From 0b2579a1b617e83da98008bac9b93c36c43a8f4b Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Wed, 24 Jan 2024 20:38:21 +0100
Subject: [PATCH 1/5] Make `PatternColumn` generic in `Cx`

---
 compiler/rustc_pattern_analysis/src/lints.rs | 43 +++++++++-----------
 compiler/rustc_pattern_analysis/src/rustc.rs |  4 --
 2 files changed, 20 insertions(+), 27 deletions(-)

diff --git a/compiler/rustc_pattern_analysis/src/lints.rs b/compiler/rustc_pattern_analysis/src/lints.rs
index 4bfe7dfb072c4..e3d218d71c301 100644
--- a/compiler/rustc_pattern_analysis/src/lints.rs
+++ b/compiler/rustc_pattern_analysis/src/lints.rs
@@ -1,12 +1,12 @@
 use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::ErrorGuaranteed;
 
+use crate::constructor::{Constructor, SplitConstructorSet};
 use crate::errors::{NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Uncovered};
-use crate::pat::PatOrWild;
-use crate::rustc::{
-    Constructor, DeconstructedPat, MatchArm, MatchCtxt, PlaceCtxt, RevealedTy, RustcMatchCheckCtxt,
-    SplitConstructorSet, WitnessPat,
-};
+use crate::pat::{DeconstructedPat, PatOrWild};
+use crate::rustc::{MatchCtxt, RevealedTy, RustcMatchCheckCtxt, WitnessPat};
+use crate::usefulness::PlaceCtxt;
+use crate::{MatchArm, TypeCx};
 
 /// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that
 /// inspect the same subvalue/place".
@@ -19,12 +19,12 @@ use crate::rustc::{
 ///
 /// This is not used in the usefulness algorithm; only in lints.
 #[derive(Debug)]
-pub(crate) struct PatternColumn<'p, 'tcx> {
-    patterns: Vec<&'p DeconstructedPat<'p, 'tcx>>,
+pub(crate) struct PatternColumn<'p, Cx: TypeCx> {
+    patterns: Vec<&'p DeconstructedPat<'p, Cx>>,
 }
 
-impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
-    pub(crate) fn new(arms: &[MatchArm<'p, 'tcx>]) -> Self {
+impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
+    pub(crate) fn new(arms: &[MatchArm<'p, Cx>]) -> Self {
         let patterns = Vec::with_capacity(arms.len());
         let mut column = PatternColumn { patterns };
         for arm in arms {
@@ -34,7 +34,7 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
     }
     /// Pushes a pattern onto the column, expanding any or-patterns into its subpatterns.
     /// Internal method, prefer [`PatternColumn::new`].
-    fn expand_and_push(&mut self, pat: PatOrWild<'p, RustcMatchCheckCtxt<'p, 'tcx>>) {
+    fn expand_and_push(&mut self, pat: PatOrWild<'p, Cx>) {
         // We flatten or-patterns and skip algorithm-generated wildcards.
         if pat.is_or_pat() {
             self.patterns.extend(
@@ -45,15 +45,12 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
         }
     }
 
-    fn head_ty(&self) -> Option<RevealedTy<'tcx>> {
-        self.patterns.first().map(|pat| *pat.ty())
+    fn head_ty(&self) -> Option<&Cx::Ty> {
+        self.patterns.first().map(|pat| pat.ty())
     }
 
     /// Do constructor splitting on the constructors of the column.
-    fn analyze_ctors(
-        &self,
-        pcx: &PlaceCtxt<'_, 'p, 'tcx>,
-    ) -> Result<SplitConstructorSet<'p, 'tcx>, ErrorGuaranteed> {
+    fn analyze_ctors(&self, pcx: &PlaceCtxt<'_, Cx>) -> Result<SplitConstructorSet<Cx>, Cx::Error> {
         let column_ctors = self.patterns.iter().map(|p| p.ctor());
         let ctors_for_ty = &pcx.ctors_for_ty()?;
         Ok(ctors_for_ty.split(column_ctors))
@@ -66,9 +63,9 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
     /// which may change the lengths.
     fn specialize(
         &self,
-        pcx: &PlaceCtxt<'_, 'p, 'tcx>,
-        ctor: &Constructor<'p, 'tcx>,
-    ) -> Vec<PatternColumn<'p, 'tcx>> {
+        pcx: &PlaceCtxt<'_, Cx>,
+        ctor: &Constructor<Cx>,
+    ) -> Vec<PatternColumn<'p, Cx>> {
         let arity = ctor.arity(pcx);
         if arity == 0 {
             return Vec::new();
@@ -96,9 +93,9 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
 #[instrument(level = "debug", skip(cx), ret)]
 fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
     cx: MatchCtxt<'a, 'p, 'tcx>,
-    column: &PatternColumn<'p, 'tcx>,
+    column: &PatternColumn<'p, RustcMatchCheckCtxt<'p, 'tcx>>,
 ) -> Result<Vec<WitnessPat<'p, 'tcx>>, ErrorGuaranteed> {
-    let Some(ty) = column.head_ty() else {
+    let Some(&ty) = column.head_ty() else {
         return Ok(Vec::new());
     };
     let pcx = &PlaceCtxt::new_dummy(cx, &ty);
@@ -143,8 +140,8 @@ fn collect_nonexhaustive_missing_variants<'a, '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>,
+    arms: &[MatchArm<'p, RustcMatchCheckCtxt<'p, 'tcx>>],
+    pat_column: &PatternColumn<'p, RustcMatchCheckCtxt<'p, 'tcx>>,
     scrut_ty: RevealedTy<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
     let rcx: &RustcMatchCheckCtxt<'_, '_> = cx.tycx;
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index 223d6cefc83fe..4fb76a891b534 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -31,10 +31,6 @@ 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, RustcMatchCheckCtxt<'p, 'tcx>>;
-pub(crate) type PlaceCtxt<'a, 'p, 'tcx> =
-    crate::usefulness::PlaceCtxt<'a, 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>>;

From cb0e8c508c98590346f5943f1ebd0e901acf87f9 Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Wed, 24 Jan 2024 21:16:57 +0100
Subject: [PATCH 2/5] Limit the use of `PlaceCtxt`

---
 .../rustc_pattern_analysis/src/constructor.rs | 16 ++++-----
 compiler/rustc_pattern_analysis/src/lib.rs    |  2 +-
 compiler/rustc_pattern_analysis/src/lints.rs  | 34 +++++++++----------
 compiler/rustc_pattern_analysis/src/pat.rs    |  7 ++--
 compiler/rustc_pattern_analysis/src/rustc.rs  |  1 -
 .../rustc_pattern_analysis/src/usefulness.rs  | 34 ++++++++-----------
 6 files changed, 42 insertions(+), 52 deletions(-)

diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index 4996015f86345..df5bf3111adc4 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -163,7 +163,6 @@ use self::MaybeInfiniteInt::*;
 use self::SliceKind::*;
 
 use crate::index;
-use crate::usefulness::PlaceCtxt;
 use crate::TypeCx;
 
 /// Whether we have seen a constructor in the column or not.
@@ -818,8 +817,8 @@ impl<Cx: TypeCx> Constructor<Cx> {
 
     /// The number of fields for this constructor. This must be kept in sync with
     /// `Fields::wildcards`.
-    pub(crate) fn arity(&self, pcx: &PlaceCtxt<'_, Cx>) -> usize {
-        pcx.ctor_arity(self)
+    pub(crate) fn arity(&self, cx: &Cx, ty: &Cx::Ty) -> usize {
+        cx.ctor_arity(self, ty)
     }
 
     /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
@@ -827,12 +826,11 @@ impl<Cx: TypeCx> Constructor<Cx> {
     /// this checks for inclusion.
     // We inline because this has a single call site in `Matrix::specialize_constructor`.
     #[inline]
-    pub(crate) fn is_covered_by(&self, pcx: &PlaceCtxt<'_, Cx>, other: &Self) -> bool {
+    pub(crate) fn is_covered_by(&self, cx: &Cx, other: &Self) -> bool {
         match (self, other) {
-            (Wildcard, _) => pcx
-                .mcx
-                .tycx
-                .bug(format_args!("Constructor splitting should not have returned `Wildcard`")),
+            (Wildcard, _) => {
+                cx.bug(format_args!("Constructor splitting should not have returned `Wildcard`"))
+            }
             // Wildcards cover anything
             (_, Wildcard) => true,
             // Only a wildcard pattern can match these special constructors.
@@ -873,7 +871,7 @@ impl<Cx: TypeCx> Constructor<Cx> {
             (Opaque(self_id), Opaque(other_id)) => self_id == other_id,
             (Opaque(..), _) | (_, Opaque(..)) => false,
 
-            _ => pcx.mcx.tycx.bug(format_args!(
+            _ => cx.bug(format_args!(
                 "trying to compare incompatible constructors {self:?} and {other:?}"
             )),
         }
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index a53d7a0d8096a..866eb67ea9971 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -183,7 +183,7 @@ pub fn analyze_match<'p, 'tcx>(
     // `if let`s. Only run if the match is exhaustive otherwise the error is redundant.
     if tycx.refutable && report.non_exhaustiveness_witnesses.is_empty() {
         let pat_column = PatternColumn::new(arms);
-        lint_nonexhaustive_missing_variants(cx, arms, &pat_column, scrut_ty)?;
+        lint_nonexhaustive_missing_variants(tycx, arms, &pat_column, scrut_ty)?;
     }
 
     Ok(report)
diff --git a/compiler/rustc_pattern_analysis/src/lints.rs b/compiler/rustc_pattern_analysis/src/lints.rs
index e3d218d71c301..4cbd5e70189f2 100644
--- a/compiler/rustc_pattern_analysis/src/lints.rs
+++ b/compiler/rustc_pattern_analysis/src/lints.rs
@@ -4,8 +4,7 @@ use rustc_span::ErrorGuaranteed;
 use crate::constructor::{Constructor, SplitConstructorSet};
 use crate::errors::{NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Uncovered};
 use crate::pat::{DeconstructedPat, PatOrWild};
-use crate::rustc::{MatchCtxt, RevealedTy, RustcMatchCheckCtxt, WitnessPat};
-use crate::usefulness::PlaceCtxt;
+use crate::rustc::{RevealedTy, RustcMatchCheckCtxt, WitnessPat};
 use crate::{MatchArm, TypeCx};
 
 /// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that
@@ -50,9 +49,9 @@ impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
     }
 
     /// Do constructor splitting on the constructors of the column.
-    fn analyze_ctors(&self, pcx: &PlaceCtxt<'_, Cx>) -> Result<SplitConstructorSet<Cx>, Cx::Error> {
+    fn analyze_ctors(&self, cx: &Cx, ty: &Cx::Ty) -> Result<SplitConstructorSet<Cx>, Cx::Error> {
         let column_ctors = self.patterns.iter().map(|p| p.ctor());
-        let ctors_for_ty = &pcx.ctors_for_ty()?;
+        let ctors_for_ty = cx.ctors_for_ty(ty)?;
         Ok(ctors_for_ty.split(column_ctors))
     }
 
@@ -63,10 +62,11 @@ impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
     /// which may change the lengths.
     fn specialize(
         &self,
-        pcx: &PlaceCtxt<'_, Cx>,
+        cx: &Cx,
+        ty: &Cx::Ty,
         ctor: &Constructor<Cx>,
     ) -> Vec<PatternColumn<'p, Cx>> {
-        let arity = ctor.arity(pcx);
+        let arity = ctor.arity(cx, ty);
         if arity == 0 {
             return Vec::new();
         }
@@ -77,7 +77,7 @@ impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
         let mut specialized_columns: Vec<_> =
             (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
         let relevant_patterns =
-            self.patterns.iter().filter(|pat| ctor.is_covered_by(pcx, pat.ctor()));
+            self.patterns.iter().filter(|pat| ctor.is_covered_by(cx, pat.ctor()));
         for pat in relevant_patterns {
             let specialized = pat.specialize(ctor, arity);
             for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
@@ -92,15 +92,14 @@ impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
 /// in a given column.
 #[instrument(level = "debug", skip(cx), ret)]
 fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
-    cx: MatchCtxt<'a, 'p, 'tcx>,
+    cx: &RustcMatchCheckCtxt<'p, 'tcx>,
     column: &PatternColumn<'p, RustcMatchCheckCtxt<'p, 'tcx>>,
 ) -> Result<Vec<WitnessPat<'p, 'tcx>>, ErrorGuaranteed> {
     let Some(&ty) = column.head_ty() else {
         return Ok(Vec::new());
     };
-    let pcx = &PlaceCtxt::new_dummy(cx, &ty);
 
-    let set = column.analyze_ctors(pcx)?;
+    let set = column.analyze_ctors(cx, &ty)?;
     if set.present.is_empty() {
         // We can't consistently handle the case where no constructors are present (since this would
         // require digging deep through any type in case there's a non_exhaustive enum somewhere),
@@ -109,20 +108,20 @@ fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
     }
 
     let mut witnesses = Vec::new();
-    if cx.tycx.is_foreign_non_exhaustive_enum(ty) {
+    if cx.is_foreign_non_exhaustive_enum(ty) {
         witnesses.extend(
             set.missing
                 .into_iter()
                 // This will list missing visible variants.
                 .filter(|c| !matches!(c, Constructor::Hidden | Constructor::NonExhaustive))
-                .map(|missing_ctor| WitnessPat::wild_from_ctor(pcx, missing_ctor)),
+                .map(|missing_ctor| WitnessPat::wild_from_ctor(cx, missing_ctor, ty)),
         )
     }
 
     // Recurse into the fields.
     for ctor in set.present {
-        let specialized_columns = column.specialize(pcx, &ctor);
-        let wild_pat = WitnessPat::wild_from_ctor(pcx, ctor);
+        let specialized_columns = column.specialize(cx, &ty, &ctor);
+        let wild_pat = WitnessPat::wild_from_ctor(cx, ctor, ty);
         for (i, col_i) in specialized_columns.iter().enumerate() {
             // Compute witnesses for each column.
             let wits_for_col_i = collect_nonexhaustive_missing_variants(cx, col_i)?;
@@ -138,18 +137,17 @@ fn collect_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
     Ok(witnesses)
 }
 
-pub(crate) fn lint_nonexhaustive_missing_variants<'a, 'p, 'tcx>(
-    cx: MatchCtxt<'a, 'p, 'tcx>,
+pub(crate) fn lint_nonexhaustive_missing_variants<'p, 'tcx>(
+    rcx: &RustcMatchCheckCtxt<'p, 'tcx>,
     arms: &[MatchArm<'p, RustcMatchCheckCtxt<'p, 'tcx>>],
     pat_column: &PatternColumn<'p, RustcMatchCheckCtxt<'p, 'tcx>>,
     scrut_ty: RevealedTy<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
-    let rcx: &RustcMatchCheckCtxt<'_, '_> = cx.tycx;
     if !matches!(
         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)?;
+        let witnesses = collect_nonexhaustive_missing_variants(rcx, pat_column)?;
         if !witnesses.is_empty() {
             // Report that a match of a `non_exhaustive` enum marked with `non_exhaustive_omitted_patterns`
             // is not exhaustive enough.
diff --git a/compiler/rustc_pattern_analysis/src/pat.rs b/compiler/rustc_pattern_analysis/src/pat.rs
index d476766d466f2..c94d8b9353589 100644
--- a/compiler/rustc_pattern_analysis/src/pat.rs
+++ b/compiler/rustc_pattern_analysis/src/pat.rs
@@ -6,7 +6,6 @@ use std::fmt;
 use smallvec::{smallvec, SmallVec};
 
 use crate::constructor::{Constructor, Slice, SliceKind};
-use crate::usefulness::PlaceCtxt;
 use crate::{Captures, TypeCx};
 
 use self::Constructor::*;
@@ -331,9 +330,9 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
     /// 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: &PlaceCtxt<'_, Cx>, ctor: Constructor<Cx>) -> Self {
-        let fields = pcx.ctor_sub_tys(&ctor).map(|ty| Self::wildcard(ty)).collect();
-        Self::new(ctor, fields, pcx.ty.clone())
+    pub(crate) fn wild_from_ctor(cx: &Cx, ctor: Constructor<Cx>, ty: Cx::Ty) -> Self {
+        let fields = cx.ctor_sub_tys(&ctor, &ty).map(|ty| Self::wildcard(ty)).collect();
+        Self::new(ctor, fields, ty)
     }
 
     pub fn ctor(&self) -> &Constructor<Cx> {
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index 4fb76a891b534..ec37e20211860 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -30,7 +30,6 @@ pub type ConstructorSet<'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, 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>>;
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index b15de1c0ca9d7..62d17ca450982 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -731,20 +731,19 @@ pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
 }
 
 /// Context that provides information local to a place under investigation.
-pub(crate) struct PlaceCtxt<'a, Cx: TypeCx> {
-    pub(crate) mcx: MatchCtxt<'a, Cx>,
+struct PlaceCtxt<'a, Cx: TypeCx> {
+    mcx: MatchCtxt<'a, Cx>,
     /// Type of the place under investigation.
-    pub(crate) ty: &'a Cx::Ty,
+    ty: &'a Cx::Ty,
 }
 
+impl<'a, Cx: TypeCx> Copy for PlaceCtxt<'a, Cx> {}
 impl<'a, Cx: TypeCx> Clone for PlaceCtxt<'a, Cx> {
     fn clone(&self) -> Self {
         Self { mcx: self.mcx, ty: self.ty }
     }
 }
 
-impl<'a, Cx: TypeCx> Copy for PlaceCtxt<'a, Cx> {}
-
 impl<'a, Cx: TypeCx> fmt::Debug for PlaceCtxt<'a, Cx> {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt.debug_struct("PlaceCtxt").field("ty", self.ty).finish()
@@ -752,24 +751,21 @@ impl<'a, Cx: TypeCx> fmt::Debug for PlaceCtxt<'a, Cx> {
 }
 
 impl<'a, Cx: TypeCx> PlaceCtxt<'a, 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, Cx>, ty: &'a Cx::Ty) -> Self {
-        PlaceCtxt { mcx, ty }
-    }
-
-    pub(crate) fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
+    fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
         self.mcx.tycx.ctor_arity(ctor, self.ty)
     }
-    pub(crate) fn ctor_sub_tys(
+    fn ctor_sub_tys(
         &'a self,
         ctor: &'a Constructor<Cx>,
     ) -> impl Iterator<Item = Cx::Ty> + ExactSizeIterator + Captures<'a> {
         self.mcx.tycx.ctor_sub_tys(ctor, self.ty)
     }
-    pub(crate) fn ctors_for_ty(&self) -> Result<ConstructorSet<Cx>, Cx::Error> {
+    fn ctors_for_ty(&self) -> Result<ConstructorSet<Cx>, Cx::Error> {
         self.mcx.tycx.ctors_for_ty(self.ty)
     }
+    fn wild_from_ctor(&self, ctor: Constructor<Cx>) -> WitnessPat<Cx> {
+        WitnessPat::wild_from_ctor(self.mcx.tycx, ctor, self.ty.clone())
+    }
 }
 
 /// Serves two purposes:
@@ -1089,7 +1085,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
             wildcard_row_is_relevant: self.wildcard_row_is_relevant && ctor_is_relevant,
         };
         for (i, row) in self.rows().enumerate() {
-            if ctor.is_covered_by(pcx, row.head().ctor()) {
+            if ctor.is_covered_by(pcx.mcx.tycx, row.head().ctor()) {
                 let new_row = row.pop_head_constructor(ctor, arity, ctor_is_relevant, i);
                 matrix.expand_and_push(new_row);
             }
@@ -1240,7 +1236,7 @@ impl<Cx: TypeCx> WitnessStack<Cx> {
     /// ```
     fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, Cx>, ctor: &Constructor<Cx>) {
         let len = self.0.len();
-        let arity = ctor.arity(pcx);
+        let arity = pcx.ctor_arity(ctor);
         let fields = self.0.drain((len - arity)..).rev().collect();
         let pat = WitnessPat::new(ctor.clone(), fields, pcx.ty.clone());
         self.0.push(pat);
@@ -1315,20 +1311,20 @@ impl<Cx: TypeCx> WitnessMatrix<Cx> {
                 *self = Self::empty();
             } else if !report_individual_missing_ctors {
                 // Report `_` as missing.
-                let pat = WitnessPat::wild_from_ctor(pcx, Constructor::Wildcard);
+                let pat = pcx.wild_from_ctor(Constructor::Wildcard);
                 self.push_pattern(pat);
             } else if missing_ctors.iter().any(|c| c.is_non_exhaustive()) {
                 // We need to report a `_` anyway, so listing other constructors would be redundant.
                 // `NonExhaustive` is displayed as `_` just like `Wildcard`, but it will be picked
                 // up by diagnostics to add a note about why `_` is required here.
-                let pat = WitnessPat::wild_from_ctor(pcx, Constructor::NonExhaustive);
+                let pat = pcx.wild_from_ctor(Constructor::NonExhaustive);
                 self.push_pattern(pat);
             } else {
                 // For each missing constructor `c`, we add a `c(_, _, _)` witness appropriately
                 // filled with wildcards.
                 let mut ret = Self::empty();
                 for ctor in missing_ctors {
-                    let pat = WitnessPat::wild_from_ctor(pcx, ctor.clone());
+                    let pat = pcx.wild_from_ctor(ctor.clone());
                     // Clone `self` and add `c(_, _, _)` to each of its witnesses.
                     let mut wit_matrix = self.clone();
                     wit_matrix.push_pattern(pat);

From 83e88c6dfcd7aecf1c49d73e2d9e761d15302341 Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Wed, 24 Jan 2024 20:57:11 +0100
Subject: [PATCH 3/5] Repurpose `MatchCtxt` for usefulness only

---
 compiler/rustc_pattern_analysis/src/lib.rs    | 18 +------------
 .../rustc_pattern_analysis/src/usefulness.rs  | 26 ++++++++++++++-----
 2 files changed, 21 insertions(+), 23 deletions(-)

diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index 866eb67ea9971..99c32de74828a 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -135,20 +135,6 @@ pub trait TypeCx: Sized + fmt::Debug {
     }
 }
 
-/// Context that provides information global to a match.
-pub struct MatchCtxt<'a, Cx: TypeCx> {
-    /// The context for type information.
-    pub tycx: &'a Cx,
-}
-
-impl<'a, Cx: TypeCx> Clone for MatchCtxt<'a, Cx> {
-    fn clone(&self) -> Self {
-        Self { tycx: self.tycx }
-    }
-}
-
-impl<'a, Cx: TypeCx> Copy for MatchCtxt<'a, Cx> {}
-
 /// The arm of a match expression.
 #[derive(Debug)]
 pub struct MatchArm<'p, Cx: TypeCx> {
@@ -175,9 +161,7 @@ pub fn analyze_match<'p, 'tcx>(
 ) -> Result<rustc::UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
     let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
-    let cx = MatchCtxt { tycx };
-
-    let report = compute_match_usefulness(cx, arms, scrut_ty, scrut_validity)?;
+    let report = compute_match_usefulness(tycx, arms, scrut_ty, scrut_validity)?;
 
     // 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.
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index 62d17ca450982..d2cf2f72a7d9b 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -719,7 +719,7 @@ use std::fmt;
 
 use crate::constructor::{Constructor, ConstructorSet, IntRange};
 use crate::pat::{DeconstructedPat, PatOrWild, WitnessPat};
-use crate::{Captures, MatchArm, MatchCtxt, TypeCx};
+use crate::{Captures, MatchArm, TypeCx};
 
 use self::ValidityConstraint::*;
 
@@ -730,9 +730,22 @@ pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
     f()
 }
 
+/// Context that provides information for usefulness checking.
+pub struct UsefulnessCtxt<'a, Cx: TypeCx> {
+    /// The context for type information.
+    pub tycx: &'a Cx,
+}
+
+impl<'a, Cx: TypeCx> Copy for UsefulnessCtxt<'a, Cx> {}
+impl<'a, Cx: TypeCx> Clone for UsefulnessCtxt<'a, Cx> {
+    fn clone(&self) -> Self {
+        Self { tycx: self.tycx }
+    }
+}
+
 /// Context that provides information local to a place under investigation.
 struct PlaceCtxt<'a, Cx: TypeCx> {
-    mcx: MatchCtxt<'a, Cx>,
+    mcx: UsefulnessCtxt<'a, Cx>,
     /// Type of the place under investigation.
     ty: &'a Cx::Ty,
 }
@@ -1358,7 +1371,7 @@ impl<Cx: TypeCx> WitnessMatrix<Cx> {
 /// We can however get false negatives because exhaustiveness does not explore all cases. See the
 /// section on relevancy at the top of the file.
 fn collect_overlapping_range_endpoints<'p, Cx: TypeCx>(
-    mcx: MatchCtxt<'_, Cx>,
+    mcx: UsefulnessCtxt<'_, Cx>,
     overlap_range: IntRange,
     matrix: &Matrix<'p, Cx>,
     specialized_matrix: &Matrix<'p, Cx>,
@@ -1431,7 +1444,7 @@ fn collect_overlapping_range_endpoints<'p, Cx: TypeCx>(
 /// This is all explained at the top of the file.
 #[instrument(level = "debug", skip(mcx, is_top_level), ret)]
 fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
-    mcx: MatchCtxt<'a, Cx>,
+    mcx: UsefulnessCtxt<'a, Cx>,
     matrix: &mut Matrix<'p, Cx>,
     is_top_level: bool,
 ) -> Result<WitnessMatrix<Cx>, Cx::Error> {
@@ -1588,13 +1601,14 @@ pub struct UsefulnessReport<'p, Cx: TypeCx> {
 }
 
 /// Computes whether a match is exhaustive and which of its arms are useful.
-#[instrument(skip(cx, arms), level = "debug")]
+#[instrument(skip(tycx, arms), level = "debug")]
 pub fn compute_match_usefulness<'p, Cx: TypeCx>(
-    cx: MatchCtxt<'_, Cx>,
+    tycx: &Cx,
     arms: &[MatchArm<'p, Cx>],
     scrut_ty: Cx::Ty,
     scrut_validity: ValidityConstraint,
 ) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> {
+    let cx = UsefulnessCtxt { tycx };
     let mut matrix = Matrix::new(arms, scrut_ty, scrut_validity);
     let non_exhaustiveness_witnesses =
         compute_exhaustiveness_and_usefulness(cx, &mut matrix, true)?;

From 6ef836246bc48387bfdbe2bde951037c71350c00 Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Wed, 24 Jan 2024 21:29:58 +0100
Subject: [PATCH 4/5] Make `PatternColumn` part of the public API

---
 .../rustc_pattern_analysis/src/constructor.rs | 10 +--
 compiler/rustc_pattern_analysis/src/lib.rs    |  4 +-
 compiler/rustc_pattern_analysis/src/lints.rs  | 87 +-----------------
 .../rustc_pattern_analysis/src/pat_column.rs  | 90 +++++++++++++++++++
 4 files changed, 101 insertions(+), 90 deletions(-)
 create mode 100644 compiler/rustc_pattern_analysis/src/pat_column.rs

diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index df5bf3111adc4..4be564b1d7b1e 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -948,10 +948,10 @@ pub enum ConstructorSet<Cx: TypeCx> {
 /// 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<Cx: TypeCx> {
-    pub(crate) present: SmallVec<[Constructor<Cx>; 1]>,
-    pub(crate) missing: Vec<Constructor<Cx>>,
-    pub(crate) missing_empty: Vec<Constructor<Cx>>,
+pub struct SplitConstructorSet<Cx: TypeCx> {
+    pub present: SmallVec<[Constructor<Cx>; 1]>,
+    pub missing: Vec<Constructor<Cx>>,
+    pub missing_empty: Vec<Constructor<Cx>>,
 }
 
 impl<Cx: TypeCx> ConstructorSet<Cx> {
@@ -960,7 +960,7 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
     /// or slices. This can get subtle; see [`SplitConstructorSet`] for details of this operation
     /// and its invariants.
     #[instrument(level = "debug", skip(self, ctors), ret)]
-    pub(crate) fn split<'a>(
+    pub fn split<'a>(
         &self,
         ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone,
     ) -> SplitConstructorSet<Cx>
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index 99c32de74828a..3d0eb117d174f 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -6,6 +6,7 @@ pub mod errors;
 #[cfg(feature = "rustc")]
 pub(crate) mod lints;
 pub mod pat;
+pub mod pat_column;
 #[cfg(feature = "rustc")]
 pub mod rustc;
 pub mod usefulness;
@@ -67,8 +68,9 @@ use rustc_span::ErrorGuaranteed;
 
 use crate::constructor::{Constructor, ConstructorSet, IntRange};
 #[cfg(feature = "rustc")]
-use crate::lints::{lint_nonexhaustive_missing_variants, PatternColumn};
+use crate::lints::lint_nonexhaustive_missing_variants;
 use crate::pat::DeconstructedPat;
+use crate::pat_column::PatternColumn;
 #[cfg(feature = "rustc")]
 use crate::rustc::RustcMatchCheckCtxt;
 #[cfg(feature = "rustc")]
diff --git a/compiler/rustc_pattern_analysis/src/lints.rs b/compiler/rustc_pattern_analysis/src/lints.rs
index 4cbd5e70189f2..3f1497540d25e 100644
--- a/compiler/rustc_pattern_analysis/src/lints.rs
+++ b/compiler/rustc_pattern_analysis/src/lints.rs
@@ -1,92 +1,11 @@
 use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::ErrorGuaranteed;
 
-use crate::constructor::{Constructor, SplitConstructorSet};
+use crate::constructor::Constructor;
 use crate::errors::{NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Uncovered};
-use crate::pat::{DeconstructedPat, PatOrWild};
+use crate::pat_column::PatternColumn;
 use crate::rustc::{RevealedTy, RustcMatchCheckCtxt, WitnessPat};
-use crate::{MatchArm, TypeCx};
-
-/// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that
-/// inspect the same subvalue/place".
-/// This is used to traverse patterns column-by-column for lints. Despite similarities with the
-/// algorithm in [`crate::usefulness`], this does a different traversal. Notably this is linear in
-/// the depth of patterns, whereas `compute_exhaustiveness_and_usefulness` is worst-case exponential
-/// (exhaustiveness is NP-complete). The core difference is that we treat sub-columns separately.
-///
-/// This must not contain an or-pattern. `expand_and_push` takes care to expand them.
-///
-/// This is not used in the usefulness algorithm; only in lints.
-#[derive(Debug)]
-pub(crate) struct PatternColumn<'p, Cx: TypeCx> {
-    patterns: Vec<&'p DeconstructedPat<'p, Cx>>,
-}
-
-impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
-    pub(crate) fn new(arms: &[MatchArm<'p, Cx>]) -> Self {
-        let patterns = Vec::with_capacity(arms.len());
-        let mut column = PatternColumn { patterns };
-        for arm in arms {
-            column.expand_and_push(PatOrWild::Pat(arm.pat));
-        }
-        column
-    }
-    /// Pushes a pattern onto the column, expanding any or-patterns into its subpatterns.
-    /// Internal method, prefer [`PatternColumn::new`].
-    fn expand_and_push(&mut self, pat: PatOrWild<'p, Cx>) {
-        // We flatten or-patterns and skip algorithm-generated wildcards.
-        if pat.is_or_pat() {
-            self.patterns.extend(
-                pat.flatten_or_pat().into_iter().filter_map(|pat_or_wild| pat_or_wild.as_pat()),
-            )
-        } else if let Some(pat) = pat.as_pat() {
-            self.patterns.push(pat)
-        }
-    }
-
-    fn head_ty(&self) -> Option<&Cx::Ty> {
-        self.patterns.first().map(|pat| pat.ty())
-    }
-
-    /// Do constructor splitting on the constructors of the column.
-    fn analyze_ctors(&self, cx: &Cx, ty: &Cx::Ty) -> Result<SplitConstructorSet<Cx>, Cx::Error> {
-        let column_ctors = self.patterns.iter().map(|p| p.ctor());
-        let ctors_for_ty = cx.ctors_for_ty(ty)?;
-        Ok(ctors_for_ty.split(column_ctors))
-    }
-
-    /// Does specialization: given a constructor, this takes the patterns from the column that match
-    /// the constructor, and outputs their fields.
-    /// 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,
-        cx: &Cx,
-        ty: &Cx::Ty,
-        ctor: &Constructor<Cx>,
-    ) -> Vec<PatternColumn<'p, Cx>> {
-        let arity = ctor.arity(cx, ty);
-        if arity == 0 {
-            return Vec::new();
-        }
-
-        // We specialize the column by `ctor`. This gives us `arity`-many columns of patterns. These
-        // columns may have different lengths in the presence of or-patterns (this is why we can't
-        // reuse `Matrix`).
-        let mut specialized_columns: Vec<_> =
-            (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
-        let relevant_patterns =
-            self.patterns.iter().filter(|pat| ctor.is_covered_by(cx, pat.ctor()));
-        for pat in relevant_patterns {
-            let specialized = pat.specialize(ctor, arity);
-            for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
-                column.expand_and_push(subpat);
-            }
-        }
-        specialized_columns
-    }
-}
+use crate::MatchArm;
 
 /// Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned
 /// in a given column.
diff --git a/compiler/rustc_pattern_analysis/src/pat_column.rs b/compiler/rustc_pattern_analysis/src/pat_column.rs
new file mode 100644
index 0000000000000..3cacfc491b981
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/src/pat_column.rs
@@ -0,0 +1,90 @@
+use crate::constructor::{Constructor, SplitConstructorSet};
+use crate::pat::{DeconstructedPat, PatOrWild};
+use crate::{Captures, MatchArm, TypeCx};
+
+/// A column of patterns in a match, where a column is the intuitive notion of "subpatterns that
+/// inspect the same subvalue/place".
+/// This is used to traverse patterns column-by-column for lints. Despite similarities with the
+/// algorithm in [`crate::usefulness`], this does a different traversal. Notably this is linear in
+/// the depth of patterns, whereas `compute_exhaustiveness_and_usefulness` is worst-case exponential
+/// (exhaustiveness is NP-complete). The core difference is that we treat sub-columns separately.
+///
+/// This is not used in the usefulness algorithm; only in lints.
+#[derive(Debug)]
+pub struct PatternColumn<'p, Cx: TypeCx> {
+    /// This must not contain an or-pattern. `expand_and_push` takes care to expand them.
+    patterns: Vec<&'p DeconstructedPat<'p, Cx>>,
+}
+
+impl<'p, Cx: TypeCx> PatternColumn<'p, Cx> {
+    pub fn new(arms: &[MatchArm<'p, Cx>]) -> Self {
+        let patterns = Vec::with_capacity(arms.len());
+        let mut column = PatternColumn { patterns };
+        for arm in arms {
+            column.expand_and_push(PatOrWild::Pat(arm.pat));
+        }
+        column
+    }
+    /// Pushes a pattern onto the column, expanding any or-patterns into its subpatterns.
+    /// Internal method, prefer [`PatternColumn::new`].
+    fn expand_and_push(&mut self, pat: PatOrWild<'p, Cx>) {
+        // We flatten or-patterns and skip algorithm-generated wildcards.
+        if pat.is_or_pat() {
+            self.patterns.extend(
+                pat.flatten_or_pat().into_iter().filter_map(|pat_or_wild| pat_or_wild.as_pat()),
+            )
+        } else if let Some(pat) = pat.as_pat() {
+            self.patterns.push(pat)
+        }
+    }
+
+    pub fn head_ty(&self) -> Option<&Cx::Ty> {
+        self.patterns.first().map(|pat| pat.ty())
+    }
+    pub fn iter<'a>(&'a self) -> impl Iterator<Item = &'p DeconstructedPat<'p, Cx>> + Captures<'a> {
+        self.patterns.iter().copied()
+    }
+
+    /// Do constructor splitting on the constructors of the column.
+    pub fn analyze_ctors(
+        &self,
+        cx: &Cx,
+        ty: &Cx::Ty,
+    ) -> Result<SplitConstructorSet<Cx>, Cx::Error> {
+        let column_ctors = self.patterns.iter().map(|p| p.ctor());
+        let ctors_for_ty = cx.ctors_for_ty(ty)?;
+        Ok(ctors_for_ty.split(column_ctors))
+    }
+
+    /// Does specialization: given a constructor, this takes the patterns from the column that match
+    /// the constructor, and outputs their fields.
+    /// 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.
+    pub fn specialize(
+        &self,
+        cx: &Cx,
+        ty: &Cx::Ty,
+        ctor: &Constructor<Cx>,
+    ) -> Vec<PatternColumn<'p, Cx>> {
+        let arity = ctor.arity(cx, ty);
+        if arity == 0 {
+            return Vec::new();
+        }
+
+        // We specialize the column by `ctor`. This gives us `arity`-many columns of patterns. These
+        // columns may have different lengths in the presence of or-patterns (this is why we can't
+        // reuse `Matrix`).
+        let mut specialized_columns: Vec<_> =
+            (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
+        let relevant_patterns =
+            self.patterns.iter().filter(|pat| ctor.is_covered_by(cx, pat.ctor()));
+        for pat in relevant_patterns {
+            let specialized = pat.specialize(ctor, arity);
+            for (subpat, column) in specialized.into_iter().zip(&mut specialized_columns) {
+                column.expand_and_push(subpat);
+            }
+        }
+        specialized_columns
+    }
+}

From 59031429c5ad9f4002e3f860ce2ef31e90368b6c Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Wed, 24 Jan 2024 21:51:17 +0100
Subject: [PATCH 5/5] Separate `PlaceCtxt` from `UsefulnessCtxt`

---
 .../rustc_pattern_analysis/src/usefulness.rs     | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index d2cf2f72a7d9b..3d45d032a9934 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -745,7 +745,7 @@ impl<'a, Cx: TypeCx> Clone for UsefulnessCtxt<'a, Cx> {
 
 /// Context that provides information local to a place under investigation.
 struct PlaceCtxt<'a, Cx: TypeCx> {
-    mcx: UsefulnessCtxt<'a, Cx>,
+    cx: &'a Cx,
     /// Type of the place under investigation.
     ty: &'a Cx::Ty,
 }
@@ -753,7 +753,7 @@ struct PlaceCtxt<'a, Cx: TypeCx> {
 impl<'a, Cx: TypeCx> Copy for PlaceCtxt<'a, Cx> {}
 impl<'a, Cx: TypeCx> Clone for PlaceCtxt<'a, Cx> {
     fn clone(&self) -> Self {
-        Self { mcx: self.mcx, ty: self.ty }
+        Self { cx: self.cx, ty: self.ty }
     }
 }
 
@@ -765,19 +765,19 @@ impl<'a, Cx: TypeCx> fmt::Debug for PlaceCtxt<'a, Cx> {
 
 impl<'a, Cx: TypeCx> PlaceCtxt<'a, Cx> {
     fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
-        self.mcx.tycx.ctor_arity(ctor, self.ty)
+        self.cx.ctor_arity(ctor, self.ty)
     }
     fn ctor_sub_tys(
         &'a self,
         ctor: &'a Constructor<Cx>,
     ) -> impl Iterator<Item = Cx::Ty> + ExactSizeIterator + Captures<'a> {
-        self.mcx.tycx.ctor_sub_tys(ctor, self.ty)
+        self.cx.ctor_sub_tys(ctor, self.ty)
     }
     fn ctors_for_ty(&self) -> Result<ConstructorSet<Cx>, Cx::Error> {
-        self.mcx.tycx.ctors_for_ty(self.ty)
+        self.cx.ctors_for_ty(self.ty)
     }
     fn wild_from_ctor(&self, ctor: Constructor<Cx>) -> WitnessPat<Cx> {
-        WitnessPat::wild_from_ctor(self.mcx.tycx, ctor, self.ty.clone())
+        WitnessPat::wild_from_ctor(self.cx, ctor, self.ty.clone())
     }
 }
 
@@ -1098,7 +1098,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
             wildcard_row_is_relevant: self.wildcard_row_is_relevant && ctor_is_relevant,
         };
         for (i, row) in self.rows().enumerate() {
-            if ctor.is_covered_by(pcx.mcx.tycx, row.head().ctor()) {
+            if ctor.is_covered_by(pcx.cx, row.head().ctor()) {
                 let new_row = row.pop_head_constructor(ctor, arity, ctor_is_relevant, i);
                 matrix.expand_and_push(new_row);
             }
@@ -1478,7 +1478,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
     };
 
     debug!("ty: {ty:?}");
-    let pcx = &PlaceCtxt { mcx, ty: &ty };
+    let pcx = &PlaceCtxt { cx: mcx.tycx, ty: &ty };
     let ctors_for_ty = pcx.ctors_for_ty()?;
 
     // Whether the place/column we are inspecting is known to contain valid data.