Auto merge of #118842 - Nadrieril:librarify-further, r=compiler-errors

Make exhaustiveness usable outside of rustc

With this PR, `rustc_pattern_analysis` compiles on stable (with the `stable` feature)! `rust-analyzer` will be able to use it to provide match-related diagnostics and refactors.

Two questions:
- Should I name the feature `nightly` instead of `rustc` for consistency with other crates? `rustc` makes more sense imo.
- `typed-arena` is an optional dependency but tidy made me add it to the allow-list anyway. Can I avoid that somehow?

r? `@compiler-errors`

14 files changed, 720 insertions, 473 deletions

diff --git a/Cargo.lock b/Cargo.lock
index dbe5b2ec6b7..117d8c2610e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4354,6 +4354,7 @@ dependencies = [
  "rustc_target",
  "smallvec",
  "tracing",
+ "typed-arena",
 ]
 
 [[package]]
@@ -5690,6 +5691,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "typed-arena"
+version = "2.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6af6ae20167a9ece4bcb41af5b80f8a1f1df981f6391189ce00fd257af04126a"
+
+[[package]]
 name = "typenum"
 version = "1.16.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index d0b4889b45f..3ea1a52ae28 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -9,6 +9,7 @@ use std::slice;
 use arrayvec::ArrayVec;
 use smallvec::{smallvec, SmallVec};
 
+#[cfg(feature = "nightly")]
 use rustc_macros::{Decodable, Encodable};
 
 use crate::{Idx, IndexVec};
@@ -111,7 +112,8 @@ macro_rules! bit_relations_inherent_impls {
 /// to or greater than the domain size. All operations that involve two bitsets
 /// will panic if the bitsets have differing domain sizes.
 ///
-#[derive(Eq, PartialEq, Hash, Decodable, Encodable)]
+#[cfg_attr(feature = "nightly", derive(Decodable, Encodable))]
+#[derive(Eq, PartialEq, Hash)]
 pub struct BitSet<T> {
     domain_size: usize,
     words: SmallVec<[Word; 2]>,
@@ -491,10 +493,21 @@ impl<T: Idx> ChunkedBitSet<T> {
         match *chunk {
             Zeros(chunk_domain_size) => {
                 if chunk_domain_size > 1 {
-                    // We take some effort to avoid copying the words.
-                    let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
-                    // SAFETY: `words` can safely be all zeroes.
-                    let mut words = unsafe { words.assume_init() };
+                    #[cfg(feature = "nightly")]
+                    let mut words = {
+                        // We take some effort to avoid copying the words.
+                        let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
+                        // SAFETY: `words` can safely be all zeroes.
+                        unsafe { words.assume_init() }
+                    };
+                    #[cfg(not(feature = "nightly"))]
+                    let mut words = {
+                        let words = mem::MaybeUninit::<[Word; CHUNK_WORDS]>::zeroed();
+                        // SAFETY: `words` can safely be all zeroes.
+                        let words = unsafe { words.assume_init() };
+                        // Unfortunate possibly-large copy
+                        Rc::new(words)
+                    };
                     let words_ref = Rc::get_mut(&mut words).unwrap();
 
                     let (word_index, mask) = chunk_word_index_and_mask(elem);
@@ -545,10 +558,21 @@ impl<T: Idx> ChunkedBitSet<T> {
             Zeros(_) => false,
             Ones(chunk_domain_size) => {
                 if chunk_domain_size > 1 {
-                    // We take some effort to avoid copying the words.
-                    let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
-                    // SAFETY: `words` can safely be all zeroes.
-                    let mut words = unsafe { words.assume_init() };
+                    #[cfg(feature = "nightly")]
+                    let mut words = {
+                        // We take some effort to avoid copying the words.
+                        let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
+                        // SAFETY: `words` can safely be all zeroes.
+                        unsafe { words.assume_init() }
+                    };
+                    #[cfg(not(feature = "nightly"))]
+                    let mut words = {
+                        let words = mem::MaybeUninit::<[Word; CHUNK_WORDS]>::zeroed();
+                        // SAFETY: `words` can safely be all zeroes.
+                        let words = unsafe { words.assume_init() };
+                        // Unfortunate possibly-large copy
+                        Rc::new(words)
+                    };
                     let words_ref = Rc::get_mut(&mut words).unwrap();
 
                     // Set only the bits in use.
@@ -1564,7 +1588,8 @@ impl<T: Idx> From<BitSet<T>> for GrowableBitSet<T> {
 ///
 /// All operations that involve a row and/or column index will panic if the
 /// index exceeds the relevant bound.
-#[derive(Clone, Eq, PartialEq, Hash, Decodable, Encodable)]
+#[cfg_attr(feature = "nightly", derive(Decodable, Encodable))]
+#[derive(Clone, Eq, PartialEq, Hash)]
 pub struct BitMatrix<R: Idx, C: Idx> {
     num_rows: usize,
     num_columns: usize,
@@ -1993,7 +2018,8 @@ impl std::fmt::Debug for FiniteBitSet<u32> {
 
 /// A fixed-sized bitset type represented by an integer type. Indices outwith than the range
 /// representable by `T` are considered set.
-#[derive(Copy, Clone, Eq, PartialEq, Decodable, Encodable)]
+#[cfg_attr(feature = "nightly", derive(Decodable, Encodable))]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub struct FiniteBitSet<T: FiniteBitSetTy>(pub T);
 
 impl<T: FiniteBitSetTy> FiniteBitSet<T> {
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index c5602392c53..185e0c7d698 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -14,7 +14,6 @@
 )]
 #![cfg_attr(feature = "nightly", allow(internal_features))]
 
-#[cfg(feature = "nightly")]
 pub mod bit_set;
 #[cfg(feature = "nightly")]
 pub mod interval;
diff --git a/compiler/rustc_mir_build/src/errors.rs b/compiler/rustc_mir_build/src/errors.rs
index db2624cac02..c66687330dc 100644
--- a/compiler/rustc_mir_build/src/errors.rs
+++ b/compiler/rustc_mir_build/src/errors.rs
@@ -6,7 +6,7 @@ use rustc_errors::{
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, Ty};
-use rustc_pattern_analysis::{cx::MatchCheckCtxt, errors::Uncovered};
+use rustc_pattern_analysis::{errors::Uncovered, rustc::RustcMatchCheckCtxt};
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 
@@ -454,7 +454,7 @@ pub enum UnusedUnsafeEnclosing {
 }
 
 pub(crate) struct NonExhaustivePatternsTypeNotEmpty<'p, 'tcx, 'm> {
-    pub cx: &'m MatchCheckCtxt<'p, 'tcx>,
+    pub cx: &'m RustcMatchCheckCtxt<'p, 'tcx>,
     pub expr_span: Span,
     pub span: Span,
     pub ty: Ty<'tcx>,
diff --git a/compiler/rustc_mir_build/src/thir/pattern/check_match.rs b/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
index 792a443c908..c435f4023af 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
@@ -1,13 +1,13 @@
-use rustc_pattern_analysis::constructor::Constructor;
-use rustc_pattern_analysis::cx::MatchCheckCtxt;
 use rustc_pattern_analysis::errors::Uncovered;
-use rustc_pattern_analysis::pat::{DeconstructedPat, WitnessPat};
-use rustc_pattern_analysis::usefulness::{Usefulness, UsefulnessReport};
+use rustc_pattern_analysis::rustc::{
+    Constructor, DeconstructedPat, RustcMatchCheckCtxt as MatchCheckCtxt, Usefulness,
+    UsefulnessReport, WitnessPat,
+};
 use rustc_pattern_analysis::{analyze_match, MatchArm};
 
 use crate::errors::*;
 
-use rustc_arena::TypedArena;
+use rustc_arena::{DroplessArena, TypedArena};
 use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -31,6 +31,7 @@ pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), Err
     let (thir, expr) = tcx.thir_body(def_id)?;
     let thir = thir.borrow();
     let pattern_arena = TypedArena::default();
+    let dropless_arena = DroplessArena::default();
     let mut visitor = MatchVisitor {
         tcx,
         thir: &*thir,
@@ -38,6 +39,7 @@ pub(crate) fn check_match(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), Err
         lint_level: tcx.local_def_id_to_hir_id(def_id),
         let_source: LetSource::None,
         pattern_arena: &pattern_arena,
+        dropless_arena: &dropless_arena,
         error: Ok(()),
     };
     visitor.visit_expr(&thir[expr]);
@@ -82,6 +84,7 @@ struct MatchVisitor<'thir, 'p, 'tcx> {
     lint_level: HirId,
     let_source: LetSource,
     pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>,
+    dropless_arena: &'p DroplessArena,
     /// Tracks if we encountered an error while checking this body. That the first function to
     /// report it stores it here. Some functions return `Result` to allow callers to short-circuit
     /// on error, but callers don't need to store it here again.
@@ -382,6 +385,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             param_env: self.param_env,
             module: self.tcx.parent_module(self.lint_level).to_def_id(),
             pattern_arena: self.pattern_arena,
+            dropless_arena: self.dropless_arena,
             match_lint_level: self.lint_level,
             whole_match_span,
             scrut_span,
@@ -425,7 +429,8 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
             let arm = &self.thir.arms[arm];
             let got_error = self.with_lint_level(arm.lint_level, |this| {
                 let Ok(pat) = this.lower_pattern(&cx, &arm.pattern) else { return true };
-                let arm = MatchArm { pat, hir_id: this.lint_level, has_guard: arm.guard.is_some() };
+                let arm =
+                    MatchArm { pat, arm_data: this.lint_level, has_guard: arm.guard.is_some() };
                 tarms.push(arm);
                 false
             });
@@ -548,7 +553,7 @@ impl<'thir, 'p, 'tcx> MatchVisitor<'thir, 'p, 'tcx> {
     ) -> Result<(MatchCheckCtxt<'p, 'tcx>, UsefulnessReport<'p, 'tcx>), ErrorGuaranteed> {
         let cx = self.new_cx(refutability, None, scrut, pat.span);
         let pat = self.lower_pattern(&cx, pat)?;
-        let arms = [MatchArm { pat, hir_id: self.lint_level, has_guard: false }];
+        let arms = [MatchArm { pat, arm_data: self.lint_level, has_guard: false }];
         let report = analyze_match(&cx, &arms, pat.ty());
         Ok((cx, report))
     }
@@ -847,34 +852,34 @@ fn report_arm_reachability<'p, 'tcx>(
         );
     };
 
-    use Usefulness::*;
     let mut catchall = None;
     for (arm, is_useful) in report.arm_usefulness.iter() {
         match is_useful {
-            Redundant => report_unreachable_pattern(arm.pat.span(), arm.hir_id, catchall),
-            Useful(redundant_spans) if redundant_spans.is_empty() => {}
+            Usefulness::Redundant => {
+                report_unreachable_pattern(*arm.pat.data(), arm.arm_data, catchall)
+            }
+            Usefulness::Useful(redundant_subpats) if redundant_subpats.is_empty() => {}
             // The arm is reachable, but contains redundant subpatterns (from or-patterns).
-            Useful(redundant_spans) => {
-                let mut redundant_spans = redundant_spans.clone();
+            Usefulness::Useful(redundant_subpats) => {
+                let mut redundant_subpats = redundant_subpats.clone();
                 // Emit lints in the order in which they occur in the file.
-                redundant_spans.sort_unstable();
-                for span in redundant_spans {
-                    report_unreachable_pattern(span, arm.hir_id, None);
+                redundant_subpats.sort_unstable_by_key(|pat| pat.data());
+                for pat in redundant_subpats {
+                    report_unreachable_pattern(*pat.data(), arm.arm_data, None);
                 }
             }
         }
         if !arm.has_guard && catchall.is_none() && pat_is_catchall(arm.pat) {
-            catchall = Some(arm.pat.span());
+            catchall = Some(*arm.pat.data());
         }
     }
 }
 
 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 fn pat_is_catchall(pat: &DeconstructedPat<'_, '_>) -> bool {
-    use Constructor::*;
     match pat.ctor() {
-        Wildcard => true,
-        Single => pat.iter_fields().all(|pat| pat_is_catchall(pat)),
+        Constructor::Wildcard => true,
+        Constructor::Struct | Constructor::Ref => pat.iter_fields().all(|pat| pat_is_catchall(pat)),
         _ => false,
     }
 }
@@ -885,7 +890,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
     thir: &Thir<'tcx>,
     scrut_ty: Ty<'tcx>,
     sp: Span,
-    witnesses: Vec<WitnessPat<'tcx>>,
+    witnesses: Vec<WitnessPat<'p, 'tcx>>,
     arms: &[ArmId],
     expr_span: Span,
 ) -> ErrorGuaranteed {
@@ -1082,10 +1087,10 @@ fn report_non_exhaustive_match<'p, 'tcx>(
 
 fn joined_uncovered_patterns<'p, 'tcx>(
     cx: &MatchCheckCtxt<'p, 'tcx>,
-    witnesses: &[WitnessPat<'tcx>],
+    witnesses: &[WitnessPat<'p, 'tcx>],
 ) -> String {
     const LIMIT: usize = 3;
-    let pat_to_str = |pat: &WitnessPat<'tcx>| cx.hoist_witness_pat(pat).to_string();
+    let pat_to_str = |pat: &WitnessPat<'p, 'tcx>| cx.hoist_witness_pat(pat).to_string();
     match witnesses {
         [] => bug!(),
         [witness] => format!("`{}`", cx.hoist_witness_pat(witness)),
@@ -1103,7 +1108,7 @@ fn joined_uncovered_patterns<'p, 'tcx>(
 
 fn collect_non_exhaustive_tys<'tcx>(
     cx: &MatchCheckCtxt<'_, 'tcx>,
-    pat: &WitnessPat<'tcx>,
+    pat: &WitnessPat<'_, 'tcx>,
     non_exhaustive_tys: &mut FxIndexSet<Ty<'tcx>>,
 ) {
     if matches!(pat.ctor(), Constructor::NonExhaustive) {
@@ -1122,7 +1127,7 @@ fn collect_non_exhaustive_tys<'tcx>(
 fn report_adt_defined_here<'tcx>(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>,
-    witnesses: &[WitnessPat<'tcx>],
+    witnesses: &[WitnessPat<'_, 'tcx>],
     point_at_non_local_ty: bool,
 ) -> Option<AdtDefinedHere<'tcx>> {
     let ty = ty.peel_refs();
@@ -1144,15 +1149,14 @@ fn report_adt_defined_here<'tcx>(
     Some(AdtDefinedHere { adt_def_span, ty, variants })
 }
 
-fn maybe_point_at_variant<'a, 'tcx: 'a>(
+fn maybe_point_at_variant<'a, 'p: 'a, 'tcx: 'p>(
     tcx: TyCtxt<'tcx>,
     def: AdtDef<'tcx>,
-    patterns: impl Iterator<Item = &'a WitnessPat<'tcx>>,
+    patterns: impl Iterator<Item = &'a WitnessPat<'p, 'tcx>>,
 ) -> Vec<Span> {
-    use Constructor::*;
     let mut covered = vec![];
     for pattern in patterns {
-        if let Variant(variant_index) = pattern.ctor() {
+        if let Constructor::Variant(variant_index) = pattern.ctor() {
             if let ty::Adt(this_def, _) = pattern.ty().kind()
                 && this_def.did() != def.did()
             {
diff --git a/compiler/rustc_pattern_analysis/Cargo.toml b/compiler/rustc_pattern_analysis/Cargo.toml
index 0639944a45c..908d00cf105 100644
--- a/compiler/rustc_pattern_analysis/Cargo.toml
+++ b/compiler/rustc_pattern_analysis/Cargo.toml
@@ -6,17 +6,40 @@ edition = "2021"
 [dependencies]
 # tidy-alphabetical-start
 rustc_apfloat = "0.2.0"
-rustc_arena = { path = "../rustc_arena" }
-rustc_data_structures = { path = "../rustc_data_structures" }
-rustc_errors = { path = "../rustc_errors" }
-rustc_fluent_macro = { path = "../rustc_fluent_macro" }
-rustc_hir = { path = "../rustc_hir" }
-rustc_index = { path = "../rustc_index" }
-rustc_macros = { path = "../rustc_macros" }
-rustc_middle = { path = "../rustc_middle" }
-rustc_session = { path = "../rustc_session" }
-rustc_span = { path = "../rustc_span" }
-rustc_target = { path = "../rustc_target" }
-smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
+rustc_arena = { path = "../rustc_arena", optional = true }
+rustc_data_structures = { path = "../rustc_data_structures", optional = true }
+rustc_errors = { path = "../rustc_errors", optional = true }
+rustc_fluent_macro = { path = "../rustc_fluent_macro", optional = true }
+rustc_hir = { path = "../rustc_hir", optional = true }
+rustc_index = { path = "../rustc_index", default-features = false }
+rustc_macros = { path = "../rustc_macros", optional = true }
+rustc_middle = { path = "../rustc_middle", optional = true }
+rustc_session = { path = "../rustc_session", optional = true }
+rustc_span = { path = "../rustc_span", optional = true }
+rustc_target = { path = "../rustc_target", optional = true }
+smallvec = { version = "1.8.1", features = ["union"] }
 tracing = "0.1"
+typed-arena = { version = "2.0.2", optional = true }
 # tidy-alphabetical-end
+
+[features]
+default = ["rustc"]
+# It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
+# we use another feature instead. The crate won't compile if one of these isn't enabled.
+rustc = [
+    "dep:rustc_arena",
+    "dep:rustc_data_structures",
+    "dep:rustc_errors",
+    "dep:rustc_fluent_macro",
+    "dep:rustc_hir",
+    "dep:rustc_macros",
+    "dep:rustc_middle",
+    "dep:rustc_session",
+    "dep:rustc_span",
+    "dep:rustc_target",
+    "smallvec/may_dangle",
+    "rustc_index/nightly",
+]
+stable = [
+    "dep:typed-arena",
+]
diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index 6486ad8b483..af0a7497a34 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -40,7 +40,7 @@
 //! - That have no non-trivial intersection with any of the constructors in the column (i.e. they're
 //!     each either disjoint with or covered by any given column constructor).
 //!
-//! We compute this in two steps: first [`crate::cx::MatchCheckCtxt::ctors_for_ty`] determines the
+//! We compute this in two steps: first [`TypeCx::ctors_for_ty`] determines the
 //! set of all possible constructors for the type. Then [`ConstructorSet::split`] looks at the
 //! column of constructors and splits the set into groups accordingly. The precise invariants of
 //! [`ConstructorSet::split`] is described in [`SplitConstructorSet`].
@@ -136,7 +136,7 @@
 //! the algorithm can't distinguish them from a nonempty constructor. The only known case where this
 //! could happen is the `[..]` pattern on `[!; N]` with `N > 0` so we must take care to not emit it.
 //!
-//! This is all handled by [`crate::cx::MatchCheckCtxt::ctors_for_ty`] and
+//! This is all handled by [`TypeCx::ctors_for_ty`] and
 //! [`ConstructorSet::split`]. The invariants of [`SplitConstructorSet`] are also of interest.
 //!
 //!
@@ -155,17 +155,15 @@ use std::iter::once;
 use smallvec::SmallVec;
 
 use rustc_apfloat::ieee::{DoubleS, IeeeFloat, SingleS};
-use rustc_data_structures::fx::FxHashSet;
-use rustc_hir::RangeEnd;
+use rustc_index::bit_set::{BitSet, GrowableBitSet};
 use rustc_index::IndexVec;
-use rustc_middle::mir::Const;
-use rustc_target::abi::VariantIdx;
 
 use self::Constructor::*;
 use self::MaybeInfiniteInt::*;
 use self::SliceKind::*;
 
-use crate::usefulness::PatCtxt;
+use crate::usefulness::PlaceCtxt;
+use crate::TypeCx;
 
 /// Whether we have seen a constructor in the column or not.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
@@ -174,6 +172,21 @@ enum Presence {
     Seen,
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum RangeEnd {
+    Included,
+    Excluded,
+}
+
+impl fmt::Display for RangeEnd {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            RangeEnd::Included => "..=",
+            RangeEnd::Excluded => "..",
+        })
+    }
+}
+
 /// A possibly infinite integer. Values are encoded such that the ordering on `u128` matches the
 /// natural order on the original type. For example, `-128i8` is encoded as `0` and `127i8` as
 /// `255`. See `signed_bias` for details.
@@ -221,7 +234,7 @@ impl MaybeInfiniteInt {
         match self {
             Finite(n) => match n.checked_sub(1) {
                 Some(m) => Finite(m),
-                None => bug!(),
+                None => panic!("Called `MaybeInfiniteInt::minus_one` on 0"),
             },
             JustAfterMax => Finite(u128::MAX),
             x => x,
@@ -234,7 +247,7 @@ impl MaybeInfiniteInt {
                 Some(m) => Finite(m),
                 None => JustAfterMax,
             },
-            JustAfterMax => bug!(),
+            JustAfterMax => panic!("Called `MaybeInfiniteInt::plus_one` on u128::MAX+1"),
             x => x,
         }
     }
@@ -253,7 +266,7 @@ pub struct IntRange {
 
 impl IntRange {
     /// Best effort; will not know that e.g. `255u8..` is a singleton.
-    pub(crate) fn is_singleton(&self) -> bool {
+    pub fn is_singleton(&self) -> bool {
         // Since `lo` and `hi` can't be the same `Infinity` and `plus_one` never changes from finite
         // to infinite, this correctly only detects ranges that contain exacly one `Finite(x)`.
         self.lo.plus_one() == self.hi
@@ -271,7 +284,7 @@ impl IntRange {
         }
         if lo >= hi {
             // This should have been caught earlier by E0030.
-            bug!("malformed range pattern: {lo:?}..{hi:?}");
+            panic!("malformed range pattern: {lo:?}..{hi:?}");
         }
         IntRange { lo, hi }
     }
@@ -432,7 +445,7 @@ impl Slice {
         let kind = match (array_len, kind) {
             // If the middle `..` has length 0, we effectively have a fixed-length pattern.
             (Some(len), VarLen(prefix, suffix)) if prefix + suffix == len => FixedLen(len),
-            (Some(len), VarLen(prefix, suffix)) if prefix + suffix > len => bug!(
+            (Some(len), VarLen(prefix, suffix)) if prefix + suffix > len => panic!(
                 "Slice pattern of length {} longer than its array length {len}",
                 prefix + suffix
             ),
@@ -532,7 +545,7 @@ impl Slice {
         // therefore `Presence::Seen` in the column.
         let mut min_var_len = usize::MAX;
         // Tracks the fixed-length slices we've seen, to mark them as `Presence::Seen`.
-        let mut seen_fixed_lens = FxHashSet::default();
+        let mut seen_fixed_lens = GrowableBitSet::new_empty();
         match &mut max_slice {
             VarLen(max_prefix_len, max_suffix_len) => {
                 // A length larger than any fixed-length slice encountered.
@@ -600,7 +613,7 @@ impl Slice {
 
         smaller_lengths.map(FixedLen).chain(once(max_slice)).map(move |kind| {
             let arity = kind.arity();
-            let seen = if min_var_len <= arity || seen_fixed_lens.contains(&arity) {
+            let seen = if min_var_len <= arity || seen_fixed_lens.contains(arity) {
                 Presence::Seen
             } else {
                 Presence::Unseen
@@ -630,12 +643,17 @@ impl OpaqueId {
 /// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
 /// `Fields`.
 #[derive(Clone, Debug, PartialEq)]
-pub enum Constructor<'tcx> {
-    /// The constructor for patterns that have a single constructor, like tuples, struct patterns,
-    /// and references. Fixed-length arrays are treated separately with `Slice`.
-    Single,
+pub enum Constructor<Cx: TypeCx> {
+    /// Tuples and structs.
+    Struct,
     /// Enum variants.
-    Variant(VariantIdx),
+    Variant(Cx::VariantIdx),
+    /// References
+    Ref,
+    /// Array and slice patterns.
+    Slice(Slice),
+    /// Union field accesses.
+    UnionField,
     /// Booleans
     Bool(bool),
     /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
@@ -644,9 +662,7 @@ pub enum Constructor<'tcx> {
     F32Range(IeeeFloat<SingleS>, IeeeFloat<SingleS>, RangeEnd),
     F64Range(IeeeFloat<DoubleS>, IeeeFloat<DoubleS>, RangeEnd),
     /// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
-    Str(Const<'tcx>),
-    /// Array and slice patterns.
-    Slice(Slice),
+    Str(Cx::StrLit),
     /// Constants that must not be matched structurally. They are treated as black boxes for the
     /// purposes of exhaustiveness: we must not inspect them, and they don't count towards making a
     /// match exhaustive.
@@ -669,12 +685,12 @@ pub enum Constructor<'tcx> {
     Missing,
 }
 
-impl<'tcx> Constructor<'tcx> {
+impl<Cx: TypeCx> Constructor<Cx> {
     pub(crate) fn is_non_exhaustive(&self) -> bool {
         matches!(self, NonExhaustive)
     }
 
-    pub(crate) fn as_variant(&self) -> Option<VariantIdx> {
+    pub(crate) fn as_variant(&self) -> Option<Cx::VariantIdx> {
         match self {
             Variant(i) => Some(*i),
             _ => None,
@@ -701,8 +717,8 @@ impl<'tcx> Constructor<'tcx> {
 
     /// The number of fields for this constructor. This must be kept in sync with
     /// `Fields::wildcards`.
-    pub(crate) fn arity(&self, pcx: &PatCtxt<'_, '_, 'tcx>) -> usize {
-        pcx.cx.ctor_arity(self, pcx.ty)
+    pub(crate) fn arity(&self, pcx: &PlaceCtxt<'_, '_, Cx>) -> usize {
+        pcx.ctor_arity(self)
     }
 
     /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
@@ -710,20 +726,20 @@ impl<'tcx> Constructor<'tcx> {
     /// this checks for inclusion.
     // We inline because this has a single call site in `Matrix::specialize_constructor`.
     #[inline]
-    pub(crate) fn is_covered_by<'p>(&self, pcx: &PatCtxt<'_, 'p, 'tcx>, other: &Self) -> bool {
+    pub(crate) fn is_covered_by<'p>(&self, pcx: &PlaceCtxt<'_, 'p, Cx>, other: &Self) -> bool {
         match (self, other) {
-            (Wildcard, _) => {
-                span_bug!(
-                    pcx.cx.scrut_span,
-                    "Constructor splitting should not have returned `Wildcard`"
-                )
-            }
+            (Wildcard, _) => pcx
+                .mcx
+                .tycx
+                .bug(format_args!("Constructor splitting should not have returned `Wildcard`")),
             // Wildcards cover anything
             (_, Wildcard) => true,
             // Only a wildcard pattern can match these special constructors.
             (Missing { .. } | NonExhaustive | Hidden, _) => false,
 
-            (Single, Single) => true,
+            (Struct, Struct) => true,
+            (Ref, Ref) => true,
+            (UnionField, UnionField) => true,
             (Variant(self_id), Variant(other_id)) => self_id == other_id,
             (Bool(self_b), Bool(other_b)) => self_b == other_b,
 
@@ -756,12 +772,9 @@ impl<'tcx> Constructor<'tcx> {
             (Opaque(self_id), Opaque(other_id)) => self_id == other_id,
             (Opaque(..), _) | (_, Opaque(..)) => false,
 
-            _ => span_bug!(
-                pcx.cx.scrut_span,
-                "trying to compare incompatible constructors {:?} and {:?}",
-                self,
-                other
-            ),
+            _ => pcx.mcx.tycx.bug(format_args!(
+                "trying to compare incompatible constructors {self:?} and {other:?}"
+            )),
         }
     }
 }
@@ -785,13 +798,16 @@ pub enum VariantVisibility {
 /// In terms of division of responsibility, [`ConstructorSet::split`] handles all of the
 /// `exhaustive_patterns` feature.
 #[derive(Debug)]
-pub enum ConstructorSet {
-    /// The type has a single constructor, e.g. `&T` or a struct. `empty` tracks whether the
-    /// constructor is empty.
-    Single { empty: bool },
+pub enum ConstructorSet<Cx: TypeCx> {
+    /// The type is a tuple or struct. `empty` tracks whether the type is empty.
+    Struct { empty: bool },
     /// This type has the following list of constructors. If `variants` is empty and
     /// `non_exhaustive` is false, don't use this; use `NoConstructors` instead.
-    Variants { variants: IndexVec<VariantIdx, VariantVisibility>, non_exhaustive: bool },
+    Variants { variants: IndexVec<Cx::VariantIdx, VariantVisibility>, non_exhaustive: bool },
+    /// The type is `&T`.
+    Ref,
+    /// The type is a union.
+    Union,
     /// Booleans.
     Bool,
     /// The type is spanned by integer values. The range or ranges give the set of allowed values.
@@ -830,25 +846,25 @@ pub enum ConstructorSet {
 /// of the `ConstructorSet` for the type, yet if we forgot to include them in `present` we would be
 /// ignoring any row with `Opaque`s in the algorithm. Hence the importance of point 4.
 #[derive(Debug)]
-pub(crate) struct SplitConstructorSet<'tcx> {
-    pub(crate) present: SmallVec<[Constructor<'tcx>; 1]>,
-    pub(crate) missing: Vec<Constructor<'tcx>>,
-    pub(crate) missing_empty: Vec<Constructor<'tcx>>,
+pub(crate) struct SplitConstructorSet<Cx: TypeCx> {
+    pub(crate) present: SmallVec<[Constructor<Cx>; 1]>,
+    pub(crate) missing: Vec<Constructor<Cx>>,
+    pub(crate) missing_empty: Vec<Constructor<Cx>>,
 }
 
-impl ConstructorSet {
+impl<Cx: TypeCx> ConstructorSet<Cx> {
     /// This analyzes a column of constructors to 1/ determine which constructors of the type (if
     /// any) are missing; 2/ split constructors to handle non-trivial intersections e.g. on ranges
     /// or slices. This can get subtle; see [`SplitConstructorSet`] for details of this operation
     /// and its invariants.
     #[instrument(level = "debug", skip(self, pcx, ctors), ret)]
-    pub(crate) fn split<'a, 'tcx>(
+    pub(crate) fn split<'a>(
         &self,
-        pcx: &PatCtxt<'_, '_, 'tcx>,
-        ctors: impl Iterator<Item = &'a Constructor<'tcx>> + Clone,
-    ) -> SplitConstructorSet<'tcx>
+        pcx: &PlaceCtxt<'_, '_, Cx>,
+        ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone,
+    ) -> SplitConstructorSet<Cx>
     where
-        'tcx: 'a,
+        Cx: 'a,
     {
         let mut present: SmallVec<[_; 1]> = SmallVec::new();
         // Empty constructors found missing.
@@ -866,22 +882,39 @@ impl ConstructorSet {
         }
 
         match self {
-            ConstructorSet::Single { empty } => {
+            ConstructorSet::Struct { empty } => {
                 if !seen.is_empty() {
-                    present.push(Single);
+                    present.push(Struct);
                 } else if *empty {
-                    missing_empty.push(Single);
+                    missing_empty.push(Struct);
+                } else {
+                    missing.push(Struct);
+                }
+            }
+            ConstructorSet::Ref => {
+                if !seen.is_empty() {
+                    present.push(Ref);
                 } else {
-                    missing.push(Single);
+                    missing.push(Ref);
+                }
+            }
+            ConstructorSet::Union => {
+                if !seen.is_empty() {
+                    present.push(UnionField);
+                } else {
+                    missing.push(UnionField);
                 }
             }
             ConstructorSet::Variants { variants, non_exhaustive } => {
-                let seen_set: FxHashSet<_> = seen.iter().map(|c| c.as_variant().unwrap()).collect();
+                let mut seen_set: BitSet<_> = BitSet::new_empty(variants.len());
+                for idx in seen.iter().map(|c| c.as_variant().unwrap()) {
+                    seen_set.insert(idx);
+                }
                 let mut skipped_a_hidden_variant = false;
 
                 for (idx, visibility) in variants.iter_enumerated() {
                     let ctor = Variant(idx);
-                    if seen_set.contains(&idx) {
+                    if seen_set.contains(idx) {
                         present.push(ctor);
                     } else {
                         // We only put visible variants directly into `missing`.
@@ -975,8 +1008,8 @@ impl ConstructorSet {
         // We have now grouped all the constructors into 3 buckets: present, missing, missing_empty.
         // In the absence of the `exhaustive_patterns` feature however, we don't count nested empty
         // types as empty. Only non-nested `!` or `enum Foo {}` are considered empty.
-        if !pcx.cx.tcx.features().exhaustive_patterns
-            && !(pcx.is_top_level && matches!(self, Self::NoConstructors))
+        if !pcx.mcx.tycx.is_exhaustive_patterns_feature_on()
+            && !(pcx.is_scrutinee && matches!(self, Self::NoConstructors))
         {
             // Treat all missing constructors as nonempty.
             // This clears `missing_empty`.
diff --git a/compiler/rustc_pattern_analysis/src/errors.rs b/compiler/rustc_pattern_analysis/src/errors.rs
index 0efa8a0ec08..88770b0c43b 100644
--- a/compiler/rustc_pattern_analysis/src/errors.rs
+++ b/compiler/rustc_pattern_analysis/src/errors.rs
@@ -1,11 +1,11 @@
-use crate::{cx::MatchCheckCtxt, pat::WitnessPat};
-
 use rustc_errors::{AddToDiagnostic, Diagnostic, SubdiagnosticMessage};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::thir::Pat;
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
 
+use crate::rustc::{RustcMatchCheckCtxt, WitnessPat};
+
 #[derive(Subdiagnostic)]
 #[label(pattern_analysis_uncovered)]
 pub struct Uncovered<'tcx> {
@@ -21,8 +21,8 @@ pub struct Uncovered<'tcx> {
 impl<'tcx> Uncovered<'tcx> {
     pub fn new<'p>(
         span: Span,
-        cx: &MatchCheckCtxt<'p, 'tcx>,
-        witnesses: Vec<WitnessPat<'tcx>>,
+        cx: &RustcMatchCheckCtxt<'p, 'tcx>,
+        witnesses: Vec<WitnessPat<'p, 'tcx>>,
     ) -> Self {
         let witness_1 = cx.hoist_witness_pat(witnesses.get(0).unwrap());
         Self {
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index 07730aa49d3..785a60e9978 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -1,54 +1,133 @@
 //! Analysis of patterns, notably match exhaustiveness checking.
 
 pub mod constructor;
-pub mod cx;
+#[cfg(feature = "rustc")]
 pub mod errors;
+#[cfg(feature = "rustc")]
 pub(crate) mod lints;
 pub mod pat;
+#[cfg(feature = "rustc")]
+pub mod rustc;
 pub mod usefulness;
 
 #[macro_use]
 extern crate tracing;
+#[cfg(feature = "rustc")]
 #[macro_use]
 extern crate rustc_middle;
 
+#[cfg(feature = "rustc")]
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
-use lints::PatternColumn;
-use rustc_hir::HirId;
+use std::fmt;
+
+use rustc_index::Idx;
+#[cfg(feature = "rustc")]
 use rustc_middle::ty::Ty;
-use usefulness::{compute_match_usefulness, UsefulnessReport};
 
-use crate::cx::MatchCheckCtxt;
-use crate::lints::{lint_nonexhaustive_missing_variants, lint_overlapping_range_endpoints};
+use crate::constructor::{Constructor, ConstructorSet};
+#[cfg(feature = "rustc")]
+use crate::lints::{
+    lint_nonexhaustive_missing_variants, lint_overlapping_range_endpoints, PatternColumn,
+};
 use crate::pat::DeconstructedPat;
+#[cfg(feature = "rustc")]
+use crate::rustc::RustcMatchCheckCtxt;
+#[cfg(feature = "rustc")]
+use crate::usefulness::{compute_match_usefulness, ValidityConstraint};
+
+// It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
+// we use another feature instead. The crate won't compile if one of these isn't enabled.
+#[cfg(feature = "rustc")]
+pub(crate) use rustc_arena::TypedArena;
+#[cfg(feature = "stable")]
+pub(crate) use typed_arena::Arena as TypedArena;
+
+pub trait Captures<'a> {}
+impl<'a, T: ?Sized> Captures<'a> for T {}
+
+/// Context that provides type information about constructors.
+///
+/// Most of the crate is parameterized on a type that implements this trait.
+pub trait TypeCx: Sized + Clone + fmt::Debug {
+    /// The type of a pattern.
+    type Ty: Copy + Clone + fmt::Debug; // FIXME: remove Copy
+    /// The index of an enum variant.
+    type VariantIdx: Clone + Idx;
+    /// A string literal
+    type StrLit: Clone + PartialEq + fmt::Debug;
+    /// Extra data to store in a match arm.
+    type ArmData: Copy + Clone + fmt::Debug;
+    /// Extra data to store in a pattern. `Default` needed when we create fictitious wildcard
+    /// patterns during analysis.
+    type PatData: Clone + Default;
+
+    fn is_opaque_ty(ty: Self::Ty) -> bool;
+    fn is_exhaustive_patterns_feature_on(&self) -> bool;
+
+    /// The number of fields for this constructor.
+    fn ctor_arity(&self, ctor: &Constructor<Self>, ty: Self::Ty) -> usize;
+
+    /// The types of the fields for this constructor. The result must have a length of
+    /// `ctor_arity()`.
+    fn ctor_sub_tys(&self, ctor: &Constructor<Self>, ty: Self::Ty) -> &[Self::Ty];
+
+    /// The set of all the constructors for `ty`.
+    ///
+    /// This must follow the invariants of `ConstructorSet`
+    fn ctors_for_ty(&self, ty: Self::Ty) -> ConstructorSet<Self>;
+
+    /// Best-effort `Debug` implementation.
+    fn debug_pat(f: &mut fmt::Formatter<'_>, pat: &DeconstructedPat<'_, Self>) -> fmt::Result;
+
+    /// Raise a bug.
+    fn bug(&self, fmt: fmt::Arguments<'_>) -> !;
+}
+
+/// Context that provides information global to a match.
+#[derive(Clone)]
+pub struct MatchCtxt<'a, 'p, Cx: TypeCx> {
+    /// The context for type information.
+    pub tycx: &'a Cx,
+    /// An arena to store the wildcards we produce during analysis.
+    pub wildcard_arena: &'a TypedArena<DeconstructedPat<'p, Cx>>,
+}
+
+impl<'a, 'p, Cx: TypeCx> Copy for MatchCtxt<'a, 'p, Cx> {}
 
 /// The arm of a match expression.
-#[derive(Clone, Copy, Debug)]
-pub struct MatchArm<'p, 'tcx> {
-    /// The pattern must have been lowered through `check_match::MatchVisitor::lower_pattern`.
-    pub pat: &'p DeconstructedPat<'p, 'tcx>,
-    pub hir_id: HirId,
+#[derive(Clone, Debug)]
+pub struct MatchArm<'p, Cx: TypeCx> {
+    pub pat: &'p DeconstructedPat<'p, Cx>,
     pub has_guard: bool,
+    pub arm_data: Cx::ArmData,
 }
 
+impl<'p, Cx: TypeCx> Copy for MatchArm<'p, Cx> {}
+
 /// The entrypoint for this crate. Computes whether a match is exhaustive and which of its arms are
 /// useful, and runs some lints.
+#[cfg(feature = "rustc")]
 pub fn analyze_match<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
-    arms: &[MatchArm<'p, 'tcx>],
+    tycx: &RustcMatchCheckCtxt<'p, 'tcx>,
+    arms: &[rustc::MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
-) -> UsefulnessReport<'p, 'tcx> {
-    let pat_column = PatternColumn::new(arms);
+) -> rustc::UsefulnessReport<'p, 'tcx> {
+    // Arena to store the extra wildcards we construct during analysis.
+    let wildcard_arena = tycx.pattern_arena;
+    let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
+    let cx = MatchCtxt { tycx, wildcard_arena };
 
-    let report = compute_match_usefulness(cx, arms, scrut_ty);
+    let report = compute_match_usefulness(cx, arms, scrut_ty, scrut_validity);
+
+    let pat_column = PatternColumn::new(arms);
 
     // Lint on ranges that overlap on their endpoints, which is likely a mistake.
     lint_overlapping_range_endpoints(cx, &pat_column);
 
     // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid hitting
     // `if let`s. Only run if the match is exhaustive otherwise the error is redundant.
-    if cx.refutable && report.non_exhaustiveness_witnesses.is_empty() {
+    if tycx.refutable && report.non_exhaustiveness_witnesses.is_empty() {
         lint_nonexhaustive_missing_variants(cx, arms, &pat_column, scrut_ty)
     }
 
diff --git a/compiler/rustc_pattern_analysis/src/lints.rs b/compiler/rustc_pattern_analysis/src/lints.rs
index 8ab559c9e7a..072ef4836a8 100644
--- a/compiler/rustc_pattern_analysis/src/lints.rs
+++ b/compiler/rustc_pattern_analysis/src/lints.rs
@@ -6,15 +6,16 @@ use rustc_session::lint;
 use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::Span;
 
-use crate::constructor::{Constructor, IntRange, MaybeInfiniteInt, SplitConstructorSet};
-use crate::cx::MatchCheckCtxt;
+use crate::constructor::{IntRange, MaybeInfiniteInt};
 use crate::errors::{
     NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Overlap,
     OverlappingRangeEndpoints, Uncovered,
 };
-use crate::pat::{DeconstructedPat, WitnessPat};
-use crate::usefulness::PatCtxt;
-use crate::MatchArm;
+use crate::rustc::{
+    Constructor, DeconstructedPat, MatchArm, MatchCtxt, PlaceCtxt, RustcMatchCheckCtxt,
+    SplitConstructorSet, WitnessPat,
+};
+use crate::TypeCx;
 
 /// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that
 /// inspect the same subvalue/place".
@@ -27,11 +28,11 @@ use crate::MatchArm;
 ///
 /// This is not used in the main algorithm; only in lints.
 #[derive(Debug)]
-pub(crate) struct PatternColumn<'p, 'tcx> {
-    patterns: Vec<&'p DeconstructedPat<'p, 'tcx>>,
+pub(crate) struct PatternColumn<'a, 'p, 'tcx> {
+    patterns: Vec<&'a DeconstructedPat<'p, 'tcx>>,
 }
 
-impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
+impl<'a, 'p, 'tcx> PatternColumn<'a, 'p, 'tcx> {
     pub(crate) fn new(arms: &[MatchArm<'p, 'tcx>]) -> Self {
         let mut patterns = Vec::with_capacity(arms.len());
         for arm in arms {
@@ -53,12 +54,11 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
         }
         // If the type is opaque and it is revealed anywhere in the column, we take the revealed
         // version. Otherwise we could encounter constructors for the revealed type and crash.
-        let is_opaque = |ty: Ty<'tcx>| matches!(ty.kind(), ty::Alias(ty::Opaque, ..));
         let first_ty = self.patterns[0].ty();
-        if is_opaque(first_ty) {
+        if RustcMatchCheckCtxt::is_opaque_ty(first_ty) {
             for pat in &self.patterns {
                 let ty = pat.ty();
-                if !is_opaque(ty) {
+                if !RustcMatchCheckCtxt::is_opaque_ty(ty) {
                     return Some(ty);
                 }
             }
@@ -67,12 +67,12 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
     }
 
     /// Do constructor splitting on the constructors of the column.
-    fn analyze_ctors(&self, pcx: &PatCtxt<'_, 'p, 'tcx>) -> SplitConstructorSet<'tcx> {
+    fn analyze_ctors(&self, pcx: &PlaceCtxt<'_, 'p, 'tcx>) -> SplitConstructorSet<'p, 'tcx> {
         let column_ctors = self.patterns.iter().map(|p| p.ctor());
-        pcx.cx.ctors_for_ty(pcx.ty).split(pcx, column_ctors)
+        pcx.ctors_for_ty().split(pcx, column_ctors)
     }
 
-    fn iter<'a>(&'a self) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Captures<'a> {
+    fn iter<'b>(&'b self) -> impl Iterator<Item = &'a DeconstructedPat<'p, 'tcx>> + Captures<'b> {
         self.patterns.iter().copied()
     }
 
@@ -81,7 +81,11 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
     /// This returns one column per field of the constructor. They usually all have the same length
     /// (the number of patterns in `self` that matched `ctor`), except that we expand or-patterns
     /// which may change the lengths.
-    fn specialize(&self, pcx: &PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Vec<Self> {
+    fn specialize(
+        &self,
+        pcx: &PlaceCtxt<'a, 'p, 'tcx>,
+        ctor: &Constructor<'p, 'tcx>,
+    ) -> Vec<PatternColumn<'a, 'p, 'tcx>> {
         let arity = ctor.arity(pcx);
         if arity == 0 {
             return Vec::new();
@@ -117,14 +121,14 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
 /// Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned
 /// in a given column.
 #[instrument(level = "debug", skip(cx), ret)]
-fn collect_nonexhaustive_missing_variants<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
-    column: &PatternColumn<'p, 'tcx>,
-) -> Vec<WitnessPat<'tcx>> {
+fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
+    cx: MatchCtxt<'a, 'p, 'tcx>,
+    column: &PatternColumn<'a, 'p, 'tcx>,
+) -> Vec<WitnessPat<'p, 'tcx>> {
     let Some(ty) = column.head_ty() else {
         return Vec::new();
     };
-    let pcx = &PatCtxt::new_dummy(cx, ty);
+    let pcx = &PlaceCtxt::new_dummy(cx, ty);
 
     let set = column.analyze_ctors(pcx);
     if set.present.is_empty() {
@@ -135,7 +139,7 @@ fn collect_nonexhaustive_missing_variants<'p, 'tcx>(
     }
 
     let mut witnesses = Vec::new();
-    if cx.is_foreign_non_exhaustive_enum(ty) {
+    if cx.tycx.is_foreign_non_exhaustive_enum(ty) {
         witnesses.extend(
             set.missing
                 .into_iter()
@@ -164,14 +168,15 @@ fn collect_nonexhaustive_missing_variants<'p, 'tcx>(
     witnesses
 }
 
-pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
+pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
+    cx: MatchCtxt<'a, 'p, 'tcx>,
     arms: &[MatchArm<'p, 'tcx>],
-    pat_column: &PatternColumn<'p, 'tcx>,
+    pat_column: &PatternColumn<'a, 'p, 'tcx>,
     scrut_ty: Ty<'tcx>,
 ) {
+    let rcx: &RustcMatchCheckCtxt<'_, '_> = cx.tycx;
     if !matches!(
-        cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, cx.match_lint_level).0,
+        rcx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, rcx.match_lint_level).0,
         rustc_session::lint::Level::Allow
     ) {
         let witnesses = collect_nonexhaustive_missing_variants(cx, pat_column);
@@ -180,13 +185,13 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
             // is not exhaustive enough.
             //
             // NB: The partner lint for structs lives in `compiler/rustc_hir_analysis/src/check/pat.rs`.
-            cx.tcx.emit_spanned_lint(
+            rcx.tcx.emit_spanned_lint(
                 NON_EXHAUSTIVE_OMITTED_PATTERNS,
-                cx.match_lint_level,
-                cx.scrut_span,
+                rcx.match_lint_level,
+                rcx.scrut_span,
                 NonExhaustiveOmittedPattern {
                     scrut_ty,
-                    uncovered: Uncovered::new(cx.scrut_span, cx, witnesses),
+                    uncovered: Uncovered::new(rcx.scrut_span, rcx, witnesses),
                 },
             );
         }
@@ -196,17 +201,17 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
         // usage of the lint.
         for arm in arms {
             let (lint_level, lint_level_source) =
-                cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.hir_id);
+                rcx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.arm_data);
             if !matches!(lint_level, rustc_session::lint::Level::Allow) {
                 let decorator = NonExhaustiveOmittedPatternLintOnArm {
                     lint_span: lint_level_source.span(),
-                    suggest_lint_on_match: cx.whole_match_span.map(|span| span.shrink_to_lo()),
+                    suggest_lint_on_match: rcx.whole_match_span.map(|span| span.shrink_to_lo()),
                     lint_level: lint_level.as_str(),
                     lint_name: "non_exhaustive_omitted_patterns",
                 };
 
                 use rustc_errors::DecorateLint;
-                let mut err = cx.tcx.sess.struct_span_warn(arm.pat.span(), "");
+                let mut err = rcx.tcx.sess.struct_span_warn(*arm.pat.data(), "");
                 err.set_primary_message(decorator.msg());
                 decorator.decorate_lint(&mut err);
                 err.emit();
@@ -217,28 +222,29 @@ pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
 
 /// Traverse the patterns to warn the user about ranges that overlap on their endpoints.
 #[instrument(level = "debug", skip(cx))]
-pub(crate) fn lint_overlapping_range_endpoints<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
-    column: &PatternColumn<'p, 'tcx>,
+pub(crate) fn lint_overlapping_range_endpoints<'a, 'p, 'tcx>(
+    cx: MatchCtxt<'a, 'p, 'tcx>,
+    column: &PatternColumn<'a, 'p, 'tcx>,
 ) {
     let Some(ty) = column.head_ty() else {
         return;
     };
-    let pcx = &PatCtxt::new_dummy(cx, ty);
+    let pcx = &PlaceCtxt::new_dummy(cx, ty);
+    let rcx: &RustcMatchCheckCtxt<'_, '_> = cx.tycx;
 
     let set = column.analyze_ctors(pcx);
 
     if matches!(ty.kind(), ty::Char | ty::Int(_) | ty::Uint(_)) {
         let emit_lint = |overlap: &IntRange, this_span: Span, overlapped_spans: &[Span]| {
-            let overlap_as_pat = cx.hoist_pat_range(overlap, ty);
+            let overlap_as_pat = rcx.hoist_pat_range(overlap, ty);
             let overlaps: Vec<_> = overlapped_spans
                 .iter()
                 .copied()
                 .map(|span| Overlap { range: overlap_as_pat.clone(), span })
                 .collect();
-            cx.tcx.emit_spanned_lint(
+            rcx.tcx.emit_spanned_lint(
                 lint::builtin::OVERLAPPING_RANGE_ENDPOINTS,
-                cx.match_lint_level,
+                rcx.match_lint_level,
                 this_span,
                 OverlappingRangeEndpoints { overlap: overlaps, range: this_span },
             );
@@ -255,7 +261,7 @@ pub(crate) fn lint_overlapping_range_endpoints<'p, 'tcx>(
                 let mut suffixes: SmallVec<[_; 1]> = Default::default();
                 // Iterate on patterns that contained `overlap`.
                 for pat in column.iter() {
-                    let this_span = pat.span();
+                    let this_span = *pat.data();
                     let Constructor::IntRange(this_range) = pat.ctor() else { continue };
                     if this_range.is_singleton() {
                         // Don't lint when one of the ranges is a singleton.
diff --git a/compiler/rustc_pattern_analysis/src/pat.rs b/compiler/rustc_pattern_analysis/src/pat.rs
index 404651124ad..0cc8477b7cd 100644
--- a/compiler/rustc_pattern_analysis/src/pat.rs
+++ b/compiler/rustc_pattern_analysis/src/pat.rs
@@ -5,16 +5,11 @@ use std::fmt;
 
 use smallvec::{smallvec, SmallVec};
 
-use rustc_data_structures::captures::Captures;
-use rustc_middle::ty::{self, Ty};
-use rustc_span::{Span, DUMMY_SP};
+use crate::constructor::{Constructor, Slice, SliceKind};
+use crate::usefulness::PlaceCtxt;
+use crate::{Captures, TypeCx};
 
 use self::Constructor::*;
-use self::SliceKind::*;
-
-use crate::constructor::{Constructor, SliceKind};
-use crate::cx::MatchCheckCtxt;
-use crate::usefulness::PatCtxt;
 
 /// Values and patterns can be represented as a constructor applied to some fields. This represents
 /// a pattern in this form.
@@ -27,34 +22,34 @@ use crate::usefulness::PatCtxt;
 /// 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`. Care must
 /// be taken when converting to/from `thir::Pat`.
-pub struct DeconstructedPat<'p, 'tcx> {
-    ctor: Constructor<'tcx>,
-    fields: &'p [DeconstructedPat<'p, 'tcx>],
-    ty: Ty<'tcx>,
-    span: Span,
+pub struct DeconstructedPat<'p, Cx: TypeCx> {
+    ctor: Constructor<Cx>,
+    fields: &'p [DeconstructedPat<'p, Cx>],
+    ty: Cx::Ty,
+    data: Cx::PatData,
     /// Whether removing this arm would change the behavior of the match expression.
     useful: Cell<bool>,
 }
 
-impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
-    pub fn wildcard(ty: Ty<'tcx>, span: Span) -> Self {
-        Self::new(Wildcard, &[], ty, span)
+impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
+    pub fn wildcard(ty: Cx::Ty, data: Cx::PatData) -> Self {
+        Self::new(Wildcard, &[], ty, data)
     }
 
     pub fn new(
-        ctor: Constructor<'tcx>,
-        fields: &'p [DeconstructedPat<'p, 'tcx>],
-        ty: Ty<'tcx>,
-        span: Span,
+        ctor: Constructor<Cx>,
+        fields: &'p [DeconstructedPat<'p, Cx>],
+        ty: Cx::Ty,
+        data: Cx::PatData,
     ) -> Self {
-        DeconstructedPat { ctor, fields, ty, span, useful: Cell::new(false) }
+        DeconstructedPat { ctor, fields, ty, data, useful: Cell::new(false) }
     }
 
     pub(crate) fn is_or_pat(&self) -> bool {
         matches!(self.ctor, Or)
     }
     /// Expand this (possibly-nested) or-pattern into its alternatives.
-    pub(crate) fn flatten_or_pat(&'p self) -> SmallVec<[&'p Self; 1]> {
+    pub(crate) fn flatten_or_pat(&self) -> SmallVec<[&Self; 1]> {
         if self.is_or_pat() {
             self.iter_fields().flat_map(|p| p.flatten_or_pat()).collect()
         } else {
@@ -62,66 +57,64 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
         }
     }
 
-    pub fn ctor(&self) -> &Constructor<'tcx> {
+    pub fn ctor(&self) -> &Constructor<Cx> {
         &self.ctor
     }
-    pub fn ty(&self) -> Ty<'tcx> {
+    pub fn ty(&self) -> Cx::Ty {
         self.ty
     }
-    pub fn span(&self) -> Span {
-        self.span
+    pub fn data(&self) -> &Cx::PatData {
+        &self.data
     }
 
     pub fn iter_fields<'a>(
         &'a self,
-    ) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Captures<'a> {
+    ) -> impl Iterator<Item = &'p DeconstructedPat<'p, Cx>> + Captures<'a> {
         self.fields.iter()
     }
 
     /// Specialize this pattern with a constructor.
     /// `other_ctor` can be different from `self.ctor`, but must be covered by it.
     pub(crate) fn specialize<'a>(
-        &'a self,
-        pcx: &PatCtxt<'_, 'p, 'tcx>,
-        other_ctor: &Constructor<'tcx>,
-    ) -> SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]> {
+        &self,
+        pcx: &PlaceCtxt<'a, 'p, Cx>,
+        other_ctor: &Constructor<Cx>,
+    ) -> SmallVec<[&'a DeconstructedPat<'p, Cx>; 2]> {
+        let wildcard_sub_tys = || {
+            let tys = pcx.ctor_sub_tys(other_ctor);
+            tys.iter()
+                .map(|ty| DeconstructedPat::wildcard(*ty, Cx::PatData::default()))
+                .map(|pat| pcx.mcx.wildcard_arena.alloc(pat) as &_)
+                .collect()
+        };
         match (&self.ctor, other_ctor) {
-            (Wildcard, _) => {
-                // We return a wildcard for each field of `other_ctor`.
-                pcx.cx.ctor_wildcard_fields(other_ctor, pcx.ty).iter().collect()
-            }
-            (Slice(self_slice), Slice(other_slice))
-                if self_slice.arity() != other_slice.arity() =>
-            {
-                // The only tricky case: two slices of different arity. Since `self_slice` covers
-                // `other_slice`, `self_slice` must be `VarLen`, i.e. of the form
-                // `[prefix, .., suffix]`. Moreover `other_slice` is guaranteed to have a larger
-                // arity. So we fill the middle part with enough wildcards to reach the length of
-                // the new, larger slice.
-                match self_slice.kind {
-                    FixedLen(_) => bug!("{:?} doesn't cover {:?}", self_slice, other_slice),
-                    VarLen(prefix, suffix) => {
-                        let (ty::Slice(inner_ty) | ty::Array(inner_ty, _)) = *self.ty.kind() else {
-                            bug!("bad slice pattern {:?} {:?}", self.ctor, self.ty);
-                        };
-                        let prefix = &self.fields[..prefix];
-                        let suffix = &self.fields[self_slice.arity() - suffix..];
-                        let wildcard: &_ = pcx
-                            .cx
-                            .pattern_arena
-                            .alloc(DeconstructedPat::wildcard(inner_ty, DUMMY_SP));
-                        let extra_wildcards = other_slice.arity() - self_slice.arity();
-                        let extra_wildcards = (0..extra_wildcards).map(|_| wildcard);
-                        prefix.iter().chain(extra_wildcards).chain(suffix).collect()
-                    }
+            // Return a wildcard for each field of `other_ctor`.
+            (Wildcard, _) => wildcard_sub_tys(),
+            // The only non-trivial case: two slices of different arity. `other_slice` is
+            // guaranteed to have a larger arity, so we fill the middle part with enough
+            // wildcards to reach the length of the new, larger slice.
+            (
+                &Slice(self_slice @ Slice { kind: SliceKind::VarLen(prefix, suffix), .. }),
+                &Slice(other_slice),
+            ) if self_slice.arity() != other_slice.arity() => {
+                // Start with a slice of wildcards of the appropriate length.
+                let mut fields: SmallVec<[_; 2]> = wildcard_sub_tys();
+                // Fill in the fields from both ends.
+                let new_arity = fields.len();
+                for i in 0..prefix {
+                    fields[i] = &self.fields[i];
+                }
+                for i in 0..suffix {
+                    fields[new_arity - 1 - i] = &self.fields[self.fields.len() - 1 - i];
                 }
+                fields
             }
             _ => self.fields.iter().collect(),
         }
     }
 
-    /// We keep track for each pattern if it was ever useful during the analysis. This is used
-    /// with `redundant_spans` to report redundant subpatterns arising from or patterns.
+    /// We keep track for each pattern if it was ever useful during the analysis. This is used with
+    /// `redundant_subpatterns` to report redundant subpatterns arising from or patterns.
     pub(crate) fn set_useful(&self) {
         self.useful.set(true)
     }
@@ -139,19 +132,19 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
         }
     }
 
-    /// Report the spans of subpatterns that were not useful, if any.
-    pub(crate) fn redundant_spans(&self) -> Vec<Span> {
-        let mut spans = Vec::new();
-        self.collect_redundant_spans(&mut spans);
-        spans
+    /// Report the subpatterns that were not useful, if any.
+    pub(crate) fn redundant_subpatterns(&self) -> Vec<&Self> {
+        let mut subpats = Vec::new();
+        self.collect_redundant_subpatterns(&mut subpats);
+        subpats
     }
-    fn collect_redundant_spans(&self, spans: &mut Vec<Span>) {
+    fn collect_redundant_subpatterns<'a>(&'a self, subpats: &mut Vec<&'a Self>) {
         // We don't look at subpatterns if we already reported the whole pattern as redundant.
         if !self.is_useful() {
-            spans.push(self.span);
+            subpats.push(self);
         } else {
             for p in self.iter_fields() {
-                p.collect_redundant_spans(spans);
+                p.collect_redundant_subpatterns(subpats);
             }
         }
     }
@@ -159,47 +152,46 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
 
 /// This is mostly copied from the `Pat` impl. This is best effort and not good enough for a
 /// `Display` impl.
-impl<'p, 'tcx> fmt::Debug for DeconstructedPat<'p, 'tcx> {
+impl<'p, Cx: TypeCx> fmt::Debug for DeconstructedPat<'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        MatchCheckCtxt::debug_pat(f, self)
+        Cx::debug_pat(f, self)
     }
 }
 
 /// Same idea as `DeconstructedPat`, except this is a fictitious pattern built up for diagnostics
 /// purposes. As such they don't use interning and can be cloned.
 #[derive(Debug, Clone)]
-pub struct WitnessPat<'tcx> {
-    ctor: Constructor<'tcx>,
-    pub(crate) fields: Vec<WitnessPat<'tcx>>,
-    ty: Ty<'tcx>,
+pub struct WitnessPat<Cx: TypeCx> {
+    ctor: Constructor<Cx>,
+    pub(crate) fields: Vec<WitnessPat<Cx>>,
+    ty: Cx::Ty,
 }
 
-impl<'tcx> WitnessPat<'tcx> {
-    pub(crate) fn new(ctor: Constructor<'tcx>, fields: Vec<Self>, ty: Ty<'tcx>) -> Self {
+impl<Cx: TypeCx> WitnessPat<Cx> {
+    pub(crate) fn new(ctor: Constructor<Cx>, fields: Vec<Self>, ty: Cx::Ty) -> Self {
         Self { ctor, fields, ty }
     }
-    pub(crate) fn wildcard(ty: Ty<'tcx>) -> Self {
+    pub(crate) fn wildcard(ty: Cx::Ty) -> Self {
         Self::new(Wildcard, Vec::new(), ty)
     }
 
     /// 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(crate) fn wild_from_ctor(pcx: &PatCtxt<'_, '_, 'tcx>, ctor: Constructor<'tcx>) -> Self {
-        let field_tys =
-            pcx.cx.ctor_wildcard_fields(&ctor, pcx.ty).iter().map(|deco_pat| deco_pat.ty());
-        let fields = field_tys.map(|ty| Self::wildcard(ty)).collect();
+    pub(crate) fn wild_from_ctor(pcx: &PlaceCtxt<'_, '_, Cx>, ctor: Constructor<Cx>) -> Self {
+        let field_tys = pcx.ctor_sub_tys(&ctor);
+        let fields = field_tys.iter().map(|ty| Self::wildcard(*ty)).collect();
         Self::new(ctor, fields, pcx.ty)
     }
 
-    pub fn ctor(&self) -> &Constructor<'tcx> {
+    pub fn ctor(&self) -> &Constructor<Cx> {
         &self.ctor
     }
-    pub fn ty(&self) -> Ty<'tcx> {
+    pub fn ty(&self) -> Cx::Ty {
         self.ty
     }
 
-    pub fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a WitnessPat<'tcx>> {
+    pub fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a WitnessPat<Cx>> {
         self.fields.iter()
     }
 }
diff --git a/compiler/rustc_pattern_analysis/src/cx.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index 8a4f39a1f4a..65c90aa9f1d 100644
--- a/compiler/rustc_pattern_analysis/src/cx.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -1,15 +1,15 @@
 use std::fmt;
 use std::iter::once;
 
-use rustc_arena::TypedArena;
+use rustc_arena::{DroplessArena, TypedArena};
 use rustc_data_structures::captures::Captures;
 use rustc_hir::def_id::DefId;
-use rustc_hir::{HirId, RangeEnd};
+use rustc_hir::HirId;
 use rustc_index::Idx;
 use rustc_index::IndexVec;
 use rustc_middle::middle::stability::EvalResult;
-use rustc_middle::mir;
 use rustc_middle::mir::interpret::Scalar;
+use rustc_middle::mir::{self, Const};
 use rustc_middle::thir::{FieldPat, Pat, PatKind, PatRange, PatRangeBoundary};
 use rustc_middle::ty::layout::IntegerExt;
 use rustc_middle::ty::{self, Ty, TyCtxt, VariantDef};
@@ -18,14 +18,31 @@ use rustc_target::abi::{FieldIdx, Integer, VariantIdx, FIRST_VARIANT};
 use smallvec::SmallVec;
 
 use crate::constructor::{
-    Constructor, ConstructorSet, IntRange, MaybeInfiniteInt, OpaqueId, Slice, SliceKind,
-    VariantVisibility,
+    IntRange, MaybeInfiniteInt, OpaqueId, RangeEnd, Slice, SliceKind, VariantVisibility,
 };
-use crate::pat::{DeconstructedPat, WitnessPat};
+use crate::TypeCx;
 
-use Constructor::*;
+use crate::constructor::Constructor::*;
 
-pub struct MatchCheckCtxt<'p, 'tcx> {
+// Re-export rustc-specific versions of all these types.
+pub type Constructor<'p, 'tcx> = crate::constructor::Constructor<RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type ConstructorSet<'p, 'tcx> =
+    crate::constructor::ConstructorSet<RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type DeconstructedPat<'p, 'tcx> =
+    crate::pat::DeconstructedPat<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type MatchArm<'p, 'tcx> = crate::MatchArm<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type MatchCtxt<'a, 'p, 'tcx> = crate::MatchCtxt<'a, 'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub(crate) type PlaceCtxt<'a, 'p, 'tcx> =
+    crate::usefulness::PlaceCtxt<'a, 'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub(crate) type SplitConstructorSet<'p, 'tcx> =
+    crate::constructor::SplitConstructorSet<RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type Usefulness<'p, 'tcx> = crate::usefulness::Usefulness<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type UsefulnessReport<'p, 'tcx> =
+    crate::usefulness::UsefulnessReport<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type WitnessPat<'p, 'tcx> = crate::pat::WitnessPat<RustcMatchCheckCtxt<'p, 'tcx>>;
+
+#[derive(Clone)]
+pub struct RustcMatchCheckCtxt<'p, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
     /// The module in which the match occurs. This is necessary for
     /// checking inhabited-ness of types because whether a type is (visibly)
@@ -35,6 +52,7 @@ pub struct MatchCheckCtxt<'p, 'tcx> {
     pub module: DefId,
     pub param_env: ty::ParamEnv<'tcx>,
     pub pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>,
+    pub dropless_arena: &'p DroplessArena,
     /// Lint level at the match.
     pub match_lint_level: HirId,
     /// The span of the whole match, if applicable.
@@ -48,8 +66,14 @@ pub struct MatchCheckCtxt<'p, 'tcx> {
     pub known_valid_scrutinee: bool,
 }
 
-impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
-    pub(super) fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
+impl<'p, 'tcx> fmt::Debug for RustcMatchCheckCtxt<'p, 'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("RustcMatchCheckCtxt").finish()
+    }
+}
+
+impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
+    pub(crate) fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
         !ty.is_inhabited_from(self.tcx, self.module, self.param_env)
     }
 
@@ -63,12 +87,18 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
         }
     }
 
-    pub(crate) fn alloc_wildcard_slice(
-        &self,
-        tys: impl IntoIterator<Item = Ty<'tcx>>,
-    ) -> &'p [DeconstructedPat<'p, 'tcx>] {
-        self.pattern_arena
-            .alloc_from_iter(tys.into_iter().map(|ty| DeconstructedPat::wildcard(ty, DUMMY_SP)))
+    /// Whether the range denotes the fictitious values before `isize::MIN` or after
+    /// `usize::MAX`/`isize::MAX` (see doc of [`IntRange::split`] for why these exist).
+    pub fn is_range_beyond_boundaries(&self, range: &IntRange, ty: Ty<'tcx>) -> bool {
+        ty.is_ptr_sized_integral() && {
+            // The two invalid ranges are `NegInfinity..isize::MIN` (represented as
+            // `NegInfinity..0`), and `{u,i}size::MAX+1..PosInfinity`. `hoist_pat_range_bdy`
+            // converts `MAX+1` to `PosInfinity`, and we couldn't have `PosInfinity` in `range.lo`
+            // otherwise.
+            let lo = self.hoist_pat_range_bdy(range.lo, ty);
+            matches!(lo, PatRangeBoundary::PosInfinity)
+                || matches!(range.hi, MaybeInfiniteInt::Finite(0))
+        }
     }
 
     // In the cases of either a `#[non_exhaustive]` field list or a non-public field, we hide
@@ -100,12 +130,12 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
     }
 
     pub(crate) fn variant_index_for_adt(
-        ctor: &Constructor<'tcx>,
+        ctor: &Constructor<'p, 'tcx>,
         adt: ty::AdtDef<'tcx>,
     ) -> VariantIdx {
         match *ctor {
             Variant(idx) => idx,
-            Single => {
+            Struct | UnionField => {
                 assert!(!adt.is_enum());
                 FIRST_VARIANT
             }
@@ -113,37 +143,36 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
         }
     }
 
-    /// Creates a new list of wildcard fields for a given constructor. The result must have a length
-    /// of `ctor.arity()`.
+    /// Returns the types of the fields for a given constructor. The result must have a length of
+    /// `ctor.arity()`.
     #[instrument(level = "trace", skip(self))]
-    pub(crate) fn ctor_wildcard_fields(
-        &self,
-        ctor: &Constructor<'tcx>,
-        ty: Ty<'tcx>,
-    ) -> &'p [DeconstructedPat<'p, 'tcx>] {
+    pub(crate) fn ctor_sub_tys(&self, ctor: &Constructor<'p, 'tcx>, ty: Ty<'tcx>) -> &[Ty<'tcx>] {
         let cx = self;
         match ctor {
-            Single | Variant(_) => match ty.kind() {
-                ty::Tuple(fs) => cx.alloc_wildcard_slice(fs.iter()),
-                ty::Ref(_, rty, _) => cx.alloc_wildcard_slice(once(*rty)),
+            Struct | Variant(_) | UnionField => match ty.kind() {
+                ty::Tuple(fs) => cx.dropless_arena.alloc_from_iter(fs.iter()),
                 ty::Adt(adt, args) => {
                     if adt.is_box() {
                         // 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.
-                        cx.alloc_wildcard_slice(once(args.type_at(0)))
+                        cx.dropless_arena.alloc_from_iter(once(args.type_at(0)))
                     } else {
                         let variant =
-                            &adt.variant(MatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
+                            &adt.variant(RustcMatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
                         let tys = cx.list_variant_nonhidden_fields(ty, variant).map(|(_, ty)| ty);
-                        cx.alloc_wildcard_slice(tys)
+                        cx.dropless_arena.alloc_from_iter(tys)
                     }
                 }
-                _ => bug!("Unexpected type for `Single` constructor: {:?}", ty),
+                _ => bug!("Unexpected type for constructor `{ctor:?}`: {ty:?}"),
+            },
+            Ref => match ty.kind() {
+                ty::Ref(_, rty, _) => cx.dropless_arena.alloc_from_iter(once(*rty)),
+                _ => bug!("Unexpected type for `Ref` constructor: {ty:?}"),
             },
             Slice(slice) => match *ty.kind() {
                 ty::Slice(ty) | ty::Array(ty, _) => {
                     let arity = slice.arity();
-                    cx.alloc_wildcard_slice((0..arity).map(|_| ty))
+                    cx.dropless_arena.alloc_from_iter((0..arity).map(|_| ty))
                 }
                 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
             },
@@ -163,13 +192,11 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
         }
     }
 
-    /// The number of fields for this constructor. This must be kept in sync with
-    /// `Fields::wildcards`.
-    pub(crate) fn ctor_arity(&self, ctor: &Constructor<'tcx>, ty: Ty<'tcx>) -> usize {
+    /// The number of fields for this constructor.
+    pub(crate) fn ctor_arity(&self, ctor: &Constructor<'p, 'tcx>, ty: Ty<'tcx>) -> usize {
         match ctor {
-            Single | Variant(_) => match ty.kind() {
+            Struct | Variant(_) | UnionField => match ty.kind() {
                 ty::Tuple(fs) => fs.len(),
-                ty::Ref(..) => 1,
                 ty::Adt(adt, ..) => {
                     if adt.is_box() {
                         // The only legal patterns of type `Box` (outside `std`) are `_` and box
@@ -177,12 +204,13 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                         1
                     } else {
                         let variant =
-                            &adt.variant(MatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
+                            &adt.variant(RustcMatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
                         self.list_variant_nonhidden_fields(ty, variant).count()
                     }
                 }
-                _ => bug!("Unexpected type for `Single` constructor: {:?}", ty),
+                _ => bug!("Unexpected type for constructor `{ctor:?}`: {ty:?}"),
             },
+            Ref => 1,
             Slice(slice) => slice.arity(),
             Bool(..)
             | IntRange(..)
@@ -202,7 +230,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
     ///
     /// See [`crate::constructor`] for considerations of emptiness.
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn ctors_for_ty(&self, ty: Ty<'tcx>) -> ConstructorSet {
+    pub fn ctors_for_ty(&self, ty: Ty<'tcx>) -> ConstructorSet<'p, 'tcx> {
         let cx = self;
         let make_uint_range = |start, end| {
             IntRange::from_range(
@@ -298,9 +326,9 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                     ConstructorSet::Variants { variants, non_exhaustive: is_declared_nonexhaustive }
                 }
             }
-            ty::Adt(..) | ty::Tuple(..) | ty::Ref(..) => {
-                ConstructorSet::Single { empty: cx.is_uninhabited(ty) }
-            }
+            ty::Adt(def, _) if def.is_union() => ConstructorSet::Union,
+            ty::Adt(..) | ty::Tuple(..) => ConstructorSet::Struct { empty: cx.is_uninhabited(ty) },
+            ty::Ref(..) => ConstructorSet::Ref,
             ty::Never => ConstructorSet::NoConstructors,
             // This type is one for which we cannot list constructors, like `str` or `f64`.
             // FIXME(Nadrieril): which of these are actually allowed?
@@ -359,13 +387,18 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                 fields = &[];
             }
             PatKind::Deref { subpattern } => {
-                ctor = Single;
                 fields = singleton(self.lower_pat(subpattern));
+                ctor = match pat.ty.kind() {
+                    // This is a box pattern.
+                    ty::Adt(adt, ..) if adt.is_box() => Struct,
+                    ty::Ref(..) => Ref,
+                    _ => bug!("pattern has unexpected type: pat: {:?}, ty: {:?}", pat, pat.ty),
+                };
             }
             PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
                 match pat.ty.kind() {
                     ty::Tuple(fs) => {
-                        ctor = Single;
+                        ctor = Struct;
                         let mut wilds: SmallVec<[_; 2]> =
                             fs.iter().map(|ty| DeconstructedPat::wildcard(ty, pat.span)).collect();
                         for pat in subpatterns {
@@ -380,7 +413,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                         // _)` 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 ,
+                        // 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
@@ -392,17 +425,18 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                         } else {
                             DeconstructedPat::wildcard(args.type_at(0), pat.span)
                         };
-                        ctor = Single;
+                        ctor = Struct;
                         fields = singleton(pat);
                     }
                     ty::Adt(adt, _) => {
                         ctor = match pat.kind {
-                            PatKind::Leaf { .. } => Single,
+                            PatKind::Leaf { .. } if adt.is_union() => UnionField,
+                            PatKind::Leaf { .. } => Struct,
                             PatKind::Variant { variant_index, .. } => Variant(variant_index),
                             _ => bug!(),
                         };
                         let variant =
-                            &adt.variant(MatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
+                            &adt.variant(RustcMatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
                         // 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>> =
                             (0..variant.fields.len()).map(|_| None).collect();
@@ -477,11 +511,11 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                         // with other `Deref` patterns. This could have been done in `const_to_pat`,
                         // but that causes issues with the rest of the matching code.
                         // So here, the constructor for a `"foo"` pattern is `&` (represented by
-                        // `Single`), and has one field. That field has constructor `Str(value)` and no
-                        // fields.
+                        // `Ref`), and has one field. That field has constructor `Str(value)` and no
+                        // subfields.
                         // Note: `t` is `str`, not `&str`.
                         let subpattern = DeconstructedPat::new(Str(*value), &[], *t, pat.span);
-                        ctor = Single;
+                        ctor = Ref;
                         fields = singleton(subpattern)
                     }
                     // All constants that can be structurally matched have already been expanded
@@ -495,12 +529,16 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
             }
             PatKind::Range(patrange) => {
                 let PatRange { lo, hi, end, .. } = patrange.as_ref();
+                let end = match end {
+                    rustc_hir::RangeEnd::Included => RangeEnd::Included,
+                    rustc_hir::RangeEnd::Excluded => RangeEnd::Excluded,
+                };
                 let ty = pat.ty;
                 ctor = match ty.kind() {
                     ty::Char | ty::Int(_) | ty::Uint(_) => {
                         let lo = cx.lower_pat_range_bdy(*lo, ty);
                         let hi = cx.lower_pat_range_bdy(*hi, ty);
-                        IntRange(IntRange::from_range(lo, hi, *end))
+                        IntRange(IntRange::from_range(lo, hi, end))
                     }
                     ty::Float(fty) => {
                         use rustc_apfloat::Float;
@@ -511,13 +549,13 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                                 use rustc_apfloat::ieee::Single;
                                 let lo = lo.map(Single::from_bits).unwrap_or(-Single::INFINITY);
                                 let hi = hi.map(Single::from_bits).unwrap_or(Single::INFINITY);
-                                F32Range(lo, hi, *end)
+                                F32Range(lo, hi, end)
                             }
                             ty::FloatTy::F64 => {
                                 use rustc_apfloat::ieee::Double;
                                 let lo = lo.map(Double::from_bits).unwrap_or(-Double::INFINITY);
                                 let hi = hi.map(Double::from_bits).unwrap_or(Double::INFINITY);
-                                F64Range(lo, hi, *end)
+                                F64Range(lo, hi, end)
                             }
                         }
                     }
@@ -597,20 +635,6 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
         }
     }
 
-    /// Whether the range denotes the fictitious values before `isize::MIN` or after
-    /// `usize::MAX`/`isize::MAX` (see doc of [`IntRange::split`] for why these exist).
-    pub fn is_range_beyond_boundaries(&self, range: &IntRange, ty: Ty<'tcx>) -> bool {
-        ty.is_ptr_sized_integral() && {
-            // The two invalid ranges are `NegInfinity..isize::MIN` (represented as
-            // `NegInfinity..0`), and `{u,i}size::MAX+1..PosInfinity`. `hoist_pat_range_bdy`
-            // converts `MAX+1` to `PosInfinity`, and we couldn't have `PosInfinity` in `range.lo`
-            // otherwise.
-            let lo = self.hoist_pat_range_bdy(range.lo, ty);
-            matches!(lo, PatRangeBoundary::PosInfinity)
-                || matches!(range.hi, MaybeInfiniteInt::Finite(0))
-        }
-    }
-
     /// Convert back to a `thir::Pat` for diagnostic purposes.
     pub(crate) fn hoist_pat_range(&self, range: &IntRange, ty: Ty<'tcx>) -> Pat<'tcx> {
         use MaybeInfiniteInt::*;
@@ -623,7 +647,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
             PatKind::Constant { value }
         } else {
             // We convert to an inclusive range for diagnostics.
-            let mut end = RangeEnd::Included;
+            let mut end = rustc_hir::RangeEnd::Included;
             let mut lo = cx.hoist_pat_range_bdy(range.lo, ty);
             if matches!(lo, PatRangeBoundary::PosInfinity) {
                 // The only reason to get `PosInfinity` here is the special case where
@@ -637,7 +661,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
             }
             let hi = if matches!(range.hi, Finite(0)) {
                 // The range encodes `..ty::MIN`, so we can't convert it to an inclusive range.
-                end = RangeEnd::Excluded;
+                end = rustc_hir::RangeEnd::Excluded;
                 range.hi
             } else {
                 range.hi.minus_one()
@@ -650,14 +674,14 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
     }
     /// Convert back to a `thir::Pat` for diagnostic purposes. This panics for patterns that don't
     /// appear in diagnostics, like float ranges.
-    pub fn hoist_witness_pat(&self, pat: &WitnessPat<'tcx>) -> Pat<'tcx> {
+    pub fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> Pat<'tcx> {
         let cx = self;
         let is_wildcard = |pat: &Pat<'_>| matches!(pat.kind, PatKind::Wild);
         let mut subpatterns = pat.iter_fields().map(|p| Box::new(cx.hoist_witness_pat(p)));
         let kind = match pat.ctor() {
             Bool(b) => PatKind::Constant { value: mir::Const::from_bool(cx.tcx, *b) },
             IntRange(range) => return self.hoist_pat_range(range, pat.ty()),
-            Single | Variant(_) => match pat.ty().kind() {
+            Struct | Variant(_) | UnionField => match pat.ty().kind() {
                 ty::Tuple(..) => PatKind::Leaf {
                     subpatterns: subpatterns
                         .enumerate()
@@ -672,7 +696,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                 }
                 ty::Adt(adt_def, args) => {
                     let variant_index =
-                        MatchCheckCtxt::variant_index_for_adt(&pat.ctor(), *adt_def);
+                        RustcMatchCheckCtxt::variant_index_for_adt(&pat.ctor(), *adt_def);
                     let variant = &adt_def.variant(variant_index);
                     let subpatterns = cx
                         .list_variant_nonhidden_fields(pat.ty(), variant)
@@ -686,13 +710,13 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                         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
-                // ignore this issue.
-                ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
                 _ => bug!("unexpected ctor for type {:?} {:?}", pat.ctor(), pat.ty()),
             },
+            // 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
+            // ignore this issue.
+            Ref => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
             Slice(slice) => {
                 match slice.kind {
                     SliceKind::FixedLen(_) => PatKind::Slice {
@@ -744,7 +768,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
     /// Best-effort `Debug` implementation.
     pub(crate) fn debug_pat(
         f: &mut fmt::Formatter<'_>,
-        pat: &DeconstructedPat<'p, 'tcx>,
+        pat: &crate::pat::DeconstructedPat<'_, Self>,
     ) -> fmt::Result {
         let mut first = true;
         let mut start_or_continue = |s| {
@@ -758,7 +782,7 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
         let mut start_or_comma = || start_or_continue(", ");
 
         match pat.ctor() {
-            Single | Variant(_) => match pat.ty().kind() {
+            Struct | Variant(_) | UnionField => match pat.ty().kind() {
                 ty::Adt(def, _) if def.is_box() => {
                     // Without `box_patterns`, the only legal pattern of type `Box` is `_` (outside
                     // of `std`). So this branch is only reachable when the feature is enabled and
@@ -767,13 +791,14 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                     write!(f, "box {subpattern:?}")
                 }
                 ty::Adt(..) | ty::Tuple(..) => {
-                    let variant = match pat.ty().kind() {
-                        ty::Adt(adt, _) => Some(
-                            adt.variant(MatchCheckCtxt::variant_index_for_adt(pat.ctor(), *adt)),
-                        ),
-                        ty::Tuple(_) => None,
-                        _ => unreachable!(),
-                    };
+                    let variant =
+                        match pat.ty().kind() {
+                            ty::Adt(adt, _) => Some(adt.variant(
+                                RustcMatchCheckCtxt::variant_index_for_adt(pat.ctor(), *adt),
+                            )),
+                            ty::Tuple(_) => None,
+                            _ => unreachable!(),
+                        };
 
                     if let Some(variant) = variant {
                         write!(f, "{}", variant.name)?;
@@ -789,15 +814,15 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
                     }
                     write!(f, ")")
                 }
-                // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
-                // be careful to detect strings here. However a string literal pattern will never
-                // be reported as a non-exhaustiveness witness, so we can ignore this issue.
-                ty::Ref(_, _, mutbl) => {
-                    let subpattern = pat.iter_fields().next().unwrap();
-                    write!(f, "&{}{:?}", mutbl.prefix_str(), subpattern)
-                }
                 _ => write!(f, "_"),
             },
+            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
+            // be careful to detect strings here. However a string literal pattern will never
+            // be reported as a non-exhaustiveness witness, so we can ignore this issue.
+            Ref => {
+                let subpattern = pat.iter_fields().next().unwrap();
+                write!(f, "&{:?}", subpattern)
+            }
             Slice(slice) => {
                 let mut subpatterns = pat.iter_fields();
                 write!(f, "[")?;
@@ -838,6 +863,45 @@ impl<'p, 'tcx> MatchCheckCtxt<'p, 'tcx> {
     }
 }
 
+impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
+    type Ty = Ty<'tcx>;
+    type VariantIdx = VariantIdx;
+    type StrLit = Const<'tcx>;
+    type ArmData = HirId;
+    type PatData = Span;
+
+    fn is_exhaustive_patterns_feature_on(&self) -> bool {
+        self.tcx.features().exhaustive_patterns
+    }
+    fn is_opaque_ty(ty: Self::Ty) -> bool {
+        matches!(ty.kind(), ty::Alias(ty::Opaque, ..))
+    }
+
+    fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: Self::Ty) -> usize {
+        self.ctor_arity(ctor, ty)
+    }
+    fn ctor_sub_tys(
+        &self,
+        ctor: &crate::constructor::Constructor<Self>,
+        ty: Self::Ty,
+    ) -> &[Self::Ty] {
+        self.ctor_sub_tys(ctor, ty)
+    }
+    fn ctors_for_ty(&self, ty: Self::Ty) -> crate::constructor::ConstructorSet<Self> {
+        self.ctors_for_ty(ty)
+    }
+
+    fn debug_pat(
+        f: &mut fmt::Formatter<'_>,
+        pat: &crate::pat::DeconstructedPat<'_, Self>,
+    ) -> fmt::Result {
+        Self::debug_pat(f, pat)
+    }
+    fn bug(&self, fmt: fmt::Arguments<'_>) -> ! {
+        span_bug!(self.scrut_span, "{}", fmt)
+    }
+}
+
 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
 fn expand_or_pat<'p, 'tcx>(pat: &'p Pat<'tcx>) -> Vec<&'p Pat<'tcx>> {
     fn expand<'p, 'tcx>(pat: &'p Pat<'tcx>, vec: &mut Vec<&'p Pat<'tcx>>) {
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index f268a551547..6b1de807797 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -242,7 +242,7 @@
 //! Therefore `usefulness(tp_1, tp_2, tq)` returns the single witness-tuple `[Variant2(Some(true), 0)]`.
 //!
 //!
-//! Computing the set of constructors for a type is done in [`MatchCheckCtxt::ctors_for_ty`]. See
+//! Computing the set of constructors for a type is done in [`TypeCx::ctors_for_ty`]. See
 //! the following sections for more accurate versions of the algorithm and corresponding links.
 //!
 //!
@@ -555,37 +555,52 @@
 use smallvec::{smallvec, SmallVec};
 use std::fmt;
 
-use rustc_data_structures::{captures::Captures, stack::ensure_sufficient_stack};
-use rustc_middle::ty::{self, Ty};
-use rustc_span::{Span, DUMMY_SP};
-
 use crate::constructor::{Constructor, ConstructorSet};
-use crate::cx::MatchCheckCtxt;
 use crate::pat::{DeconstructedPat, WitnessPat};
-use crate::MatchArm;
+use crate::{Captures, MatchArm, MatchCtxt, TypeCx, TypedArena};
 
 use self::ValidityConstraint::*;
 
-#[derive(Copy, Clone)]
-pub(crate) struct PatCtxt<'a, 'p, 'tcx> {
-    pub(crate) cx: &'a MatchCheckCtxt<'p, 'tcx>,
-    /// Type of the current column under investigation.
-    pub(crate) ty: Ty<'tcx>,
-    /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
-    /// subpattern.
-    pub(crate) is_top_level: bool,
+#[cfg(feature = "rustc")]
+use rustc_data_structures::stack::ensure_sufficient_stack;
+#[cfg(not(feature = "rustc"))]
+pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
+    f()
 }
 
-impl<'a, 'p, 'tcx> PatCtxt<'a, 'p, 'tcx> {
-    /// A `PatCtxt` when code other than `is_useful` needs one.
-    pub(crate) fn new_dummy(cx: &'a MatchCheckCtxt<'p, 'tcx>, ty: Ty<'tcx>) -> Self {
-        PatCtxt { cx, ty, is_top_level: false }
+/// Context that provides information local to a place under investigation.
+#[derive(Clone)]
+pub(crate) struct PlaceCtxt<'a, 'p, Cx: TypeCx> {
+    pub(crate) mcx: MatchCtxt<'a, 'p, Cx>,
+    /// Type of the place under investigation.
+    pub(crate) ty: Cx::Ty,
+    /// Whether the place is the original scrutinee place, as opposed to a subplace of it.
+    pub(crate) is_scrutinee: bool,
+}
+
+impl<'a, 'p, Cx: TypeCx> PlaceCtxt<'a, 'p, Cx> {
+    /// A `PlaceCtxt` when code other than `is_useful` needs one.
+    #[cfg_attr(not(feature = "rustc"), allow(dead_code))]
+    pub(crate) fn new_dummy(mcx: MatchCtxt<'a, 'p, Cx>, ty: Cx::Ty) -> Self {
+        PlaceCtxt { mcx, ty, is_scrutinee: false }
+    }
+
+    pub(crate) fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
+        self.mcx.tycx.ctor_arity(ctor, self.ty)
+    }
+    pub(crate) fn ctor_sub_tys(&self, ctor: &Constructor<Cx>) -> &[Cx::Ty] {
+        self.mcx.tycx.ctor_sub_tys(ctor, self.ty)
+    }
+    pub(crate) fn ctors_for_ty(&self) -> ConstructorSet<Cx> {
+        self.mcx.tycx.ctors_for_ty(self.ty)
     }
 }
 
-impl<'a, 'p, 'tcx> fmt::Debug for PatCtxt<'a, 'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> Copy for PlaceCtxt<'a, 'p, Cx> {}
+
+impl<'a, 'p, Cx: TypeCx> fmt::Debug for PlaceCtxt<'a, 'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("PatCtxt").field("ty", &self.ty).finish()
+        f.debug_struct("PlaceCtxt").field("ty", &self.ty).finish()
     }
 }
 
@@ -595,7 +610,7 @@ impl<'a, 'p, 'tcx> fmt::Debug for PatCtxt<'a, 'p, 'tcx> {
 /// - in the matrix, track whether a given place (aka column) is known to contain a valid value or
 ///     not.
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
-enum ValidityConstraint {
+pub enum ValidityConstraint {
     ValidOnly,
     MaybeInvalid,
     /// Option for backwards compatibility: the place is not known to be valid but we allow omitting
@@ -604,7 +619,7 @@ enum ValidityConstraint {
 }
 
 impl ValidityConstraint {
-    fn from_bool(is_valid_only: bool) -> Self {
+    pub fn from_bool(is_valid_only: bool) -> Self {
         if is_valid_only { ValidOnly } else { MaybeInvalid }
     }
 
@@ -629,12 +644,9 @@ impl ValidityConstraint {
     ///
     /// Pending further opsem decisions, the current behavior is: validity is preserved, except
     /// inside `&` and union fields where validity is reset to `MaybeInvalid`.
-    fn specialize<'tcx>(self, pcx: &PatCtxt<'_, '_, 'tcx>, ctor: &Constructor<'tcx>) -> Self {
+    fn specialize<Cx: TypeCx>(self, ctor: &Constructor<Cx>) -> Self {
         // We preserve validity except when we go inside a reference or a union field.
-        if matches!(ctor, Constructor::Single)
-            && (matches!(pcx.ty.kind(), ty::Ref(..))
-                || matches!(pcx.ty.kind(), ty::Adt(def, ..) if def.is_union()))
-        {
+        if matches!(ctor, Constructor::Ref | Constructor::UnionField) {
             // Validity of `x: &T` does not imply validity of `*x: T`.
             MaybeInvalid
         } else {
@@ -654,14 +666,18 @@ impl fmt::Display for ValidityConstraint {
 }
 
 /// Represents a pattern-tuple under investigation.
+// The three lifetimes are:
+// - 'a allocated by us
+// - 'p coming from the input
+// - Cx global compilation context
 #[derive(Clone)]
-struct PatStack<'p, 'tcx> {
+struct PatStack<'a, 'p, Cx: TypeCx> {
     // Rows of len 1 are very common, which is why `SmallVec[_; 2]` works well.
-    pats: SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]>,
+    pats: SmallVec<[&'a DeconstructedPat<'p, Cx>; 2]>,
 }
 
-impl<'p, 'tcx> PatStack<'p, 'tcx> {
-    fn from_pattern(pat: &'p DeconstructedPat<'p, 'tcx>) -> Self {
+impl<'a, 'p, Cx: TypeCx> PatStack<'a, 'p, Cx> {
+    fn from_pattern(pat: &'a DeconstructedPat<'p, Cx>) -> Self {
         PatStack { pats: smallvec![pat] }
     }
 
@@ -673,17 +689,17 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
         self.pats.len()
     }
 
-    fn head(&self) -> &'p DeconstructedPat<'p, 'tcx> {
+    fn head(&self) -> &'a DeconstructedPat<'p, Cx> {
         self.pats[0]
     }
 
-    fn iter(&self) -> impl Iterator<Item = &DeconstructedPat<'p, 'tcx>> {
+    fn iter<'b>(&'b self) -> impl Iterator<Item = &'a DeconstructedPat<'p, Cx>> + Captures<'b> {
         self.pats.iter().copied()
     }
 
     // Recursively expand the first or-pattern into its subpatterns. Only useful if the pattern is
     // an or-pattern. Panics if `self` is empty.
-    fn expand_or_pat<'a>(&'a self) -> impl Iterator<Item = PatStack<'p, 'tcx>> + Captures<'a> {
+    fn expand_or_pat<'b>(&'b self) -> impl Iterator<Item = PatStack<'a, 'p, Cx>> + Captures<'b> {
         self.head().flatten_or_pat().into_iter().map(move |pat| {
             let mut new = self.clone();
             new.pats[0] = pat;
@@ -695,9 +711,9 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
-        pcx: &PatCtxt<'_, 'p, 'tcx>,
-        ctor: &Constructor<'tcx>,
-    ) -> PatStack<'p, 'tcx> {
+        pcx: &PlaceCtxt<'a, 'p, Cx>,
+        ctor: &Constructor<Cx>,
+    ) -> PatStack<'a, 'p, Cx> {
         // We pop the head pattern and push the new fields extracted from the arguments of
         // `self.head()`.
         let mut new_pats = self.head().specialize(pcx, ctor);
@@ -706,7 +722,7 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     }
 }
 
-impl<'p, 'tcx> fmt::Debug for PatStack<'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> fmt::Debug for PatStack<'a, 'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // We pretty-print similarly to the `Debug` impl of `Matrix`.
         write!(f, "+")?;
@@ -719,9 +735,9 @@ impl<'p, 'tcx> fmt::Debug for PatStack<'p, 'tcx> {
 
 /// A row of the matrix.
 #[derive(Clone)]
-struct MatrixRow<'p, 'tcx> {
+struct MatrixRow<'a, 'p, Cx: TypeCx> {
     // The patterns in the row.
-    pats: PatStack<'p, 'tcx>,
+    pats: PatStack<'a, 'p, Cx>,
     /// Whether the original arm had a guard. This is inherited when specializing.
     is_under_guard: bool,
     /// When we specialize, we remember which row of the original matrix produced a given row of the
@@ -734,7 +750,7 @@ struct MatrixRow<'p, 'tcx> {
     useful: bool,
 }
 
-impl<'p, 'tcx> MatrixRow<'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> MatrixRow<'a, 'p, Cx> {
     fn is_empty(&self) -> bool {
         self.pats.is_empty()
     }
@@ -743,17 +759,17 @@ impl<'p, 'tcx> MatrixRow<'p, 'tcx> {
         self.pats.len()
     }
 
-    fn head(&self) -> &'p DeconstructedPat<'p, 'tcx> {
+    fn head(&self) -> &'a DeconstructedPat<'p, Cx> {
         self.pats.head()
     }
 
-    fn iter(&self) -> impl Iterator<Item = &DeconstructedPat<'p, 'tcx>> {
+    fn iter<'b>(&'b self) -> impl Iterator<Item = &'a DeconstructedPat<'p, Cx>> + Captures<'b> {
         self.pats.iter()
     }
 
     // Recursively expand the first or-pattern into its subpatterns. Only useful if the pattern is
     // an or-pattern. Panics if `self` is empty.
-    fn expand_or_pat<'a>(&'a self) -> impl Iterator<Item = MatrixRow<'p, 'tcx>> + Captures<'a> {
+    fn expand_or_pat<'b>(&'b self) -> impl Iterator<Item = MatrixRow<'a, 'p, Cx>> + Captures<'b> {
         self.pats.expand_or_pat().map(|patstack| MatrixRow {
             pats: patstack,
             parent_row: self.parent_row,
@@ -766,10 +782,10 @@ impl<'p, 'tcx> MatrixRow<'p, 'tcx> {
     /// Only call if `ctor.is_covered_by(self.head().ctor())` is true.
     fn pop_head_constructor(
         &self,
-        pcx: &PatCtxt<'_, 'p, 'tcx>,
-        ctor: &Constructor<'tcx>,
+        pcx: &PlaceCtxt<'a, 'p, Cx>,
+        ctor: &Constructor<Cx>,
         parent_row: usize,
-    ) -> MatrixRow<'p, 'tcx> {
+    ) -> MatrixRow<'a, 'p, Cx> {
         MatrixRow {
             pats: self.pats.pop_head_constructor(pcx, ctor),
             parent_row,
@@ -779,7 +795,7 @@ impl<'p, 'tcx> MatrixRow<'p, 'tcx> {
     }
 }
 
-impl<'p, 'tcx> fmt::Debug for MatrixRow<'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> fmt::Debug for MatrixRow<'a, 'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.pats.fmt(f)
     }
@@ -796,22 +812,22 @@ impl<'p, 'tcx> fmt::Debug for MatrixRow<'p, 'tcx> {
 /// specializing `(,)` and `Some` on a pattern of type `(Option<u32>, bool)`, the first column of
 /// the matrix will correspond to `scrutinee.0.Some.0` and the second column to `scrutinee.1`.
 #[derive(Clone)]
-struct Matrix<'p, 'tcx> {
+struct Matrix<'a, 'p, Cx: TypeCx> {
     /// Vector of rows. The rows must form a rectangular 2D array. Moreover, all the patterns of
     /// each column must have the same type. Each column corresponds to a place within the
     /// scrutinee.
-    rows: Vec<MatrixRow<'p, 'tcx>>,
+    rows: Vec<MatrixRow<'a, 'p, Cx>>,
     /// Stores an extra fictitious row full of wildcards. Mostly used to keep track of the type of
     /// each column. This must obey the same invariants as the real rows.
-    wildcard_row: PatStack<'p, 'tcx>,
+    wildcard_row: PatStack<'a, 'p, Cx>,
     /// Track for each column/place whether it contains a known valid value.
     place_validity: SmallVec<[ValidityConstraint; 2]>,
 }
 
-impl<'p, 'tcx> Matrix<'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> Matrix<'a, 'p, Cx> {
     /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
     /// expands it. Internal method, prefer [`Matrix::new`].
-    fn expand_and_push(&mut self, row: MatrixRow<'p, 'tcx>) {
+    fn expand_and_push(&mut self, row: MatrixRow<'a, 'p, Cx>) {
         if !row.is_empty() && row.head().is_or_pat() {
             // Expand nested or-patterns.
             for new_row in row.expand_or_pat() {
@@ -823,16 +839,14 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
     }
 
     /// Build a new matrix from an iterator of `MatchArm`s.
-    fn new<'a>(
-        cx: &MatchCheckCtxt<'p, 'tcx>,
-        arms: &[MatchArm<'p, 'tcx>],
-        scrut_ty: Ty<'tcx>,
+    fn new(
+        wildcard_arena: &'a TypedArena<DeconstructedPat<'p, Cx>>,
+        arms: &'a [MatchArm<'p, Cx>],
+        scrut_ty: Cx::Ty,
         scrut_validity: ValidityConstraint,
-    ) -> Self
-    where
-        'p: 'a,
-    {
-        let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty, DUMMY_SP));
+    ) -> Self {
+        let wild_pattern =
+            wildcard_arena.alloc(DeconstructedPat::wildcard(scrut_ty, Default::default()));
         let wildcard_row = PatStack::from_pattern(wild_pattern);
         let mut matrix = Matrix {
             rows: Vec::with_capacity(arms.len()),
@@ -851,7 +865,7 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         matrix
     }
 
-    fn head_ty(&self) -> Option<Ty<'tcx>> {
+    fn head_ty(&self) -> Option<Cx::Ty> {
         if self.column_count() == 0 {
             return None;
         }
@@ -859,11 +873,10 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         let mut ty = self.wildcard_row.head().ty();
         // If the type is opaque and it is revealed anywhere in the column, we take the revealed
         // version. Otherwise we could encounter constructors for the revealed type and crash.
-        let is_opaque = |ty: Ty<'tcx>| matches!(ty.kind(), ty::Alias(ty::Opaque, ..));
-        if is_opaque(ty) {
+        if Cx::is_opaque_ty(ty) {
             for pat in self.heads() {
                 let pat_ty = pat.ty();
-                if !is_opaque(pat_ty) {
+                if !Cx::is_opaque_ty(pat_ty) {
                     ty = pat_ty;
                     break;
                 }
@@ -875,34 +888,34 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         self.wildcard_row.len()
     }
 
-    fn rows<'a>(
-        &'a self,
-    ) -> impl Iterator<Item = &'a MatrixRow<'p, 'tcx>> + Clone + DoubleEndedIterator + ExactSizeIterator
+    fn rows<'b>(
+        &'b self,
+    ) -> impl Iterator<Item = &'b MatrixRow<'a, 'p, Cx>> + Clone + DoubleEndedIterator + ExactSizeIterator
     {
         self.rows.iter()
     }
-    fn rows_mut<'a>(
-        &'a mut self,
-    ) -> impl Iterator<Item = &'a mut MatrixRow<'p, 'tcx>> + DoubleEndedIterator + ExactSizeIterator
+    fn rows_mut<'b>(
+        &'b mut self,
+    ) -> impl Iterator<Item = &'b mut MatrixRow<'a, 'p, Cx>> + DoubleEndedIterator + ExactSizeIterator
     {
         self.rows.iter_mut()
     }
 
     /// Iterate over the first pattern of each row.
-    fn heads<'a>(
-        &'a self,
-    ) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Clone + Captures<'a> {
+    fn heads<'b>(
+        &'b self,
+    ) -> impl Iterator<Item = &'b DeconstructedPat<'p, Cx>> + Clone + Captures<'a> {
         self.rows().map(|r| r.head())
     }
 
     /// This computes `specialize(ctor, self)`. See top of the file for explanations.
     fn specialize_constructor(
         &self,
-        pcx: &PatCtxt<'_, 'p, 'tcx>,
-        ctor: &Constructor<'tcx>,
-    ) -> Matrix<'p, 'tcx> {
+        pcx: &PlaceCtxt<'a, 'p, Cx>,
+        ctor: &Constructor<Cx>,
+    ) -> Matrix<'a, 'p, Cx> {
         let wildcard_row = self.wildcard_row.pop_head_constructor(pcx, ctor);
-        let new_validity = self.place_validity[0].specialize(pcx, ctor);
+        let new_validity = self.place_validity[0].specialize(ctor);
         let new_place_validity = std::iter::repeat(new_validity)
             .take(ctor.arity(pcx))
             .chain(self.place_validity[1..].iter().copied())
@@ -929,7 +942,7 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
 /// + _     + [_, _, tail @ ..] +
 /// | ✓     | ?                 | // column validity
 /// ```
-impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
+impl<'a, 'p, Cx: TypeCx> fmt::Debug for Matrix<'a, 'p, Cx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "\n")?;
 
@@ -1020,17 +1033,17 @@ impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
 ///
 /// See the top of the file for more detailed explanations and examples.
 #[derive(Debug, Clone)]
-struct WitnessStack<'tcx>(Vec<WitnessPat<'tcx>>);
+struct WitnessStack<Cx: TypeCx>(Vec<WitnessPat<Cx>>);
 
-impl<'tcx> WitnessStack<'tcx> {
+impl<Cx: TypeCx> WitnessStack<Cx> {
     /// Asserts that the witness contains a single pattern, and returns it.
-    fn single_pattern(self) -> WitnessPat<'tcx> {
+    fn single_pattern(self) -> WitnessPat<Cx> {
         assert_eq!(self.0.len(), 1);
         self.0.into_iter().next().unwrap()
     }
 
     /// Reverses specialization by the `Missing` constructor by pushing a whole new pattern.
-    fn push_pattern(&mut self, pat: WitnessPat<'tcx>) {
+    fn push_pattern(&mut self, pat: WitnessPat<Cx>) {
         self.0.push(pat);
     }
 
@@ -1048,7 +1061,7 @@ impl<'tcx> WitnessStack<'tcx> {
     /// pats: [(false, "foo"), _, true]
     /// result: [Enum::Variant { a: (false, "foo"), b: _ }, true]
     /// ```
-    fn apply_constructor(&mut self, pcx: &PatCtxt<'_, '_, 'tcx>, ctor: &Constructor<'tcx>) {
+    fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, '_, Cx>, ctor: &Constructor<Cx>) {
         let len = self.0.len();
         let arity = ctor.arity(pcx);
         let fields = self.0.drain((len - arity)..).rev().collect();
@@ -1067,9 +1080,9 @@ impl<'tcx> WitnessStack<'tcx> {
 /// Just as the `Matrix` starts with a single column, by the end of the algorithm, this has a single
 /// column, which contains the patterns that are missing for the match to be exhaustive.
 #[derive(Debug, Clone)]
-struct WitnessMatrix<'tcx>(Vec<WitnessStack<'tcx>>);
+struct WitnessMatrix<Cx: TypeCx>(Vec<WitnessStack<Cx>>);
 
-impl<'tcx> WitnessMatrix<'tcx> {
+impl<Cx: TypeCx> WitnessMatrix<Cx> {
     /// New matrix with no witnesses.
     fn empty() -> Self {
         WitnessMatrix(vec![])
@@ -1084,12 +1097,12 @@ impl<'tcx> WitnessMatrix<'tcx> {
         self.0.is_empty()
     }
     /// Asserts that there is a single column and returns the patterns in it.
-    fn single_column(self) -> Vec<WitnessPat<'tcx>> {
+    fn single_column(self) -> Vec<WitnessPat<Cx>> {
         self.0.into_iter().map(|w| w.single_pattern()).collect()
     }
 
     /// Reverses specialization by the `Missing` constructor by pushing a whole new pattern.
-    fn push_pattern(&mut self, pat: WitnessPat<'tcx>) {
+    fn push_pattern(&mut self, pat: WitnessPat<Cx>) {
         for witness in self.0.iter_mut() {
             witness.push_pattern(pat.clone())
         }
@@ -1098,9 +1111,9 @@ impl<'tcx> WitnessMatrix<'tcx> {
     /// Reverses specialization by `ctor`. See the section on `unspecialize` at the top of the file.
     fn apply_constructor(
         &mut self,
-        pcx: &PatCtxt<'_, '_, 'tcx>,
-        missing_ctors: &[Constructor<'tcx>],
-        ctor: &Constructor<'tcx>,
+        pcx: &PlaceCtxt<'_, '_, Cx>,
+        missing_ctors: &[Constructor<Cx>],
+        ctor: &Constructor<Cx>,
         report_individual_missing_ctors: bool,
     ) {
         if self.is_empty() {
@@ -1160,12 +1173,12 @@ impl<'tcx> WitnessMatrix<'tcx> {
 /// - unspecialization, where we lift the results from the previous step into results for this step
 ///     (using `apply_constructor` and by updating `row.useful` for each parent row).
 /// This is all explained at the top of the file.
-#[instrument(level = "debug", skip(cx, is_top_level), ret)]
-fn compute_exhaustiveness_and_usefulness<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
-    matrix: &mut Matrix<'p, 'tcx>,
+#[instrument(level = "debug", skip(mcx, is_top_level), ret)]
+fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
+    mcx: MatchCtxt<'a, 'p, Cx>,
+    matrix: &mut Matrix<'a, 'p, Cx>,
     is_top_level: bool,
-) -> WitnessMatrix<'tcx> {
+) -> WitnessMatrix<Cx> {
     debug_assert!(matrix.rows().all(|r| r.len() == matrix.column_count()));
 
     let Some(ty) = matrix.head_ty() else {
@@ -1185,7 +1198,7 @@ fn compute_exhaustiveness_and_usefulness<'p, 'tcx>(
     };
 
     debug!("ty: {ty:?}");
-    let pcx = &PatCtxt { cx, ty, is_top_level };
+    let pcx = &PlaceCtxt { mcx, ty, is_scrutinee: is_top_level };
 
     // Whether the place/column we are inspecting is known to contain valid data.
     let place_validity = matrix.place_validity[0];
@@ -1194,7 +1207,7 @@ fn compute_exhaustiveness_and_usefulness<'p, 'tcx>(
 
     // Analyze the constructors present in this column.
     let ctors = matrix.heads().map(|p| p.ctor());
-    let ctors_for_ty = &cx.ctors_for_ty(ty);
+    let ctors_for_ty = pcx.ctors_for_ty();
     let is_integers = matches!(ctors_for_ty, ConstructorSet::Integers { .. }); // For diagnostics.
     let split_set = ctors_for_ty.split(pcx, ctors);
     let all_missing = split_set.present.is_empty();
@@ -1228,7 +1241,7 @@ fn compute_exhaustiveness_and_usefulness<'p, 'tcx>(
         // Dig into rows that match `ctor`.
         let mut spec_matrix = matrix.specialize_constructor(pcx, &ctor);
         let mut witnesses = ensure_sufficient_stack(|| {
-            compute_exhaustiveness_and_usefulness(cx, &mut spec_matrix, false)
+            compute_exhaustiveness_and_usefulness(mcx, &mut spec_matrix, false)
         });
 
         let counts_for_exhaustiveness = match ctor {
@@ -1270,34 +1283,34 @@ fn compute_exhaustiveness_and_usefulness<'p, 'tcx>(
 
 /// Indicates whether or not a given arm is useful.
 #[derive(Clone, Debug)]
-pub enum Usefulness {
+pub enum Usefulness<'p, Cx: TypeCx> {
     /// The arm is useful. This additionally carries a set of or-pattern branches that have been
     /// found to be redundant despite the overall arm being useful. Used only in the presence of
     /// or-patterns, otherwise it stays empty.
-    Useful(Vec<Span>),
+    Useful(Vec<&'p DeconstructedPat<'p, Cx>>),
     /// The arm is redundant and can be removed without changing the behavior of the match
     /// expression.
     Redundant,
 }
 
 /// The output of checking a match for exhaustiveness and arm usefulness.
-pub struct UsefulnessReport<'p, 'tcx> {
+pub struct UsefulnessReport<'p, Cx: TypeCx> {
     /// For each arm of the input, whether that arm is useful after the arms above it.
-    pub arm_usefulness: Vec<(MatchArm<'p, 'tcx>, Usefulness)>,
+    pub arm_usefulness: Vec<(MatchArm<'p, Cx>, Usefulness<'p, Cx>)>,
     /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
     /// exhaustiveness.
-    pub non_exhaustiveness_witnesses: Vec<WitnessPat<'tcx>>,
+    pub non_exhaustiveness_witnesses: Vec<WitnessPat<Cx>>,
 }
 
 /// Computes whether a match is exhaustive and which of its arms are useful.
 #[instrument(skip(cx, arms), level = "debug")]
-pub(crate) fn compute_match_usefulness<'p, 'tcx>(
-    cx: &MatchCheckCtxt<'p, 'tcx>,
-    arms: &[MatchArm<'p, 'tcx>],
-    scrut_ty: Ty<'tcx>,
-) -> UsefulnessReport<'p, 'tcx> {
-    let scrut_validity = ValidityConstraint::from_bool(cx.known_valid_scrutinee);
-    let mut matrix = Matrix::new(cx, arms, scrut_ty, scrut_validity);
+pub fn compute_match_usefulness<'p, Cx: TypeCx>(
+    cx: MatchCtxt<'_, 'p, Cx>,
+    arms: &[MatchArm<'p, Cx>],
+    scrut_ty: Cx::Ty,
+    scrut_validity: ValidityConstraint,
+) -> UsefulnessReport<'p, Cx> {
+    let mut matrix = Matrix::new(cx.wildcard_arena, arms, scrut_ty, scrut_validity);
     let non_exhaustiveness_witnesses = compute_exhaustiveness_and_usefulness(cx, &mut matrix, true);
 
     let non_exhaustiveness_witnesses: Vec<_> = non_exhaustiveness_witnesses.single_column();
@@ -1308,7 +1321,7 @@ pub(crate) fn compute_match_usefulness<'p, 'tcx>(
             debug!(?arm);
             // We warn when a pattern is not useful.
             let usefulness = if arm.pat.is_useful() {
-                Usefulness::Useful(arm.pat.redundant_spans())
+                Usefulness::Useful(arm.pat.redundant_subpatterns())
             } else {
                 Usefulness::Redundant
             };
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 3bfe811b58e..3c00027b9fd 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -357,6 +357,7 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "tracing-tree",
     "twox-hash",
     "type-map",
+    "typed-arena",
     "typenum",
     "unic-langid",
     "unic-langid-impl",