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diff --git a/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
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+++ b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
@@ -0,0 +1,894 @@
+//! [`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 { .. } => panic!("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,
+
+            _ => panic!(
+                "bug: 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 => {
+                panic!("bug: 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() {
+                    // If `exhaustive_patterns` is enabled, we exclude variants known to be
+                    // uninhabited.
+                    unhandled()
+                } 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<'a>(
+        cx: &MatchCheckCtx<'_>,
+        tys: impl IntoIterator<Item = &'a 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)
+    }
+
+    pub(crate) fn wildcards(pcx: PatCtxt<'_>, constructor: &Constructor) -> Self {
+        let ty = pcx.ty;
+        let cx = pcx.cx;
+        let wildcard_from_ty = |ty| cx.alloc_pat(Pat::wildcard_from_ty(ty));
+
+        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)
+                }
+                TyKind::Ref(.., rty) => Fields::from_single_pattern(wildcard_from_ty(rty)),
+                TyKind::Adt(AdtId(adt), substs) => {
+                    let adt_is_box = false; // TODO(iDawer): implement this
+                    if adt_is_box {
+                        // 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;
+                        let field_ty_arena = cx.db.field_types(variant_id);
+                        let field_tys =
+                            || field_ty_arena.iter().map(|(_, binders)| binders.skip_binders());
+                        // 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
+                            unimplemented!("exhaustive_patterns feature")
+                        }
+                    }
+                }
+                _ => panic!("Unexpected type for `Single` constructor: {:?}", ty),
+            },
+            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(..) => {
+                    panic!("bug: 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),
+            Opaque => panic!("bug: we should not try to apply an opaque constructor"),
+            Missing => {
+                panic!("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): these alocations and clones are so unfortunate (+1 for switching to references)
+        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()
+}