() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (false, _) => {},
+                    (true, false) => {},
+                    (_, true) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                    (true, false) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_binding_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, _x) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_binding_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, _x) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, ((), true)) => {},
+                    (false, ((), false)) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, _) => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, _) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::A {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_ref_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match &Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_ref_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match &Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::A(false) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::B => (),
+                    _ => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_2_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(_) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn or_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(true) | Either::A(false) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match (Either::A(false), Either2::C) {
+                    (Either::A(true), _) | (Either::A(false), _) => (),
+                    (Either::B(true, _), Either2::C) => (),
+                    (Either::B(false, _), Either2::C) => (),
+                    (Either::B(_, _), Either2::D) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn mismatched_types() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match Either::A {
+                    Either2::C => (),
+                    Either2::D => (),
+                }
+            }
+        ";
+
+        // Match arms with the incorrect type are filtered out.
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn mismatched_types_with_different_arity() {
+        let content = r"
+            fn test_fn() {
+                match (true, false) {
+                    (true, false, true) => (),
+                    (true) => (),
+                }
+            }
+        ";
+
+        // Match arms with the incorrect type are filtered out.
+        check_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_not_in_scope() {
+        let content = r"
+            fn test_fn() {
+                match Foo::Bar {
+                    Foo::Baz => (),
+                }
+            }
+        ";
+
+        // The enum is not in scope so we don't perform exhaustiveness
+        // checking, but we want to be sure we don't panic here (and
+        // we don't create a diagnostic).
+        check_no_diagnostic(content);
+    }
+}
+
+#[cfg(test)]
+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
+    //! test module should document known false negatives. Eventually we will have a complete
+    //! implementation of match checking and this module will be empty.
+    //!
+    //! The reasons for documenting known false negatives:
+    //!
+    //!   1. It acts as a backlog of work that can be done to improve the behavior of the system.
+    //!   2. It ensures the code doesn't panic when handling these cases.
+
+    use super::tests::*;
+
+    #[test]
+    fn integers() {
+        let content = r"
+            fn test_fn() {
+                match 5 {
+                    10 => (),
+                    11..20 => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently check integer exhaustiveness.
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_record() {
+        let content = r"
+            enum Either {
+                A { foo: u32 },
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A { foo: 5 } => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently handle enum record types.
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn internal_or() {
+        let content = r"
+            fn test_fn() {
+                enum Either {
+                    A(bool),
+                    B,
+                }
+                match Either::B {
+                    Either::A(true | false) => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We do not currently handle patterns with internal `or`s.
+        check_no_diagnostic(content);
+    }
+}
diff --git a/crates/ra_hir_ty/src/diagnostics.rs b/crates/ra_hir_ty/src/diagnostics.rs
index 0f8522021..8cbce1168 100644
--- a/crates/ra_hir_ty/src/diagnostics.rs
+++ b/crates/ra_hir_ty/src/diagnostics.rs
@@ -6,7 +6,7 @@ use hir_expand::{db::AstDatabase, name::Name, HirFileId, InFile};
 use ra_syntax::{ast, AstNode, AstPtr, SyntaxNodePtr};
 use stdx::format_to;
 
-pub use hir_def::diagnostics::UnresolvedModule;
+pub use hir_def::{diagnostics::UnresolvedModule, expr::MatchArm};
 pub use hir_expand::diagnostics::{AstDiagnostic, Diagnostic, DiagnosticSink};
 
 #[derive(Debug)]
@@ -63,6 +63,25 @@ impl AstDiagnostic for MissingFields {
 }
 
 #[derive(Debug)]
+pub struct MissingMatchArms {
+    pub file: HirFileId,
+    pub match_expr: AstPtr<ast::Expr>,
+    pub arms: AstPtr<ast::MatchArmList>,
+}
+
+impl Diagnostic for MissingMatchArms {
+    fn message(&self) -> String {
+        String::from("Missing match arm")
+    }
+    fn source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.match_expr.into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
 pub struct MissingOkInTailExpr {
     pub file: HirFileId,
     pub expr: AstPtr<ast::Expr>,
diff --git a/crates/ra_hir_ty/src/expr.rs b/crates/ra_hir_ty/src/expr.rs
index b7b476b4c..1e7395b16 100644
--- a/crates/ra_hir_ty/src/expr.rs
+++ b/crates/ra_hir_ty/src/expr.rs
@@ -14,9 +14,10 @@ use rustc_hash::FxHashSet;
 
 use crate::{
     db::HirDatabase,
-    diagnostics::{MissingFields, MissingOkInTailExpr},
+    diagnostics::{MissingFields, MissingMatchArms, MissingOkInTailExpr},
     utils::variant_data,
     ApplicationTy, InferenceResult, Ty, TypeCtor,
+    _match::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
 };
 
 pub use hir_def::{
@@ -52,15 +53,99 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
         for e in body.exprs.iter() {
             if let (id, Expr::RecordLit { path, fields, spread }) = e {
                 self.validate_record_literal(id, path, fields, *spread, db);
+            } else if let (id, Expr::Match { expr, arms }) = e {
+                self.validate_match(id, *expr, arms, db, self.infer.clone());
             }
         }
 
         let body_expr = &body[body.body_expr];
-        if let Expr::Block { statements: _, tail: Some(t) } = body_expr {
+        if let Expr::Block { tail: Some(t), .. } = body_expr {
             self.validate_results_in_tail_expr(body.body_expr, *t, db);
         }
     }
 
+    fn validate_match(
+        &mut self,
+        id: ExprId,
+        match_expr: ExprId,
+        arms: &[MatchArm],
+        db: &dyn HirDatabase,
+        infer: Arc<InferenceResult>,
+    ) {
+        let (body, source_map): (Arc<Body>, Arc<BodySourceMap>) =
+            db.body_with_source_map(self.func.into());
+
+        let match_expr_ty = match infer.type_of_expr.get(match_expr) {
+            Some(ty) => ty,
+            // If we can't resolve the type of the match expression
+            // we cannot perform exhaustiveness checks.
+            None => return,
+        };
+
+        let cx = MatchCheckCtx { body, infer: infer.clone(), db };
+        let pats = arms.iter().map(|arm| arm.pat);
+
+        let mut seen = Matrix::empty();
+        for pat in pats {
+            // We skip any patterns whose type we cannot resolve.
+            //
+            // This could lead to false positives in this diagnostic, so
+            // it might be better to skip the entire diagnostic if we either
+            // cannot resolve a match arm or determine that the match arm has
+            // the wrong type.
+            if let Some(pat_ty) = infer.type_of_pat.get(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
+                // block here.
+                //
+                // When comparing the types, we also have to consider that rustc
+                // will automatically de-reference the match expression type if
+                // necessary.
+                //
+                // FIXME we should use the type checker for this.
+                if pat_ty == match_expr_ty
+                    || match_expr_ty
+                        .as_reference()
+                        .map(|(match_expr_ty, _)| match_expr_ty == pat_ty)
+                        .unwrap_or(false)
+                {
+                    // 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);
+                }
+            }
+        }
+
+        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,
+        }
+
+        if let Ok(source_ptr) = source_map.expr_syntax(id) {
+            if let Some(expr) = source_ptr.value.left() {
+                let root = source_ptr.file_syntax(db.upcast());
+                if let ast::Expr::MatchExpr(match_expr) = expr.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 validate_record_literal(
         &mut self,
         id: ExprId,
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index 86acd27f8..69bbb4307 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -21,9 +21,13 @@ impl<'a> InferenceContext<'a> {
         subpats: &[PatId],
         expected: &Ty,
         default_bm: BindingMode,
+        id: PatId,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
         let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        if let Some(variant) = def {
+            self.write_variant_resolution(id.into(), variant);
+        }
         self.unify(&ty, expected);
 
         let substs = ty.substs().unwrap_or_else(Substs::empty);
@@ -152,7 +156,7 @@ impl<'a> InferenceContext<'a> {
                 Ty::apply_one(TypeCtor::Ref(*mutability), subty)
             }
             Pat::TupleStruct { path: p, args: subpats } => {
-                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm)
+                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm, pat)
             }
             Pat::Record { path: p, args: fields } => {
                 self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index 318652c61..2b6bc0f79 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -67,8 +67,16 @@ impl<'a> InferenceContext<'a> {
             ValueNs::FunctionId(it) => it.into(),
             ValueNs::ConstId(it) => it.into(),
             ValueNs::StaticId(it) => it.into(),
-            ValueNs::StructId(it) => it.into(),
-            ValueNs::EnumVariantId(it) => it.into(),
+            ValueNs::StructId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
+            ValueNs::EnumVariantId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
         };
 
         let ty = self.db.value_ty(typable);
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 717399b6d..18f74d3b1 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -43,6 +43,7 @@ mod tests;
 #[cfg(test)]
 mod test_db;
 mod marks;
+mod _match;
 
 use std::ops::Deref;
 use std::sync::Arc;
diff --git a/crates/ra_hir_ty/src/test_db.rs b/crates/ra_hir_ty/src/test_db.rs
index 208096aab..3a4d58bf9 100644
--- a/crates/ra_hir_ty/src/test_db.rs
+++ b/crates/ra_hir_ty/src/test_db.rs
@@ -105,8 +105,9 @@ impl TestDB {
     }
 
     // FIXME: don't duplicate this
-    pub fn diagnostics(&self) -> String {
+    pub fn diagnostics(&self) -> (String, u32) {
         let mut buf = String::new();
+        let mut count = 0;
         let crate_graph = self.crate_graph();
         for krate in crate_graph.iter() {
             let crate_def_map = self.crate_def_map(krate);
@@ -133,13 +134,14 @@ impl TestDB {
                 let infer = self.infer(f.into());
                 let mut sink = DiagnosticSink::new(|d| {
                     format_to!(buf, "{:?}: {}\n", d.syntax_node(self).text(), d.message());
+                    count += 1;
                 });
                 infer.add_diagnostics(self, f, &mut sink);
                 let mut validator = ExprValidator::new(f, infer, &mut sink);
                 validator.validate_body(self);
             }
         }
-        buf
+        (buf, count)
     }
 }
 
diff --git a/crates/ra_hir_ty/src/tests.rs b/crates/ra_hir_ty/src/tests.rs
index 027e5a8f8..e4a103d1b 100644
--- a/crates/ra_hir_ty/src/tests.rs
+++ b/crates/ra_hir_ty/src/tests.rs
@@ -309,7 +309,8 @@ fn no_such_field_diagnostics() {
         }
         ",
     )
-    .diagnostics();
+    .diagnostics()
+    .0;
 
     assert_snapshot!(diagnostics, @r###"
     "baz: 62": no such field
diff --git a/crates/ra_ide/src/diagnostics.rs b/crates/ra_ide/src/diagnostics.rs
index c1d7ddaf2..901ad104c 100644
--- a/crates/ra_ide/src/diagnostics.rs
+++ b/crates/ra_ide/src/diagnostics.rs
@@ -101,6 +101,14 @@ pub(crate) fn diagnostics(db: &RootDatabase, file_id: FileId) -> Vec<Diagnostic>
             fix,
         })
     })
+    .on::<hir::diagnostics::MissingMatchArms, _>(|d| {
+        res.borrow_mut().push(Diagnostic {
+            range: d.highlight_range(),
+            message: d.message(),
+            severity: Severity::Error,
+            fix: None,
+        })
+    })
     .on::<hir::diagnostics::MissingOkInTailExpr, _>(|d| {
         let node = d.ast(db);
         let replacement = format!("Ok({})", node.syntax());
@@ -291,7 +299,7 @@ mod tests {
     fn check_no_diagnostic(content: &str) {
         let (analysis, file_id) = single_file(content);
         let diagnostics = analysis.diagnostics(file_id).unwrap();
-        assert_eq!(diagnostics.len(), 0);
+        assert_eq!(diagnostics.len(), 0, "expected no diagnostic, found one");
     }
 
     #[test]