Merge #10484

10484: internal: Update match checking algorithm r=lnicola a=iDawer

Sync match checking algorithm with rust-lang/rust f31622a50 2021-11-12 (https://github.com/rust-lang/rust/pull/90813)

This update brings huge simplification to the match checking and introduces an easy to use machinery for pattern destructuring and also:

1. Add a function to do post-inference normalization `hir_ty::infer::normalize(...)`.
2. Store binding modes in `InferenceResult`.

Todo:

- [x] Rebase & test (https://github.com/rust-analyzer/rust-analyzer/pull/10484#issuecomment-996669665)

Co-authored-by: Dawer <7803845+iDawer@users.noreply.github.com>
Co-authored-by: iDawer <ilnur.iskhakov.oss@outlook.com>

9 files changed, 699 insertions, 886 deletions

diff --git a/Cargo.lock b/Cargo.lock
index cd4324cf3c4..7d6446ea6b8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -559,6 +559,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "tracing-tree",
+ "typed-arena",
 ]
 
 [[package]]
@@ -1776,6 +1777,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "typed-arena"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0685c84d5d54d1c26f7d3eb96cd41550adb97baed141a761cf335d3d33bcd0ae"
+
+[[package]]
 name = "ungrammar"
 version = "1.14.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/crates/hir_ty/Cargo.toml b/crates/hir_ty/Cargo.toml
index a1a7063d48e..f5e55757071 100644
--- a/crates/hir_ty/Cargo.toml
+++ b/crates/hir_ty/Cargo.toml
@@ -23,6 +23,7 @@ chalk-ir = "0.75"
 chalk-recursive = { version = "0.75", default-features = false }
 la-arena = { version = "0.3.0", path = "../../lib/arena" }
 once_cell = { version = "1.5.0" }
+typed-arena = "2.0.1"
 
 stdx = { path = "../stdx", version = "0.0.0" }
 hir_def = { path = "../hir_def", version = "0.0.0" }
diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index d3a01001237..a8c4026e31f 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -2,7 +2,7 @@
 //! through the body using inference results: mismatched arg counts, missing
 //! fields, etc.
 
-use std::{cell::RefCell, sync::Arc};
+use std::sync::Arc;
 
 use hir_def::{
     expr::Statement, path::path, resolver::HasResolver, type_ref::Mutability, AssocItemId,
@@ -11,12 +11,14 @@ use hir_def::{
 use hir_expand::name;
 use itertools::Either;
 use rustc_hash::FxHashSet;
+use typed_arena::Arena;
 
 use crate::{
     db::HirDatabase,
     diagnostics::match_check::{
         self,
-        usefulness::{compute_match_usefulness, expand_pattern, MatchCheckCtx, PatternArena},
+        deconstruct_pat::DeconstructedPat,
+        usefulness::{compute_match_usefulness, MatchCheckCtx},
     },
     AdtId, InferenceResult, Interner, Ty, TyExt, TyKind,
 };
@@ -275,15 +277,20 @@ impl ExprValidator {
     ) {
         let body = db.body(self.owner);
 
-        let match_expr_ty = if infer.type_of_expr[match_expr].is_unknown() {
+        let match_expr_ty = &infer[match_expr];
+        if match_expr_ty.is_unknown() {
             return;
-        } else {
-            &infer.type_of_expr[match_expr]
-        };
+        }
 
-        let pattern_arena = RefCell::new(PatternArena::new());
+        let pattern_arena = Arena::new();
+        let cx = MatchCheckCtx {
+            module: self.owner.module(db.upcast()),
+            body: self.owner,
+            db,
+            pattern_arena: &pattern_arena,
+        };
 
-        let mut m_arms = Vec::new();
+        let mut m_arms = Vec::with_capacity(arms.len());
         let mut has_lowering_errors = false;
         for arm in arms {
             if let Some(pat_ty) = infer.type_of_pat.get(arm.pat) {
@@ -308,13 +315,7 @@ impl ExprValidator {
                     // check the usefulness of each pattern as we added it
                     // to the matrix here.
                     let m_arm = match_check::MatchArm {
-                        pat: self.lower_pattern(
-                            arm.pat,
-                            &mut pattern_arena.borrow_mut(),
-                            db,
-                            &body,
-                            &mut has_lowering_errors,
-                        ),
+                        pat: self.lower_pattern(&cx, arm.pat, db, &body, &mut has_lowering_errors),
                         has_guard: arm.guard.is_some(),
                     };
                     m_arms.push(m_arm);
@@ -332,17 +333,10 @@ impl ExprValidator {
             return;
         }
 
-        let cx = MatchCheckCtx {
-            module: self.owner.module(db.upcast()),
-            match_expr,
-            infer: &infer,
-            db,
-            pattern_arena: &pattern_arena,
-        };
-        let report = compute_match_usefulness(&cx, &m_arms);
+        let report = compute_match_usefulness(&cx, &m_arms, match_expr_ty);
 
         // FIXME Report unreacheble arms
-        // https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/check_match.rs#L200-L201
+        // https://github.com/rust-lang/rust/blob/f31622a50/compiler/rustc_mir_build/src/thir/pattern/check_match.rs#L200
 
         let witnesses = report.non_exhaustiveness_witnesses;
         // FIXME Report witnesses
@@ -352,17 +346,17 @@ impl ExprValidator {
         }
     }
 
-    fn lower_pattern(
+    fn lower_pattern<'p>(
         &self,
+        cx: &MatchCheckCtx<'_, 'p>,
         pat: PatId,
-        pattern_arena: &mut PatternArena,
         db: &dyn HirDatabase,
         body: &Body,
         have_errors: &mut bool,
-    ) -> match_check::PatId {
+    ) -> &'p DeconstructedPat<'p> {
         let mut patcx = match_check::PatCtxt::new(db, &self.infer, body);
         let pattern = patcx.lower_pattern(pat);
-        let pattern = pattern_arena.alloc(expand_pattern(pattern));
+        let pattern = cx.pattern_arena.alloc(DeconstructedPat::from_pat(cx, &pattern));
         if !patcx.errors.is_empty() {
             *have_errors = true;
         }
diff --git a/crates/hir_ty/src/diagnostics/match_check.rs b/crates/hir_ty/src/diagnostics/match_check.rs
index b3cc83b3404..87a85797613 100644
--- a/crates/hir_ty/src/diagnostics/match_check.rs
+++ b/crates/hir_ty/src/diagnostics/match_check.rs
@@ -5,25 +5,26 @@
 //!
 //! It is modeled on the rustc module `rustc_mir_build::thir::pattern`.
 
-mod deconstruct_pat;
 mod pat_util;
 
+pub(crate) mod deconstruct_pat;
 pub(crate) mod usefulness;
 
-use hir_def::{body::Body, EnumVariantId, LocalFieldId, VariantId};
-use la_arena::Idx;
+use hir_def::{body::Body, expr::PatId, EnumVariantId, LocalFieldId, VariantId};
+use stdx::never;
 
-use crate::{db::HirDatabase, InferenceResult, Interner, Substitution, Ty, TyKind};
+use crate::{
+    db::HirDatabase, infer::BindingMode, InferenceResult, Interner, Substitution, Ty, TyKind,
+};
 
 use self::pat_util::EnumerateAndAdjustIterator;
 
 pub(crate) use self::usefulness::MatchArm;
 
-pub(crate) type PatId = Idx<Pat>;
-
 #[derive(Clone, Debug)]
 pub(crate) enum PatternError {
     Unimplemented,
+    UnexpectedType,
     UnresolvedVariant,
     MissingField,
     ExtraFields,
@@ -41,12 +42,6 @@ pub(crate) struct Pat {
     pub(crate) kind: Box<PatKind>,
 }
 
-impl Pat {
-    pub(crate) fn wildcard_from_ty(ty: Ty) -> Self {
-        Pat { ty, kind: Box::new(PatKind::Wild) }
-    }
-}
-
 /// Close relative to `rustc_mir_build::thir::pattern::PatKind`
 #[derive(Clone, Debug, PartialEq)]
 pub(crate) enum PatKind {
@@ -100,7 +95,7 @@ impl<'a> PatCtxt<'a> {
         Self { db, infer, body, errors: Vec::new() }
     }
 
-    pub(crate) fn lower_pattern(&mut self, pat: hir_def::expr::PatId) -> Pat {
+    pub(crate) fn lower_pattern(&mut self, pat: PatId) -> Pat {
         // XXX(iDawer): Collecting pattern adjustments feels imprecise to me.
         // When lowering of & and box patterns are implemented this should be tested
         // in a manner of `match_ergonomics_issue_9095` test.
@@ -116,7 +111,7 @@ impl<'a> PatCtxt<'a> {
         )
     }
 
-    fn lower_pattern_unadjusted(&mut self, pat: hir_def::expr::PatId) -> Pat {
+    fn lower_pattern_unadjusted(&mut self, pat: PatId) -> Pat {
         let mut ty = &self.infer[pat];
         let variant = self.infer.variant_resolution_for_pat(pat);
 
@@ -138,9 +133,16 @@ impl<'a> PatCtxt<'a> {
                 PatKind::Leaf { subpatterns }
             }
 
-            hir_def::expr::Pat::Bind { subpat, .. } => {
-                if let TyKind::Ref(.., rty) = ty.kind(Interner) {
-                    ty = rty;
+            hir_def::expr::Pat::Bind { ref name, subpat, .. } => {
+                let bm = self.infer.pat_binding_modes[&pat];
+                match (bm, ty.kind(Interner)) {
+                    (BindingMode::Ref(_), TyKind::Ref(.., rty)) => ty = rty,
+                    (BindingMode::Ref(_), _) => {
+                        never!("`ref {}` has wrong type {:?}", name, ty);
+                        self.errors.push(PatternError::UnexpectedType);
+                        return Pat { ty: ty.clone(), kind: PatKind::Wild.into() };
+                    }
+                    _ => (),
                 }
                 PatKind::Binding { subpattern: self.lower_opt_pattern(subpat) }
             }
@@ -189,7 +191,7 @@ impl<'a> PatCtxt<'a> {
 
     fn lower_tuple_subpats(
         &mut self,
-        pats: &[hir_def::expr::PatId],
+        pats: &[PatId],
         expected_len: usize,
         ellipsis: Option<usize>,
     ) -> Vec<FieldPat> {
@@ -207,17 +209,17 @@ impl<'a> PatCtxt<'a> {
             .collect()
     }
 
-    fn lower_patterns(&mut self, pats: &[hir_def::expr::PatId]) -> Vec<Pat> {
+    fn lower_patterns(&mut self, pats: &[PatId]) -> Vec<Pat> {
         pats.iter().map(|&p| self.lower_pattern(p)).collect()
     }
 
-    fn lower_opt_pattern(&mut self, pat: Option<hir_def::expr::PatId>) -> Option<Pat> {
+    fn lower_opt_pattern(&mut self, pat: Option<PatId>) -> Option<Pat> {
         pat.map(|p| self.lower_pattern(p))
     }
 
     fn lower_variant_or_leaf(
         &mut self,
-        pat: hir_def::expr::PatId,
+        pat: PatId,
         ty: &Ty,
         subpatterns: Vec<FieldPat>,
     ) -> PatKind {
@@ -244,7 +246,7 @@ impl<'a> PatCtxt<'a> {
         kind
     }
 
-    fn lower_path(&mut self, pat: hir_def::expr::PatId, _path: &hir_def::path::Path) -> Pat {
+    fn lower_path(&mut self, pat: PatId, _path: &hir_def::path::Path) -> Pat {
         let ty = &self.infer[pat];
 
         let pat_from_kind = |kind| Pat { ty: ty.clone(), kind: Box::new(kind) };
diff --git a/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
index eb7d5e5e482..84ded517ba7 100644
--- a/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
+++ b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
@@ -42,6 +42,7 @@
 //! wildcards, see [`SplitWildcard`]; for integer ranges, see [`SplitIntRange`].
 
 use std::{
+    cell::Cell,
     cmp::{max, min},
     iter::once,
     ops::RangeInclusive,
@@ -52,15 +53,32 @@ use smallvec::{smallvec, SmallVec};
 use stdx::never;
 use syntax::SmolStr;
 
-use crate::{AdtId, Interner, Scalar, Ty, TyExt, TyKind};
+use crate::{infer::normalize, AdtId, Interner, Scalar, Ty, TyExt, TyKind};
 
 use super::{
-    usefulness::{MatchCheckCtx, PatCtxt},
-    FieldPat, Pat, PatId, PatKind,
+    usefulness::{helper::Captures, MatchCheckCtx, PatCtxt},
+    Pat, PatKind,
 };
 
 use self::Constructor::*;
 
+/// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
+fn expand_or_pat(pat: &Pat) -> Vec<&Pat> {
+    fn expand<'p>(pat: &'p Pat, vec: &mut Vec<&'p Pat>) {
+        if let PatKind::Or { pats } = pat.kind.as_ref() {
+            for pat in pats {
+                expand(pat, vec);
+            }
+        } else {
+            vec.push(pat)
+        }
+    }
+
+    let mut pats = Vec::new();
+    expand(pat, &mut pats);
+    pats
+}
+
 /// [Constructor] uses this in umimplemented variants.
 /// It allows porting match expressions from upstream algorithm without losing semantics.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -241,6 +259,10 @@ pub(super) struct Slice {
 }
 
 impl Slice {
+    fn arity(self) -> usize {
+        unimplemented!()
+    }
+
     /// See `Constructor::is_covered_by`
     fn is_covered_by(self, _other: Self) -> bool {
         unimplemented!() // never called as Slice contains Void
@@ -278,10 +300,13 @@ pub(super) enum Constructor {
     /// 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,
+    /// for [`SplitWildcard`]. The carried `bool` is used for the `non_exhaustive_omitted_patterns`
+    /// lint.
+    Missing { nonexhaustive_enum_missing_real_variants: bool },
     /// Wildcard pattern.
     Wildcard,
+    /// Or-pattern.
+    Or,
 }
 
 impl Constructor {
@@ -289,6 +314,10 @@ impl Constructor {
         matches!(self, Wildcard)
     }
 
+    pub(super) fn is_non_exhaustive(&self) -> bool {
+        matches!(self, NonExhaustive)
+    }
+
     fn as_int_range(&self) -> Option<&IntRange> {
         match self {
             IntRange(range) => Some(range),
@@ -303,6 +332,14 @@ impl Constructor {
         }
     }
 
+    pub(super) fn is_unstable_variant(&self, _pcx: PatCtxt<'_, '_>) -> bool {
+        false //FIXME: implement this
+    }
+
+    pub(super) fn is_doc_hidden_variant(&self, _pcx: PatCtxt<'_, '_>) -> bool {
+        false //FIXME: implement this
+    }
+
     fn variant_id_for_adt(&self, adt: hir_def::AdtId) -> VariantId {
         match *self {
             Variant(id) => id.into(),
@@ -318,16 +355,39 @@ impl Constructor {
         }
     }
 
-    /// 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 { .. } => {
-                never!("Or-pattern should have been expanded earlier on.");
-                Wildcard
+    /// The number of fields for this constructor. This must be kept in sync with
+    /// `Fields::wildcards`.
+    pub(super) fn arity(&self, pcx: PatCtxt<'_, '_>) -> usize {
+        match self {
+            Single | Variant(_) => match *pcx.ty.kind(Interner) {
+                TyKind::Tuple(arity, ..) => arity,
+                TyKind::Ref(..) => 1,
+                TyKind::Adt(adt, ..) => {
+                    if adt_is_box(adt.0, pcx.cx) {
+                        // The only legal patterns of type `Box` (outside `std`) are `_` and box
+                        // patterns. If we're here we can assume this is a box pattern.
+                        1
+                    } else {
+                        let variant = self.variant_id_for_adt(adt.0);
+                        Fields::list_variant_nonhidden_fields(pcx.cx, pcx.ty, variant).count()
+                    }
+                }
+                _ => {
+                    never!("Unexpected type for `Single` constructor: {:?}", pcx.ty);
+                    0
+                }
+            },
+            Slice(slice) => slice.arity(),
+            Str(..)
+            | FloatRange(..)
+            | IntRange(..)
+            | NonExhaustive
+            | Opaque
+            | Missing { .. }
+            | Wildcard => 0,
+            Or => {
+                never!("The `Or` constructor doesn't have a fixed arity");
+                0
             }
         }
     }
@@ -347,7 +407,7 @@ impl Constructor {
     /// matrix, unless all of them are.
     pub(super) fn split<'a>(
         &self,
-        pcx: PatCtxt<'_>,
+        pcx: PatCtxt<'_, '_>,
         ctors: impl Iterator<Item = &'a Constructor> + Clone,
     ) -> SmallVec<[Self; 1]> {
         match self {
@@ -375,13 +435,13 @@ impl Constructor {
     /// 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 {
+    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,
+            (Missing { .. } | Wildcard, _) => false,
 
             (Single, Single) => true,
             (Variant(self_id), Variant(other_id)) => self_id == other_id,
@@ -411,7 +471,7 @@ impl Constructor {
     /// 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 {
+    fn is_covered_by_any(&self, _pcx: PatCtxt<'_, '_>, used_ctors: &[Constructor]) -> bool {
         if used_ctors.is_empty() {
             return false;
         }
@@ -431,7 +491,7 @@ impl Constructor {
                 .any(|other| slice.is_covered_by(other)),
             // This constructor is never covered by anything else
             NonExhaustive => false,
-            Str(..) | FloatRange(..) | Opaque | Missing | Wildcard => {
+            Str(..) | FloatRange(..) | Opaque | Missing { .. } | Wildcard | Or => {
                 never!("found unexpected ctor in all_ctors: {:?}", self);
                 true
             }
@@ -463,7 +523,7 @@ pub(super) struct SplitWildcard {
 }
 
 impl SplitWildcard {
-    pub(super) fn new(pcx: PatCtxt<'_>) -> Self {
+    pub(super) fn new(pcx: PatCtxt<'_, '_>) -> Self {
         let cx = pcx.cx;
         let make_range = |start, end, scalar| IntRange(IntRange::from_range(start, end, scalar));
 
@@ -483,7 +543,7 @@ impl SplitWildcard {
             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) => {
+            &TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ..) => {
                 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
@@ -502,27 +562,35 @@ impl SplitWildcard {
                 //
                 // 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);
+                let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(pcx.ty);
+
+                let is_exhaustive_pat_feature = cx.feature_exhaustive_patterns();
 
                 // 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()
+                    && !is_exhaustive_pat_feature
                     && !pcx.is_top_level;
 
+                let mut ctors: SmallVec<[_; 1]> = enum_data
+                    .variants
+                    .iter()
+                    .filter(|&(_, _v)| {
+                        // If `exhaustive_patterns` is enabled, we exclude variants known to be
+                        // uninhabited.
+                        let is_uninhabited = is_exhaustive_pat_feature
+                            && unimplemented!("after MatchCheckCtx.feature_exhaustive_patterns()");
+                        !is_uninhabited
+                    })
+                    .map(|(local_id, _)| Variant(EnumVariantId { parent: enum_id, local_id }))
+                    .collect();
+
                 if is_secretly_empty || is_declared_nonexhaustive {
-                    smallvec![NonExhaustive]
-                } else if cx.feature_exhaustive_patterns() {
-                    unimplemented!() // see MatchCheckCtx.feature_exhaustive_patterns()
-                } else {
-                    enum_data
-                        .variants
-                        .iter()
-                        .map(|(local_id, ..)| Variant(EnumVariantId { parent: enum_id, local_id }))
-                        .collect()
+                    ctors.push(NonExhaustive);
                 }
+                ctors
             }
             TyKind::Scalar(Scalar::Char) => unhandled(),
             TyKind::Scalar(Scalar::Int(..) | Scalar::Uint(..)) => unhandled(),
@@ -535,6 +603,7 @@ impl SplitWildcard {
             // This type is one for which we cannot list constructors, like `str` or `f64`.
             _ => smallvec![NonExhaustive],
         };
+
         SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
     }
 
@@ -542,7 +611,7 @@ impl SplitWildcard {
     /// do what you want.
     pub(super) fn split<'a>(
         &mut self,
-        pcx: PatCtxt<'_>,
+        pcx: PatCtxt<'_, '_>,
         ctors: impl Iterator<Item = &'a Constructor> + Clone,
     ) {
         // Since `all_ctors` never contains wildcards, this won't recurse further.
@@ -552,21 +621,21 @@ impl SplitWildcard {
     }
 
     /// Whether there are any value constructors for this type that are not present in the matrix.
-    fn any_missing(&self, pcx: PatCtxt<'_>) -> bool {
+    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>(
+    pub(super) fn iter_missing<'a, 'p>(
         &'a self,
-        pcx: PatCtxt<'a>,
-    ) -> impl Iterator<Item = &'a Constructor> {
+        pcx: PatCtxt<'a, 'p>,
+    ) -> impl Iterator<Item = &'a Constructor> + Captures<'p> {
         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]> {
+    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,
@@ -597,7 +666,15 @@ impl SplitWildcard {
             // 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
+                if pcx.is_non_exhaustive {
+                    Missing {
+                        nonexhaustive_enum_missing_real_variants: self
+                            .iter_missing(pcx)
+                            .any(|c| !(c.is_non_exhaustive() || c.is_unstable_variant(pcx))),
+                    }
+                } else {
+                    Missing { nonexhaustive_enum_missing_real_variants: false }
+                }
             } else {
                 Wildcard
             };
@@ -611,291 +688,334 @@ impl SplitWildcard {
 
 /// 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]>),
+/// This is constructed for a constructor using [`Fields::wildcards()`]. The idea is that
+/// [`Fields::wildcards()`] constructs a list of fields where all entries are wildcards, and then
+/// given a pattern we fill some of the fields with its subpatterns.
+/// In the following example `Fields::wildcards` returns `[_, _, _, _]`. Then in
+/// `extract_pattern_arguments` we fill some of the entries, and the result is
+/// `[Some(0), _, _, _]`.
+/// ```rust
+/// let x: [Option<u8>; 4] = foo();
+/// match x {
+///     [Some(0), ..] => {}
+/// }
+/// ```
+///
+/// Note that the number of fields of a constructor may not match the fields declared in the
+/// original struct/variant. This happens if a private or `non_exhaustive` field is uninhabited,
+/// because the code mustn't observe that it is uninhabited. In that case that field is not
+/// included in `fields`. For that reason, when you have a `mir::Field` you must use
+/// `index_with_declared_idx`.
+#[derive(Clone, Copy)]
+pub(super) struct Fields<'p> {
+    fields: &'p [DeconstructedPat<'p>],
 }
 
-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])
+impl<'p> Fields<'p> {
+    fn empty() -> Self {
+        Fields { fields: &[] }
     }
 
-    /// Convenience; internal use.
-    fn wildcards_from_tys(cx: &MatchCheckCtx<'_>, tys: impl IntoIterator<Item = Ty>) -> Self {
-        let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
-        let pats = wilds.map(|pat| cx.alloc_pat(pat)).collect();
-        Fields::Vec(pats)
+    fn singleton(cx: &MatchCheckCtx<'_, 'p>, field: DeconstructedPat<'p>) -> Self {
+        let field = cx.pattern_arena.alloc(field);
+        Fields { fields: std::slice::from_ref(field) }
     }
 
-    /// Creates a new list of wildcard fields for a given constructor.
-    pub(crate) fn wildcards(pcx: PatCtxt<'_>, constructor: &Constructor) -> Self {
-        let ty = pcx.ty;
-        let cx = pcx.cx;
-        let wildcard_from_ty = |ty: &Ty| cx.alloc_pat(Pat::wildcard_from_ty(ty.clone()));
+    pub(super) fn from_iter(
+        cx: &MatchCheckCtx<'_, 'p>,
+        fields: impl IntoIterator<Item = DeconstructedPat<'p>>,
+    ) -> Self {
+        let fields: &[_] = cx.pattern_arena.alloc_extend(fields);
+        Fields { fields }
+    }
+
+    fn wildcards_from_tys(cx: &MatchCheckCtx<'_, 'p>, tys: impl IntoIterator<Item = Ty>) -> Self {
+        Fields::from_iter(cx, tys.into_iter().map(DeconstructedPat::wildcard))
+    }
+
+    // In the cases of either a `#[non_exhaustive]` field list or a non-public field, we hide
+    // uninhabited fields in order not to reveal the uninhabitedness of the whole variant.
+    // This lists the fields we keep along with their types.
+    fn list_variant_nonhidden_fields<'a>(
+        cx: &'a MatchCheckCtx<'a, 'p>,
+        ty: &'a Ty,
+        variant: VariantId,
+    ) -> impl Iterator<Item = (LocalFieldId, Ty)> + Captures<'a> + Captures<'p> {
+        let (adt, substs) = ty.as_adt().unwrap();
+
+        let adt_is_local = variant.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, cx) && !adt_is_local;
+
+        let visibility = cx.db.field_visibilities(variant);
+        let field_ty = cx.db.field_types(variant);
+        let fields_len = variant.variant_data(cx.db.upcast()).fields().len() as u32;
+
+        (0..fields_len).map(|idx| LocalFieldId::from_raw(idx.into())).filter_map(move |fid| {
+            let ty = field_ty[fid].clone().substitute(Interner, substs);
+            let ty = normalize(cx.db, cx.body, ty);
+            let is_visible = matches!(adt, hir_def::AdtId::EnumId(..))
+                || visibility[fid].is_visible_from(cx.db.upcast(), cx.module);
+            let is_uninhabited = cx.is_uninhabited(&ty);
+
+            if is_uninhabited && (!is_visible || is_non_exhaustive) {
+                None
+            } else {
+                Some((fid, ty))
+            }
+        })
+    }
 
+    /// Creates a new list of wildcard fields for a given constructor. The result must have a
+    /// length of `constructor.arity()`.
+    pub(crate) fn wildcards(
+        cx: &MatchCheckCtx<'_, 'p>,
+        ty: &Ty,
+        constructor: &Constructor,
+    ) -> Self {
         let ret = match constructor {
             Single | Variant(_) => match ty.kind(Interner) {
                 TyKind::Tuple(_, substs) => {
                     let tys = substs.iter(Interner).map(|ty| ty.assert_ty_ref(Interner));
                     Fields::wildcards_from_tys(cx, tys.cloned())
                 }
-                TyKind::Ref(.., rty) => Fields::from_single_pattern(wildcard_from_ty(rty)),
+                TyKind::Ref(.., rty) => Fields::wildcards_from_tys(cx, once(rty.clone())),
                 &TyKind::Adt(AdtId(adt), ref substs) => {
                     if adt_is_box(adt, cx) {
-                        // Use T as the sub pattern type of Box<T>.
-                        let subst_ty = substs.at(Interner, 0).assert_ty_ref(Interner);
-                        Fields::from_single_pattern(wildcard_from_ty(subst_ty))
+                        // The only legal patterns of type `Box` (outside `std`) are `_` and box
+                        // patterns. If we're here we can assume this is a box pattern.
+                        let subst_ty = substs.at(Interner, 0).assert_ty_ref(Interner).clone();
+                        Fields::wildcards_from_tys(cx, once(subst_ty))
                     } else {
-                        let variant_id = constructor.variant_id_for_adt(adt);
-                        let adt_is_local =
-                            variant_id.module(cx.db.upcast()).krate() == cx.module.krate();
-                        // Whether we must not match the fields of this variant exhaustively.
-                        let is_non_exhaustive =
-                            is_field_list_non_exhaustive(variant_id, cx) && !adt_is_local;
-
-                        cov_mark::hit!(match_check_wildcard_expanded_to_substitutions);
-                        let field_ty_data = cx.db.field_types(variant_id);
-                        let field_tys = || {
-                            field_ty_data
-                                .iter()
-                                .map(|(_, binders)| binders.clone().substitute(Interner, substs))
-                        };
-
-                        // In the following cases, we don't need to filter out any fields. This is
-                        // the vast majority of real cases, since uninhabited fields are uncommon.
-                        let has_no_hidden_fields = (matches!(adt, hir_def::AdtId::EnumId(_))
-                            && !is_non_exhaustive)
-                            || !field_tys().any(|ty| cx.is_uninhabited(&ty));
-
-                        if has_no_hidden_fields {
-                            Fields::wildcards_from_tys(cx, field_tys())
-                        } else {
-                            //FIXME(iDawer): see MatchCheckCtx::is_uninhabited, has_no_hidden_fields is always true
-                            unimplemented!("exhaustive_patterns feature")
-                        }
+                        let variant = constructor.variant_id_for_adt(adt);
+                        let tys = Fields::list_variant_nonhidden_fields(cx, ty, variant)
+                            .map(|(_, ty)| ty);
+                        Fields::wildcards_from_tys(cx, tys)
                     }
                 }
                 ty_kind => {
                     never!("Unexpected type for `Single` constructor: {:?}", ty_kind);
-                    Fields::from_single_pattern(wildcard_from_ty(ty))
+                    Fields::wildcards_from_tys(cx, once(ty.clone()))
                 }
             },
             Slice(..) => {
                 unimplemented!()
             }
-            Str(..) | FloatRange(..) | IntRange(..) | NonExhaustive | Opaque | Missing
-            | Wildcard => Fields::Vec(Default::default()),
+            Str(..)
+            | FloatRange(..)
+            | IntRange(..)
+            | NonExhaustive
+            | Opaque
+            | Missing { .. }
+            | Wildcard => Fields::empty(),
+            Or => {
+                never!("called `Fields::wildcards` on an `Or` ctor");
+                Fields::empty()
+            }
         };
         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((hir_def::AdtId::EnumId(_), substs)) = pcx.ty.as_adt() {
-                        let enum_variant = match ctor {
-                            &Variant(id) => id,
-                            _ => unreachable!(),
-                        };
-                        PatKind::Variant { substs: substs.clone(), enum_variant, 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(..) => {
-                    never!("bad slice pattern {:?} {:?}", ctor, pcx.ty);
-                    PatKind::Wild
-                }
-                _ => PatKind::Wild,
-            },
-            Constructor::Slice(_) => UNHANDLED,
-            Str(_) => UNHANDLED,
-            FloatRange(..) => UNHANDLED,
-            Constructor::IntRange(_) => UNHANDLED,
-            NonExhaustive => PatKind::Wild,
-            Wildcard => return Pat::wildcard_from_ty(pcx.ty.clone()),
-            Opaque => {
-                never!("we should not try to apply an opaque constructor");
-                PatKind::Wild
-            }
-            Missing => {
-                never!(
-                    "trying to apply the `Missing` constructor; \
-                    this should have been done in `apply_constructors`",
-                );
-                PatKind::Wild
-            }
-        };
-
-        Pat { ty: pcx.ty.clone(), kind: Box::new(pat) }
+    /// Returns the list of patterns.
+    pub(super) fn iter_patterns<'a>(
+        &'a self,
+    ) -> impl Iterator<Item = &'p DeconstructedPat<'p>> + Captures<'a> {
+        self.fields.iter()
     }
+}
 
-    /// 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(),
-        }
-    }
+/// Values and patterns can be represented as a constructor applied to some fields. This represents
+/// a pattern in this form.
+/// This also keeps track of whether the pattern has been found reachable during analysis. For this
+/// reason we should be careful not to clone patterns for which we care about that. Use
+/// `clone_and_forget_reachability` if you're sure.
+pub(crate) struct DeconstructedPat<'p> {
+    ctor: Constructor,
+    fields: Fields<'p>,
+    ty: Ty,
+    reachable: Cell<bool>,
+}
 
-    /// 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(),
-        }
+impl<'p> DeconstructedPat<'p> {
+    pub(super) fn wildcard(ty: Ty) -> Self {
+        Self::new(Wildcard, Fields::empty(), ty)
     }
 
-    pub(super) fn into_patterns(self) -> SmallVec<[PatId; 2]> {
-        match self {
-            Fields::Vec(pats) => pats,
-        }
+    pub(super) fn new(ctor: Constructor, fields: Fields<'p>, ty: Ty) -> Self {
+        DeconstructedPat { ctor, fields, ty, reachable: Cell::new(false) }
     }
 
-    /// 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)),
-        )
+    /// Construct a pattern that matches everything that starts with this constructor.
+    /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
+    /// `Some(_)`.
+    pub(super) fn wild_from_ctor(pcx: PatCtxt<'_, 'p>, ctor: Constructor) -> Self {
+        let fields = Fields::wildcards(pcx.cx, pcx.ty, &ctor);
+        DeconstructedPat::new(ctor, fields, pcx.ty.clone())
     }
 
-    /// 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();
+    /// Clone this value. This method emphasizes that cloning loses reachability information and
+    /// should be done carefully.
+    pub(super) fn clone_and_forget_reachability(&self) -> Self {
+        DeconstructedPat::new(self.ctor.clone(), self.fields, self.ty.clone())
+    }
 
-        match &mut fields {
-            Fields::Vec(pats) => {
-                for (i, pat) in new_pats {
-                    if let Some(p) = pats.get_mut(i) {
-                        *p = pat;
+    pub(crate) fn from_pat(cx: &MatchCheckCtx<'_, 'p>, pat: &Pat) -> Self {
+        let mkpat = |pat| DeconstructedPat::from_pat(cx, pat);
+        let ctor;
+        let fields;
+        match pat.kind.as_ref() {
+            PatKind::Binding { subpattern: Some(subpat) } => return mkpat(subpat),
+            PatKind::Binding { subpattern: None } | PatKind::Wild => {
+                ctor = Wildcard;
+                fields = Fields::empty();
+            }
+            PatKind::Deref { subpattern } => {
+                ctor = Single;
+                fields = Fields::singleton(cx, mkpat(subpattern));
+            }
+            PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
+                match pat.ty.kind(Interner) {
+                    TyKind::Tuple(_, substs) => {
+                        ctor = Single;
+                        let mut wilds: SmallVec<[_; 2]> = substs
+                            .iter(Interner)
+                            .map(|arg| arg.assert_ty_ref(Interner).clone())
+                            .map(DeconstructedPat::wildcard)
+                            .collect();
+                        for pat in subpatterns {
+                            let idx: u32 = pat.field.into_raw().into();
+                            wilds[idx as usize] = mkpat(&pat.pattern);
+                        }
+                        fields = Fields::from_iter(cx, wilds)
+                    }
+                    TyKind::Adt(adt, substs) if adt_is_box(adt.0, cx) => {
+                        // The only legal patterns of type `Box` (outside `std`) are `_` and box
+                        // patterns. If we're here we can assume this is a box pattern.
+                        // FIXME(Nadrieril): A `Box` can in theory be matched either with `Box(_,
+                        // _)` or a box pattern. As a hack to avoid an ICE with the former, we
+                        // ignore other fields than the first one. This will trigger an error later
+                        // anyway.
+                        // See https://github.com/rust-lang/rust/issues/82772 ,
+                        // explanation: https://github.com/rust-lang/rust/pull/82789#issuecomment-796921977
+                        // The problem is that we can't know from the type whether we'll match
+                        // normally or through box-patterns. We'll have to figure out a proper
+                        // solution when we introduce generalized deref patterns. Also need to
+                        // prevent mixing of those two options.
+                        let pat =
+                            subpatterns.iter().find(|pat| pat.field.into_raw() == 0u32.into());
+                        let field = if let Some(pat) = pat {
+                            mkpat(&pat.pattern)
+                        } else {
+                            let ty = substs.at(Interner, 0).assert_ty_ref(Interner).clone();
+                            DeconstructedPat::wildcard(ty)
+                        };
+                        ctor = Single;
+                        fields = Fields::singleton(cx, field)
+                    }
+                    &TyKind::Adt(adt, _) => {
+                        ctor = match pat.kind.as_ref() {
+                            PatKind::Leaf { .. } => Single,
+                            PatKind::Variant { enum_variant, .. } => Variant(*enum_variant),
+                            _ => {
+                                never!();
+                                Wildcard
+                            }
+                        };
+                        let variant = ctor.variant_id_for_adt(adt.0);
+                        let fields_len = variant.variant_data(cx.db.upcast()).fields().len();
+                        // For each field in the variant, we store the relevant index into `self.fields` if any.
+                        let mut field_id_to_id: Vec<Option<usize>> = vec![None; fields_len];
+                        let tys = Fields::list_variant_nonhidden_fields(cx, &pat.ty, variant)
+                            .enumerate()
+                            .map(|(i, (fid, ty))| {
+                                let field_idx: u32 = fid.into_raw().into();
+                                field_id_to_id[field_idx as usize] = Some(i);
+                                ty
+                            });
+                        let mut wilds: SmallVec<[_; 2]> =
+                            tys.map(DeconstructedPat::wildcard).collect();
+                        for pat in subpatterns {
+                            let field_idx: u32 = pat.field.into_raw().into();
+                            if let Some(i) = field_id_to_id[field_idx as usize] {
+                                wilds[i] = mkpat(&pat.pattern);
+                            }
+                        }
+                        fields = Fields::from_iter(cx, wilds);
+                    }
+                    _ => {
+                        never!("pattern has unexpected type: pat: {:?}, ty: {:?}", pat, &pat.ty);
+                        ctor = Wildcard;
+                        fields = Fields::empty();
                     }
                 }
             }
+            &PatKind::LiteralBool { value } => {
+                ctor = IntRange(IntRange::from_bool(value));
+                fields = Fields::empty();
+            }
+            PatKind::Or { .. } => {
+                ctor = Or;
+                let pats: SmallVec<[_; 2]> = expand_or_pat(pat).into_iter().map(mkpat).collect();
+                fields = Fields::from_iter(cx, pats)
+            }
         }
-        fields
+        DeconstructedPat::new(ctor, fields, pat.ty.clone())
     }
 
-    /// 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();
+    // // FIXME(iDawer): implement reporting of noncovered patterns
+    // pub(crate) fn to_pat(&self, _cx: &MatchCheckCtx<'_, 'p>) -> Pat {
+    //     Pat { ty: self.ty.clone(), kind: PatKind::Wild.into() }
+    // }
 
-        match self {
-            Fields::Vec(_) => Fields::Vec(pats),
-        }
+    pub(super) fn is_or_pat(&self) -> bool {
+        matches!(self.ctor, Or)
     }
 
-    /// Replaces contained fields with the arguments of the given pattern. Only use on a pattern
-    /// that is compatible with the constructor used to build `self`.
-    /// This is meant to be used on the result of `Fields::wildcards()`. The idea is that
-    /// `wildcards` constructs a list of fields where all entries are wildcards, and the pattern
-    /// provided to this function fills some of the fields with non-wildcards.
-    /// In the following example `Fields::wildcards` would return `[_, _, _, _]`. If we call
-    /// `replace_with_pattern_arguments` on it with the pattern, the result will be `[Some(0), _,
-    /// _, _]`.
-    /// ```rust
-    /// let x: [Option<u8>; 4] = foo();
-    /// match x {
-    ///     [Some(0), ..] => {}
-    /// }
-    /// ```
-    /// This is guaranteed to preserve the number of patterns in `self`.
-    pub(super) fn replace_with_pattern_arguments(
-        &self,
-        pat: PatId,
-        cx: &MatchCheckCtx<'_>,
-    ) -> Self {
-        // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
-        // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
-        let mut arena = cx.pattern_arena.borrow_mut();
-        match arena[pat].kind.as_ref() {
-            PatKind::Deref { subpattern } => {
-                assert_eq!(self.len(), 1);
-                let subpattern = subpattern.clone();
-                Fields::from_single_pattern(arena.alloc(subpattern))
+    pub(super) fn ctor(&self) -> &Constructor {
+        &self.ctor
+    }
+
+    pub(super) fn ty(&self) -> &Ty {
+        &self.ty
+    }
+
+    pub(super) fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a DeconstructedPat<'a>> + 'a {
+        self.fields.iter_patterns()
+    }
+
+    /// Specialize this pattern with a constructor.
+    /// `other_ctor` can be different from `self.ctor`, but must be covered by it.
+    pub(super) fn specialize<'a>(
+        &'a self,
+        cx: &MatchCheckCtx<'_, 'p>,
+        other_ctor: &Constructor,
+    ) -> SmallVec<[&'p DeconstructedPat<'p>; 2]> {
+        match (&self.ctor, other_ctor) {
+            (Wildcard, _) => {
+                // We return a wildcard for each field of `other_ctor`.
+                Fields::wildcards(cx, &self.ty, other_ctor).iter_patterns().collect()
             }
-            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)
+            (Slice(self_slice), Slice(other_slice))
+                if self_slice.arity() != other_slice.arity() =>
+            {
+                unimplemented!()
             }
-
-            PatKind::Wild
-            | PatKind::Binding { .. }
-            | PatKind::LiteralBool { .. }
-            | PatKind::Or { .. } => self.clone(),
+            _ => self.fields.iter_patterns().collect(),
         }
     }
+
+    /// We keep track for each pattern if it was ever reachable during the analysis. This is used
+    /// with `unreachable_spans` to report unreachable subpatterns arising from or patterns.
+    pub(super) fn set_reachable(&self) {
+        self.reachable.set(true)
+    }
+    pub(super) fn is_reachable(&self) -> bool {
+        self.reachable.get()
+    }
 }
 
-fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_>) -> bool {
+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(),
@@ -904,7 +1024,7 @@ fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_>) -
     cx.db.attrs(attr_def_id).by_key("non_exhaustive").exists()
 }
 
-fn adt_is_box(adt: hir_def::AdtId, cx: &MatchCheckCtx<'_>) -> bool {
+fn adt_is_box(adt: hir_def::AdtId, cx: &MatchCheckCtx<'_, '_>) -> bool {
     use hir_def::lang_item::LangItemTarget;
     match cx.db.lang_item(cx.module.krate(), SmolStr::new_inline("owned_box")) {
         Some(LangItemTarget::StructId(box_id)) => adt == box_id.into(),
diff --git a/crates/hir_ty/src/diagnostics/match_check/usefulness.rs b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
index 43545708e5f..e8a13955d21 100644
--- a/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
+++ b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
@@ -1,5 +1,5 @@
-//! Based on rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22)
-//! <https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs>
+//! Based on rust-lang/rust (last sync f31622a50 2021-11-12)
+//! <https://github.com/rust-lang/rust/blob/f31622a50/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs>
 //!
 //! -----
 //!
@@ -271,33 +271,27 @@
 //! The details are not necessary to understand this file, so we explain them in
 //! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function.
 
-use std::{cell::RefCell, iter::FromIterator};
+use std::iter::once;
 
-use hir_def::{expr::ExprId, HasModule, ModuleId};
-use la_arena::Arena;
-use once_cell::unsync::OnceCell;
-use rustc_hash::FxHashMap;
+use hir_def::{AdtId, DefWithBodyId, HasModule, ModuleId};
 use smallvec::{smallvec, SmallVec};
+use typed_arena::Arena;
 
-use crate::{db::HirDatabase, InferenceResult, Interner, Ty};
+use crate::{db::HirDatabase, Ty, TyExt};
 
-use super::{
-    deconstruct_pat::{Constructor, Fields, SplitWildcard},
-    Pat, PatId, PatKind, PatternFoldable, PatternFolder,
-};
+use super::deconstruct_pat::{Constructor, DeconstructedPat, Fields, SplitWildcard};
 
-use self::{helper::PatIdExt, Usefulness::*, WitnessPreference::*};
+use self::{helper::Captures, ArmType::*, Usefulness::*};
 
-pub(crate) struct MatchCheckCtx<'a> {
+pub(crate) struct MatchCheckCtx<'a, 'p> {
     pub(crate) module: ModuleId,
-    pub(crate) match_expr: ExprId,
-    pub(crate) infer: &'a InferenceResult,
+    pub(crate) body: DefWithBodyId,
     pub(crate) db: &'a dyn HirDatabase,
     /// Lowered patterns from arms plus generated by the check.
-    pub(crate) pattern_arena: &'a RefCell<PatternArena>,
+    pub(crate) pattern_arena: &'p Arena<DeconstructedPat<'p>>,
 }
 
-impl<'a> MatchCheckCtx<'a> {
+impl<'a, 'p> MatchCheckCtx<'a, 'p> {
     pub(super) fn is_uninhabited(&self, _ty: &Ty) -> bool {
         // FIXME(iDawer) implement exhaustive_patterns feature. More info in:
         // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085
@@ -305,12 +299,16 @@ impl<'a> MatchCheckCtx<'a> {
     }
 
     /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
-    pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool {
-        let has_non_exhaustive_attr =
-            self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists();
-        let is_local =
-            hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
-        has_non_exhaustive_attr && !is_local
+    pub(super) fn is_foreign_non_exhaustive_enum(&self, ty: &Ty) -> bool {
+        match ty.as_adt() {
+            Some((adt @ AdtId::EnumId(_), _)) => {
+                let has_non_exhaustive_attr =
+                    self.db.attrs(adt.into()).by_key("non_exhaustive").exists();
+                let is_local = adt.module(self.db.upcast()).krate() == self.module.krate();
+                has_non_exhaustive_attr && !is_local
+            }
+            _ => false,
+        }
     }
 
     // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
@@ -318,92 +316,34 @@ impl<'a> MatchCheckCtx<'a> {
         // FIXME see MatchCheckCtx::is_uninhabited
         false
     }
-
-    pub(super) fn alloc_pat(&self, pat: Pat) -> PatId {
-        self.pattern_arena.borrow_mut().alloc(pat)
-    }
-
-    /// Get type of a pattern. Handles expanded patterns.
-    pub(super) fn type_of(&self, pat: PatId) -> Ty {
-        self.pattern_arena.borrow()[pat].ty.clone()
-    }
 }
 
 #[derive(Copy, Clone)]
-pub(super) struct PatCtxt<'a> {
-    pub(super) cx: &'a MatchCheckCtx<'a>,
+pub(super) struct PatCtxt<'a, 'p> {
+    pub(super) cx: &'a MatchCheckCtx<'a, 'p>,
     /// Type of the current column under investigation.
     pub(super) ty: &'a Ty,
     /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
     /// subpattern.
     pub(super) is_top_level: bool,
-}
-
-pub(crate) fn expand_pattern(pat: Pat) -> Pat {
-    LiteralExpander.fold_pattern(&pat)
-}
-
-struct LiteralExpander;
-
-impl PatternFolder for LiteralExpander {
-    fn fold_pattern(&mut self, pat: &Pat) -> Pat {
-        match (pat.ty.kind(Interner), pat.kind.as_ref()) {
-            (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self),
-            _ => pat.super_fold_with(self),
-        }
-    }
-}
-
-impl Pat {
-    fn _is_wildcard(&self) -> bool {
-        matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild)
-    }
-}
-
-impl PatIdExt for PatId {
-    fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool {
-        matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. })
-    }
-
-    /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
-    fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self> {
-        fn expand(pat: PatId, vec: &mut Vec<PatId>, pat_arena: &mut PatternArena) {
-            if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() {
-                // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
-                // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
-                let pats = pats.clone();
-                for pat in pats {
-                    let pat = pat_arena.alloc(pat.clone());
-                    expand(pat, vec, pat_arena);
-                }
-            } else {
-                vec.push(pat)
-            }
-        }
-
-        let mut pat_arena = cx.pattern_arena.borrow_mut();
-        let mut pats = Vec::new();
-        expand(self, &mut pats, &mut pat_arena);
-        pats
-    }
+    /// Wether the current pattern is from a `non_exhaustive` enum.
+    pub(super) is_non_exhaustive: bool,
 }
 
 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
 /// works well.
 #[derive(Clone)]
-pub(super) struct PatStack {
-    pats: SmallVec<[PatId; 2]>,
-    /// Cache for the constructor of the head
-    head_ctor: OnceCell<Constructor>,
+pub(super) struct PatStack<'p> {
+    pats: SmallVec<[&'p DeconstructedPat<'p>; 2]>,
 }
 
-impl PatStack {
-    fn from_pattern(pat: PatId) -> Self {
+impl<'p> PatStack<'p> {
+    fn from_pattern(pat: &'p DeconstructedPat<'p>) -> Self {
         Self::from_vec(smallvec![pat])
     }
 
-    fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self {
-        PatStack { pats: vec, head_ctor: OnceCell::new() }
+    fn from_vec(vec: SmallVec<[&'p DeconstructedPat<'p>; 2]>) -> Self {
+        PatStack { pats: vec }
     }
 
     fn is_empty(&self) -> bool {
@@ -414,73 +354,42 @@ impl PatStack {
         self.pats.len()
     }
 
-    fn head(&self) -> PatId {
+    fn head(&self) -> &'p DeconstructedPat<'p> {
         self.pats[0]
     }
 
-    #[inline]
-    fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor {
-        self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head()))
-    }
-
     // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an
     // or-pattern. Panics if `self` is empty.
-    fn expand_or_pat(&self, cx: &MatchCheckCtx<'_>) -> impl Iterator<Item = PatStack> + '_ {
-        self.head().expand_or_pat(cx).into_iter().map(move |pat| {
+    fn expand_or_pat(&self) -> impl Iterator<Item = PatStack<'p>> + Captures<'_> {
+        self.head().iter_fields().map(move |pat| {
             let mut new_patstack = PatStack::from_pattern(pat);
             new_patstack.pats.extend_from_slice(&self.pats[1..]);
             new_patstack
         })
     }
 
-    /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations.
+    /// This computes `S(self.head().ctor(), self)`. See top of the file for explanations.
     ///
     /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
     /// fields filled with wild patterns.
     ///
     /// This is roughly the inverse of `Constructor::apply`.
-    fn pop_head_constructor(
-        &self,
-        ctor_wild_subpatterns: &Fields,
-        cx: &MatchCheckCtx<'_>,
-    ) -> PatStack {
+    fn pop_head_constructor(&self, cx: &MatchCheckCtx<'_, 'p>, ctor: &Constructor) -> PatStack<'p> {
         // We pop the head pattern and push the new fields extracted from the arguments of
         // `self.head()`.
-        let mut new_fields =
-            ctor_wild_subpatterns.replace_with_pattern_arguments(self.head(), cx).into_patterns();
+        let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(cx, ctor);
         new_fields.extend_from_slice(&self.pats[1..]);
         PatStack::from_vec(new_fields)
     }
 }
 
-impl Default for PatStack {
-    fn default() -> Self {
-        Self::from_vec(smallvec![])
-    }
-}
-
-impl PartialEq for PatStack {
-    fn eq(&self, other: &Self) -> bool {
-        self.pats == other.pats
-    }
-}
-
-impl FromIterator<PatId> for PatStack {
-    fn from_iter<T>(iter: T) -> Self
-    where
-        T: IntoIterator<Item = PatId>,
-    {
-        Self::from_vec(iter.into_iter().collect())
-    }
-}
-
 /// A 2D matrix.
 #[derive(Clone)]
-pub(super) struct Matrix {
-    patterns: Vec<PatStack>,
+pub(super) struct Matrix<'p> {
+    patterns: Vec<PatStack<'p>>,
 }
 
-impl Matrix {
+impl<'p> Matrix<'p> {
     fn empty() -> Self {
         Matrix { patterns: vec![] }
     }
@@ -492,295 +401,29 @@ impl Matrix {
 
     /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
     /// expands it.
-    fn push(&mut self, row: PatStack, cx: &MatchCheckCtx<'_>) {
-        if !row.is_empty() && row.head().is_or_pat(cx) {
-            for row in row.expand_or_pat(cx) {
-                self.patterns.push(row);
-            }
+    fn push(&mut self, row: PatStack<'p>) {
+        if !row.is_empty() && row.head().is_or_pat() {
+            self.patterns.extend(row.expand_or_pat());
         } else {
             self.patterns.push(row);
         }
     }
 
     /// Iterate over the first component of each row
-    fn heads(&self) -> impl Iterator<Item = PatId> + '_ {
+    fn heads(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p>> + Clone + Captures<'_> {
         self.patterns.iter().map(|r| r.head())
     }
 
-    /// Iterate over the first constructor of each row.
-    fn head_ctors<'a>(
-        &'a self,
-        cx: &'a MatchCheckCtx<'_>,
-    ) -> impl Iterator<Item = &'a Constructor> + Clone {
-        self.patterns.iter().map(move |r| r.head_ctor(cx))
-    }
-
     /// This computes `S(constructor, self)`. See top of the file for explanations.
-    fn specialize_constructor(
-        &self,
-        pcx: PatCtxt<'_>,
-        ctor: &Constructor,
-        ctor_wild_subpatterns: &Fields,
-    ) -> Matrix {
-        let rows = self
-            .patterns
-            .iter()
-            .filter(|r| ctor.is_covered_by(pcx, r.head_ctor(pcx.cx)))
-            .map(|r| r.pop_head_constructor(ctor_wild_subpatterns, pcx.cx));
-        Matrix::from_iter(rows, pcx.cx)
-    }
-
-    fn from_iter(rows: impl IntoIterator<Item = PatStack>, cx: &MatchCheckCtx<'_>) -> Matrix {
+    fn specialize_constructor(&self, pcx: PatCtxt<'_, 'p>, ctor: &Constructor) -> Matrix<'p> {
         let mut matrix = Matrix::empty();
-        for x in rows {
-            // Using `push` ensures we correctly expand or-patterns.
-            matrix.push(x, cx);
-        }
-        matrix
-    }
-}
-
-/// Given a pattern or a pattern-stack, this struct captures a set of its subpatterns. We use that
-/// to track reachable sub-patterns arising from or-patterns. In the absence of or-patterns this
-/// will always be either `Empty` (the whole pattern is unreachable) or `Full` (the whole pattern
-/// is reachable). When there are or-patterns, some subpatterns may be reachable while others
-/// aren't. In this case the whole pattern still counts as reachable, but we will lint the
-/// unreachable subpatterns.
-///
-/// This supports a limited set of operations, so not all possible sets of subpatterns can be
-/// represented. That's ok, we only want the ones that make sense for our usage.
-///
-/// What we're doing is illustrated by this:
-/// ```
-/// match (true, 0) {
-///     (true, 0) => {}
-///     (_, 1) => {}
-///     (true | false, 0 | 1) => {}
-/// }
-/// ```
-/// When we try the alternatives of the `true | false` or-pattern, the last `0` is reachable in the
-/// `false` alternative but not the `true`. So overall it is reachable. By contrast, the last `1`
-/// is not reachable in either alternative, so we want to signal this to the user.
-/// Therefore we take the union of sets of reachable patterns coming from different alternatives in
-/// order to figure out which subpatterns are overall reachable.
-///
-/// Invariant: we try to construct the smallest representation we can. In particular if
-/// `self.is_empty()` we ensure that `self` is `Empty`, and same with `Full`. This is not important
-/// for correctness currently.
-#[derive(Debug, Clone)]
-enum SubPatSet {
-    /// The empty set. This means the pattern is unreachable.
-    Empty,
-    /// The set containing the full pattern.
-    Full,
-    /// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each
-    /// of its subpatterns. Missing entries in the map are implicitly full, because that's the
-    /// common case.
-    Seq { subpats: FxHashMap<usize, SubPatSet> },
-    /// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing
-    /// entries in the map are implicitly empty. Note: we always flatten nested or-patterns.
-    Alt {
-        subpats: FxHashMap<usize, SubPatSet>,
-        /// Counts the total number of alternatives in the pattern
-        alt_count: usize,
-        /// We keep the pattern around to retrieve spans.
-        pat: PatId,
-    },
-}
-
-impl SubPatSet {
-    fn full() -> Self {
-        SubPatSet::Full
-    }
-
-    fn empty() -> Self {
-        SubPatSet::Empty
-    }
-
-    fn is_empty(&self) -> bool {
-        match self {
-            SubPatSet::Empty => true,
-            SubPatSet::Full => false,
-            // If any subpattern in a sequence is unreachable, the whole pattern is unreachable.
-            SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_empty()),
-            // An or-pattern is reachable if any of its alternatives is.
-            SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_empty()),
-        }
-    }
-
-    fn is_full(&self) -> bool {
-        match self {
-            SubPatSet::Empty => false,
-            SubPatSet::Full => true,
-            // The whole pattern is reachable only when all its alternatives are.
-            SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_full()),
-            // The whole or-pattern is reachable only when all its alternatives are.
-            SubPatSet::Alt { subpats, alt_count, .. } => {
-                subpats.len() == *alt_count && subpats.values().all(|set| set.is_full())
-            }
-        }
-    }
-
-    /// Union `self` with `other`, mutating `self`.
-    fn union(&mut self, other: Self) {
-        use SubPatSet::*;
-        // Union with full stays full; union with empty changes nothing.
-        if self.is_full() || other.is_empty() {
-            return;
-        } else if self.is_empty() {
-            *self = other;
-            return;
-        } else if other.is_full() {
-            *self = Full;
-            return;
-        }
-
-        match (&mut *self, other) {
-            (Seq { subpats: s_set }, Seq { subpats: mut o_set }) => {
-                s_set.retain(|i, s_sub_set| {
-                    // Missing entries count as full.
-                    let o_sub_set = o_set.remove(i).unwrap_or(Full);
-                    s_sub_set.union(o_sub_set);
-                    // We drop full entries.
-                    !s_sub_set.is_full()
-                });
-                // Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since
-                // unioning with full returns full, we can drop those entries.
-            }
-            (Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => {
-                s_set.retain(|i, s_sub_set| {
-                    // Missing entries count as empty.
-                    let o_sub_set = o_set.remove(i).unwrap_or(Empty);
-                    s_sub_set.union(o_sub_set);
-                    // We drop empty entries.
-                    !s_sub_set.is_empty()
-                });
-                // Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since
-                // unioning with empty changes nothing, we can take those entries as is.
-                s_set.extend(o_set);
-            }
-            _ => panic!("bug"),
-        }
-
-        if self.is_full() {
-            *self = Full;
-        }
-    }
-
-    /// Returns a list of the unreachable subpatterns. If `self` is empty (i.e. the
-    /// whole pattern is unreachable) we return `None`.
-    fn list_unreachable_subpatterns(&self, cx: &MatchCheckCtx<'_>) -> Option<Vec<PatId>> {
-        /// Panics if `set.is_empty()`.
-        fn fill_subpats(
-            set: &SubPatSet,
-            unreachable_pats: &mut Vec<PatId>,
-            cx: &MatchCheckCtx<'_>,
-        ) {
-            match set {
-                SubPatSet::Empty => panic!("bug"),
-                SubPatSet::Full => {}
-                SubPatSet::Seq { subpats } => {
-                    for sub_set in subpats.values() {
-                        fill_subpats(sub_set, unreachable_pats, cx);
-                    }
-                }
-                SubPatSet::Alt { subpats, pat, alt_count, .. } => {
-                    let expanded = pat.expand_or_pat(cx);
-                    for (i, &expanded) in expanded.iter().enumerate().take(*alt_count) {
-                        let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Empty);
-                        if sub_set.is_empty() {
-                            // Found an unreachable subpattern.
-                            unreachable_pats.push(expanded);
-                        } else {
-                            fill_subpats(sub_set, unreachable_pats, cx);
-                        }
-                    }
-                }
-            }
-        }
-
-        if self.is_empty() {
-            return None;
-        }
-        if self.is_full() {
-            // No subpatterns are unreachable.
-            return Some(Vec::new());
-        }
-        let mut unreachable_pats = Vec::new();
-        fill_subpats(self, &mut unreachable_pats, cx);
-        Some(unreachable_pats)
-    }
-
-    /// When `self` refers to a patstack that was obtained from specialization, after running
-    /// `unspecialize` it will refer to the original patstack before specialization.
-    fn unspecialize(self, arity: usize) -> Self {
-        use SubPatSet::*;
-        match self {
-            Full => Full,
-            Empty => Empty,
-            Seq { subpats } => {
-                // We gather the first `arity` subpatterns together and shift the remaining ones.
-                let mut new_subpats = FxHashMap::default();
-                let mut new_subpats_first_col = FxHashMap::default();
-                for (i, sub_set) in subpats {
-                    if i < arity {
-                        // The first `arity` indices are now part of the pattern in the first
-                        // column.
-                        new_subpats_first_col.insert(i, sub_set);
-                    } else {
-                        // Indices after `arity` are simply shifted
-                        new_subpats.insert(i - arity + 1, sub_set);
-                    }
-                }
-                // If `new_subpats_first_col` has no entries it counts as full, so we can omit it.
-                if !new_subpats_first_col.is_empty() {
-                    new_subpats.insert(0, Seq { subpats: new_subpats_first_col });
-                }
-                Seq { subpats: new_subpats }
+        for row in &self.patterns {
+            if ctor.is_covered_by(pcx, row.head().ctor()) {
+                let new_row = row.pop_head_constructor(pcx.cx, ctor);
+                matrix.push(new_row);
             }
-            Alt { .. } => panic!("bug"), // `self` is a patstack
-        }
-    }
-
-    /// When `self` refers to a patstack that was obtained from splitting an or-pattern, after
-    /// running `unspecialize` it will refer to the original patstack before splitting.
-    ///
-    /// For example:
-    /// ```
-    /// match Some(true) {
-    ///     Some(true) => {}
-    ///     None | Some(true | false) => {}
-    /// }
-    /// ```
-    /// Here `None` would return the full set and `Some(true | false)` would return the set
-    /// containing `false`. After `unsplit_or_pat`, we want the set to contain `None` and `false`.
-    /// This is what this function does.
-    fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
-        use SubPatSet::*;
-        if self.is_empty() {
-            return Empty;
         }
-
-        // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0
-        // | 1)`.
-        let set_first_col = match &mut self {
-            Full => Full,
-            Seq { subpats } => subpats.remove(&0).unwrap_or(Full),
-            Empty => unreachable!(),
-            Alt { .. } => panic!("bug"), // `self` is a patstack
-        };
-        let mut subpats_first_col = FxHashMap::default();
-        subpats_first_col.insert(alt_id, set_first_col);
-        let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count };
-
-        let mut subpats = match self {
-            Full => FxHashMap::default(),
-            Seq { subpats } => subpats,
-            Empty => unreachable!(),
-            Alt { .. } => panic!("bug"), // `self` is a patstack
-        };
-        subpats.insert(0, set_first_col);
-        Seq { subpats }
+        matrix
     }
 }
 
@@ -789,31 +432,34 @@ impl SubPatSet {
 /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
 /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
 /// witnesses of non-exhaustiveness when there are any.
-/// Which variant to use is dictated by `WitnessPreference`.
-#[derive(Clone, Debug)]
-enum Usefulness {
-    /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates
-    /// the whole pattern is unreachable. If not, this indicates that the pattern is reachable but
-    /// that some sub-patterns may be unreachable (due to or-patterns). In the absence of
-    /// or-patterns this will always be either `Empty` (the whole pattern is unreachable) or `Full`
-    /// (the whole pattern is reachable).
-    NoWitnesses(SubPatSet),
+/// Which variant to use is dictated by `ArmType`.
+enum Usefulness<'p> {
+    /// If we don't care about witnesses, simply remember if the pattern was useful.
+    NoWitnesses { useful: bool },
     /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole
     /// pattern is unreachable.
-    WithWitnesses(Vec<Witness>),
+    WithWitnesses(Vec<Witness<'p>>),
 }
 
-impl Usefulness {
-    fn new_useful(preference: WitnessPreference) -> Self {
+impl<'p> Usefulness<'p> {
+    fn new_useful(preference: ArmType) -> Self {
         match preference {
-            ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
-            LeaveOutWitness => NoWitnesses(SubPatSet::full()),
+            // A single (empty) witness of reachability.
+            FakeExtraWildcard => WithWitnesses(vec![Witness(vec![])]),
+            RealArm => NoWitnesses { useful: true },
         }
     }
-    fn new_not_useful(preference: WitnessPreference) -> Self {
+    fn new_not_useful(preference: ArmType) -> Self {
         match preference {
-            ConstructWitness => WithWitnesses(vec![]),
-            LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
+            FakeExtraWildcard => WithWitnesses(vec![]),
+            RealArm => NoWitnesses { useful: false },
+        }
+    }
+
+    fn is_useful(&self) -> bool {
+        match self {
+            Usefulness::NoWitnesses { useful } => *useful,
+            Usefulness::WithWitnesses(witnesses) => !witnesses.is_empty(),
         }
     }
 
@@ -823,89 +469,96 @@ impl Usefulness {
             (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {}
             (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o),
             (WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
-            (NoWitnesses(s), NoWitnesses(o)) => s.union(o),
-            _ => unreachable!(),
-        }
-    }
-
-    /// When trying several branches and each returns a `Usefulness`, we need to combine the
-    /// results together.
-    fn merge(pref: WitnessPreference, usefulnesses: impl Iterator<Item = Self>) -> Self {
-        let mut ret = Self::new_not_useful(pref);
-        for u in usefulnesses {
-            ret.extend(u);
-            if let NoWitnesses(subpats) = &ret {
-                if subpats.is_full() {
-                    // Once we reach the full set, more unions won't change the result.
-                    return ret;
-                }
+            (NoWitnesses { useful: s_useful }, NoWitnesses { useful: o_useful }) => {
+                *s_useful = *s_useful || o_useful
             }
-        }
-        ret
-    }
-
-    /// After calculating the usefulness for a branch of an or-pattern, call this to make this
-    /// usefulness mergeable with those from the other branches.
-    fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
-        match self {
-            NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)),
-            WithWitnesses(_) => panic!("bug"),
+            _ => unreachable!(),
         }
     }
 
-    /// After calculating usefulness after a specialization, call this to recontruct a usefulness
+    /// After calculating usefulness after a specialization, call this to reconstruct a usefulness
     /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
     /// with the results of specializing with the other constructors.
     fn apply_constructor(
         self,
-        pcx: PatCtxt<'_>,
-        matrix: &Matrix,
+        pcx: PatCtxt<'_, 'p>,
+        matrix: &Matrix<'p>,
         ctor: &Constructor,
-        ctor_wild_subpatterns: &Fields,
     ) -> Self {
         match self {
-            WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses),
+            NoWitnesses { .. } => self,
+            WithWitnesses(ref witnesses) if witnesses.is_empty() => self,
             WithWitnesses(witnesses) => {
-                let new_witnesses = if matches!(ctor, Constructor::Missing) {
-                    let mut split_wildcard = SplitWildcard::new(pcx);
-                    split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
-                    // Construct for each missing constructor a "wild" version of this
-                    // constructor, that matches everything that can be built with
-                    // it. For example, if `ctor` is a `Constructor::Variant` for
-                    // `Option::Some`, we get the pattern `Some(_)`.
-                    let new_patterns: Vec<_> = split_wildcard
-                        .iter_missing(pcx)
-                        .map(|missing_ctor| {
-                            Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
-                        })
-                        .collect();
+                let new_witnesses = if let Constructor::Missing { .. } = ctor {
+                    // We got the special `Missing` constructor, so each of the missing constructors
+                    // gives a new pattern that is not caught by the match. We list those patterns.
+                    let new_patterns = if pcx.is_non_exhaustive {
+                        // Here we don't want the user to try to list all variants, we want them to add
+                        // a wildcard, so we only suggest that.
+                        vec![DeconstructedPat::wildcard(pcx.ty.clone())]
+                    } else {
+                        let mut split_wildcard = SplitWildcard::new(pcx);
+                        split_wildcard.split(pcx, matrix.heads().map(DeconstructedPat::ctor));
+
+                        // This lets us know if we skipped any variants because they are marked
+                        // `doc(hidden)` or they are unstable feature gate (only stdlib types).
+                        let mut hide_variant_show_wild = false;
+                        // Construct for each missing constructor a "wild" version of this
+                        // constructor, that matches everything that can be built with
+                        // it. For example, if `ctor` is a `Constructor::Variant` for
+                        // `Option::Some`, we get the pattern `Some(_)`.
+                        let mut new: Vec<DeconstructedPat<'_>> = split_wildcard
+                            .iter_missing(pcx)
+                            .filter_map(|missing_ctor| {
+                                // Check if this variant is marked `doc(hidden)`
+                                if missing_ctor.is_doc_hidden_variant(pcx)
+                                    || missing_ctor.is_unstable_variant(pcx)
+                                {
+                                    hide_variant_show_wild = true;
+                                    return None;
+                                }
+                                Some(DeconstructedPat::wild_from_ctor(pcx, missing_ctor.clone()))
+                            })
+                            .collect();
+
+                        if hide_variant_show_wild {
+                            new.push(DeconstructedPat::wildcard(pcx.ty.clone()))
+                        }
+
+                        new
+                    };
+
                     witnesses
                         .into_iter()
                         .flat_map(|witness| {
                             new_patterns.iter().map(move |pat| {
-                                let mut witness = witness.clone();
-                                witness.0.push(pat.clone());
-                                witness
+                                Witness(
+                                    witness
+                                        .0
+                                        .iter()
+                                        .chain(once(pat))
+                                        .map(DeconstructedPat::clone_and_forget_reachability)
+                                        .collect(),
+                                )
                             })
                         })
                         .collect()
                 } else {
                     witnesses
                         .into_iter()
-                        .map(|witness| witness.apply_constructor(pcx, ctor, ctor_wild_subpatterns))
+                        .map(|witness| witness.apply_constructor(pcx, ctor))
                         .collect()
                 };
                 WithWitnesses(new_witnesses)
             }
-            NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
         }
     }
 }
 
 #[derive(Copy, Clone, Debug)]
-enum WitnessPreference {
-    ConstructWitness,
-    LeaveOutWitness,
+enum ArmType {
+    FakeExtraWildcard,
+    RealArm,
 }
 
 /// A witness of non-exhaustiveness for error reporting, represented
@@ -941,12 +594,11 @@ enum WitnessPreference {
 ///     `Witness(vec![Pair(Some(_), true)])`
 ///
 /// The final `Pair(Some(_), true)` is then the resulting witness.
-#[derive(Clone, Debug)]
-pub(crate) struct Witness(Vec<Pat>);
+pub(crate) struct Witness<'p>(Vec<DeconstructedPat<'p>>);
 
-impl Witness {
+impl<'p> Witness<'p> {
     /// Asserts that the witness contains a single pattern, and returns it.
-    fn single_pattern(self) -> Pat {
+    fn single_pattern(self) -> DeconstructedPat<'p> {
         assert_eq!(self.0.len(), 1);
         self.0.into_iter().next().unwrap()
     }
@@ -964,17 +616,13 @@ impl Witness {
     ///
     /// left_ty: struct X { a: (bool, &'static str), b: usize}
     /// pats: [(false, "foo"), 42]  => X { a: (false, "foo"), b: 42 }
-    fn apply_constructor(
-        mut self,
-        pcx: PatCtxt<'_>,
-        ctor: &Constructor,
-        ctor_wild_subpatterns: &Fields,
-    ) -> Self {
+    fn apply_constructor(mut self, pcx: PatCtxt<'_, 'p>, ctor: &Constructor) -> Self {
         let pat = {
             let len = self.0.len();
-            let arity = ctor_wild_subpatterns.len();
+            let arity = ctor.arity(pcx);
             let pats = self.0.drain((len - arity)..).rev();
-            ctor_wild_subpatterns.replace_fields(pcx.cx, pats).apply(pcx, ctor)
+            let fields = Fields::from_iter(pcx.cx, pats);
+            DeconstructedPat::new(ctor.clone(), fields, pcx.ty.clone())
         };
 
         self.0.push(pat);
@@ -1005,14 +653,14 @@ impl Witness {
 /// `is_under_guard` is used to inform if the pattern has a guard. If it
 /// has one it must not be inserted into the matrix. This shouldn't be
 /// relied on for soundness.
-fn is_useful(
-    cx: &MatchCheckCtx<'_>,
-    matrix: &Matrix,
-    v: &PatStack,
-    witness_preference: WitnessPreference,
+fn is_useful<'p>(
+    cx: &MatchCheckCtx<'_, 'p>,
+    matrix: &Matrix<'p>,
+    v: &PatStack<'p>,
+    witness_preference: ArmType,
     is_under_guard: bool,
     is_top_level: bool,
-) -> Usefulness {
+) -> Usefulness<'p> {
     let Matrix { patterns: rows, .. } = matrix;
 
     // The base case. We are pattern-matching on () and the return value is
@@ -1029,69 +677,62 @@ fn is_useful(
         return ret;
     }
 
-    assert!(rows.iter().all(|r| r.len() == v.len()));
+    debug_assert!(rows.iter().all(|r| r.len() == v.len()));
 
-    // FIXME(Nadrieril): Hack to work around type normalization issues (see rust-lang/rust#72476).
-    let ty = matrix.heads().next().map_or(cx.type_of(v.head()), |r| cx.type_of(r));
-    let pcx = PatCtxt { cx, ty: &ty, is_top_level };
+    let ty = v.head().ty();
+    let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
+    let pcx = PatCtxt { cx, ty, is_top_level, is_non_exhaustive };
 
     // If the first pattern is an or-pattern, expand it.
-    let ret = if v.head().is_or_pat(cx) {
-        //expanding or-pattern
-        let v_head = v.head();
-        let vs: Vec<_> = v.expand_or_pat(cx).collect();
-        let alt_count = vs.len();
+    let mut ret = Usefulness::new_not_useful(witness_preference);
+    if v.head().is_or_pat() {
         // We try each or-pattern branch in turn.
         let mut matrix = matrix.clone();
-        let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| {
+        for v in v.expand_or_pat() {
             let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false);
+            ret.extend(usefulness);
             // If pattern has a guard don't add it to the matrix.
             if !is_under_guard {
                 // We push the already-seen patterns into the matrix in order to detect redundant
                 // branches like `Some(_) | Some(0)`.
-                matrix.push(v, cx);
+                matrix.push(v);
             }
-            usefulness.unsplit_or_pat(i, alt_count, v_head)
-        });
-        Usefulness::merge(witness_preference, usefulnesses)
+        }
     } else {
-        let v_ctor = v.head_ctor(cx);
-        // if let Constructor::IntRange(ctor_range) = v_ctor {
-        //     // Lint on likely incorrect range patterns (#63987)
-        //     ctor_range.lint_overlapping_range_endpoints(
-        //         pcx,
-        //         matrix.head_ctors_and_spans(cx),
-        //         matrix.column_count().unwrap_or(0),
-        //         hir_id,
-        //     )
-        // }
+        let v_ctor = v.head().ctor();
+
+        // FIXME: implement `overlapping_range_endpoints` lint
 
         // We split the head constructor of `v`.
-        let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx));
+        let split_ctors = v_ctor.split(pcx, matrix.heads().map(DeconstructedPat::ctor));
         // For each constructor, we compute whether there's a value that starts with it that would
         // witness the usefulness of `v`.
         let start_matrix = matrix;
-        let usefulnesses = split_ctors.into_iter().map(|ctor| {
-            // debug!("specialize({:?})", ctor);
+        for ctor in split_ctors {
             // We cache the result of `Fields::wildcards` because it is used a lot.
-            let ctor_wild_subpatterns = Fields::wildcards(pcx, &ctor);
-            let spec_matrix =
-                start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns);
-            let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx);
+            let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor);
+            let v = v.pop_head_constructor(cx, &ctor);
             let usefulness =
                 is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
-            usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
-        });
-        Usefulness::merge(witness_preference, usefulnesses)
+            let usefulness = usefulness.apply_constructor(pcx, start_matrix, &ctor);
+
+            // FIXME: implement `non_exhaustive_omitted_patterns` lint
+
+            ret.extend(usefulness);
+        }
     };
 
+    if ret.is_useful() {
+        v.head().set_reachable();
+    }
+
     ret
 }
 
 /// The arm of a match expression.
 #[derive(Clone, Copy)]
-pub(crate) struct MatchArm {
-    pub(crate) pat: PatId,
+pub(crate) struct MatchArm<'p> {
+    pub(crate) pat: &'p DeconstructedPat<'p>,
     pub(crate) has_guard: bool,
 }
 
@@ -1101,18 +742,19 @@ pub(crate) enum Reachability {
     /// The arm is reachable. This additionally carries a set of or-pattern branches that have been
     /// found to be unreachable despite the overall arm being reachable. Used only in the presence
     /// of or-patterns, otherwise it stays empty.
-    Reachable(Vec<PatId>),
+    // FIXME: store ureachable subpattern IDs
+    Reachable,
     /// The arm is unreachable.
     Unreachable,
 }
 
 /// The output of checking a match for exhaustiveness and arm reachability.
-pub(crate) struct UsefulnessReport {
+pub(crate) struct UsefulnessReport<'p> {
     /// For each arm of the input, whether that arm is reachable after the arms above it.
-    pub(crate) _arm_usefulness: Vec<(MatchArm, Reachability)>,
+    pub(crate) _arm_usefulness: Vec<(MatchArm<'p>, Reachability)>,
     /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
     /// exhaustiveness.
-    pub(crate) non_exhaustiveness_witnesses: Vec<Pat>,
+    pub(crate) non_exhaustiveness_witnesses: Vec<DeconstructedPat<'p>>,
 }
 
 /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
@@ -1120,53 +762,41 @@ pub(crate) struct UsefulnessReport {
 ///
 /// Note: the input patterns must have been lowered through
 /// `check_match::MatchVisitor::lower_pattern`.
-pub(crate) fn compute_match_usefulness(
-    cx: &MatchCheckCtx<'_>,
-    arms: &[MatchArm],
-) -> UsefulnessReport {
+pub(crate) fn compute_match_usefulness<'p>(
+    cx: &MatchCheckCtx<'_, 'p>,
+    arms: &[MatchArm<'p>],
+    scrut_ty: &Ty,
+) -> UsefulnessReport<'p> {
     let mut matrix = Matrix::empty();
     let arm_usefulness = arms
         .iter()
         .copied()
         .map(|arm| {
             let v = PatStack::from_pattern(arm.pat);
-            let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true);
+            is_useful(cx, &matrix, &v, RealArm, arm.has_guard, true);
             if !arm.has_guard {
-                matrix.push(v, cx);
+                matrix.push(v);
             }
-            let reachability = match usefulness {
-                NoWitnesses(subpats) if subpats.is_empty() => Reachability::Unreachable,
-                NoWitnesses(subpats) => {
-                    Reachability::Reachable(subpats.list_unreachable_subpatterns(cx).unwrap())
-                }
-                WithWitnesses(..) => panic!("bug"),
+            let reachability = if arm.pat.is_reachable() {
+                Reachability::Reachable
+            } else {
+                Reachability::Unreachable
             };
             (arm, reachability)
         })
         .collect();
 
-    let wild_pattern =
-        cx.pattern_arena.borrow_mut().alloc(Pat::wildcard_from_ty(cx.infer[cx.match_expr].clone()));
+    let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty.clone()));
     let v = PatStack::from_pattern(wild_pattern);
-    let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, false, true);
+    let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, false, true);
     let non_exhaustiveness_witnesses = match usefulness {
         WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(),
-        NoWitnesses(_) => panic!("bug"),
+        NoWitnesses { .. } => panic!("bug"),
     };
     UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
 }
 
-pub(crate) type PatternArena = Arena<Pat>;
-
-mod helper {
-    use super::MatchCheckCtx;
-
-    pub(super) trait PatIdExt: Sized {
-        // fn is_wildcard(self, cx: &MatchCheckCtx<'_>) -> bool;
-        fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool;
-        fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self>;
-    }
-
+pub(crate) mod helper {
     // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs
     /// "Signaling" trait used in impl trait to tag lifetimes that you may
     /// need to capture but don't really need for other reasons.
diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
index 1bc19323da9..54c3590f04b 100644
--- a/crates/hir_ty/src/infer.rs
+++ b/crates/hir_ty/src/infer.rs
@@ -16,7 +16,7 @@
 use std::ops::Index;
 use std::sync::Arc;
 
-use chalk_ir::{cast::Cast, DebruijnIndex, Mutability, Safety, Scalar};
+use chalk_ir::{cast::Cast, DebruijnIndex, Mutability, Safety, Scalar, TypeFlags};
 use hir_def::{
     body::Body,
     data::{ConstData, FunctionData, StaticData},
@@ -70,6 +70,26 @@ pub(crate) fn infer_query(db: &dyn HirDatabase, def: DefWithBodyId) -> Arc<Infer
     Arc::new(ctx.resolve_all())
 }
 
+/// Fully normalize all the types found within `ty` in context of `owner` body definition.
+///
+/// This is appropriate to use only after type-check: it assumes
+/// that normalization will succeed, for example.
+pub(crate) fn normalize(db: &dyn HirDatabase, owner: DefWithBodyId, ty: Ty) -> Ty {
+    if !ty.data(Interner).flags.intersects(TypeFlags::HAS_PROJECTION) {
+        return ty;
+    }
+    let krate = owner.module(db.upcast()).krate();
+    let trait_env = owner
+        .as_generic_def_id()
+        .map_or_else(|| Arc::new(TraitEnvironment::empty(krate)), |d| db.trait_environment(d));
+    let mut table = unify::InferenceTable::new(db, trait_env.clone());
+
+    let ty_with_vars = table.normalize_associated_types_in(ty);
+    table.resolve_obligations_as_possible();
+    table.propagate_diverging_flag();
+    table.resolve_completely(ty_with_vars)
+}
+
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 enum ExprOrPatId {
     ExprId(ExprId),
@@ -80,7 +100,7 @@ impl_from!(ExprId, PatId for ExprOrPatId);
 /// Binding modes inferred for patterns.
 /// <https://doc.rust-lang.org/reference/patterns.html#binding-modes>
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
-enum BindingMode {
+pub enum BindingMode {
     Move,
     Ref(Mutability),
 }
@@ -272,6 +292,7 @@ pub struct InferenceResult {
     standard_types: InternedStandardTypes,
     /// Stores the types which were implicitly dereferenced in pattern binding modes.
     pub pat_adjustments: FxHashMap<PatId, Vec<Adjustment>>,
+    pub pat_binding_modes: FxHashMap<PatId, BindingMode>,
     pub expr_adjustments: FxHashMap<ExprId, Vec<Adjustment>>,
 }
 
diff --git a/crates/hir_ty/src/infer/pat.rs b/crates/hir_ty/src/infer/pat.rs
index 64e72abf0f3..50fd2dd7494 100644
--- a/crates/hir_ty/src/infer/pat.rs
+++ b/crates/hir_ty/src/infer/pat.rs
@@ -204,6 +204,8 @@ impl<'a> InferenceContext<'a> {
                 } else {
                     BindingMode::convert(*mode)
                 };
+                self.result.pat_binding_modes.insert(pat, mode);
+
                 let inner_ty = match subpat {
                     Some(subpat) => self.infer_pat(*subpat, &expected, default_bm),
                     None => expected,
diff --git a/crates/ide_diagnostics/src/handlers/missing_match_arms.rs b/crates/ide_diagnostics/src/handlers/missing_match_arms.rs
index 947b0f2e2e6..6e2764e59ff 100644
--- a/crates/ide_diagnostics/src/handlers/missing_match_arms.rs
+++ b/crates/ide_diagnostics/src/handlers/missing_match_arms.rs
@@ -821,7 +821,6 @@ fn main() {
 
     #[test]
     fn pattern_type_is_of_substitution() {
-        cov_mark::check!(match_check_wildcard_expanded_to_substitutions);
         check_diagnostics_no_bails(
             r#"
 struct Foo<T>(T);
@@ -864,6 +863,43 @@ fn main() {
         );
     }
 
+    #[test]
+    fn normalize_field_ty() {
+        check_diagnostics_no_bails(
+            r"
+trait Trait { type Projection; }
+enum E {Foo, Bar}
+struct A;
+impl Trait for A { type Projection = E; }
+struct Next<T: Trait>(T::Projection);
+static __: () = {
+    let n: Next<A> = Next(E::Foo);
+    match n { Next(E::Foo) => {} }
+    //    ^ error: missing match arm
+    match n { Next(E::Foo | E::Bar) => {} }
+    match n { Next(E::Foo | _     ) => {} }
+    match n { Next(_      | E::Bar) => {} }
+    match n {      _ | Next(E::Bar) => {} }
+    match &n { Next(E::Foo | E::Bar) => {} }
+    match &n {      _ | Next(E::Bar) => {} }
+};",
+        );
+    }
+
+    #[test]
+    fn binding_mode_by_ref() {
+        check_diagnostics_no_bails(
+            r"
+enum E{ A, B }
+fn foo() {
+    match &E::A {
+        E::A => {}
+        x => {}
+    }
+}",
+        );
+    }
+
     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