Auto merge of #99843 - oli-obk:is_useful_perf, r=compiler-errors

Some `is_useful` cleanups

#98582 was reverted because it was a perf regression.

https://github.com/rust-lang/rust/pull/99806 reintroduces the changes, but this PR picks individual ones that have no regressions.

2 files changed, 43 insertions, 46 deletions

diff --git a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
index 60db98073a3..98eb6e170ab 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs
@@ -261,7 +261,7 @@ impl IntRange {
     /// Lint on likely incorrect range patterns (#63987)
     pub(super) fn lint_overlapping_range_endpoints<'a, 'p: 'a, 'tcx: 'a>(
         &self,
-        pcx: PatCtxt<'_, 'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         pats: impl Iterator<Item = &'a DeconstructedPat<'p, 'tcx>>,
         column_count: usize,
         hir_id: HirId,
@@ -696,7 +696,7 @@ impl<'tcx> Constructor<'tcx> {
     /// `EvalResult::Deny { .. }`.
     ///
     /// This means that the variant has a stdlib unstable feature marking it.
-    pub(super) fn is_unstable_variant(&self, pcx: PatCtxt<'_, '_, 'tcx>) -> bool {
+    pub(super) fn is_unstable_variant(&self, pcx: &PatCtxt<'_, '_, 'tcx>) -> bool {
         if let Constructor::Variant(idx) = self && let ty::Adt(adt, _) = pcx.ty.kind() {
             let variant_def_id = adt.variant(*idx).def_id;
             // Filter variants that depend on a disabled unstable feature.
@@ -710,7 +710,7 @@ impl<'tcx> Constructor<'tcx> {
 
     /// Checks if the `Constructor` is a `Constructor::Variant` with a `#[doc(hidden)]`
     /// attribute from a type not local to the current crate.
-    pub(super) fn is_doc_hidden_variant(&self, pcx: PatCtxt<'_, '_, 'tcx>) -> bool {
+    pub(super) fn is_doc_hidden_variant(&self, pcx: &PatCtxt<'_, '_, 'tcx>) -> bool {
         if let Constructor::Variant(idx) = self && let ty::Adt(adt, _) = pcx.ty.kind() {
             let variant_def_id = adt.variants()[*idx].def_id;
             return pcx.cx.tcx.is_doc_hidden(variant_def_id) && !variant_def_id.is_local();
@@ -731,7 +731,7 @@ impl<'tcx> Constructor<'tcx> {
 
     /// The number of fields for this constructor. This must be kept in sync with
     /// `Fields::wildcards`.
-    pub(super) fn arity(&self, pcx: PatCtxt<'_, '_, 'tcx>) -> usize {
+    pub(super) fn arity(&self, pcx: &PatCtxt<'_, '_, 'tcx>) -> usize {
         match self {
             Single | Variant(_) => match pcx.ty.kind() {
                 ty::Tuple(fs) => fs.len(),
@@ -775,7 +775,7 @@ impl<'tcx> Constructor<'tcx> {
     /// matrix, unless all of them are.
     pub(super) fn split<'a>(
         &self,
-        pcx: PatCtxt<'_, '_, 'tcx>,
+        pcx: &PatCtxt<'_, '_, 'tcx>,
         ctors: impl Iterator<Item = &'a Constructor<'tcx>> + Clone,
     ) -> SmallVec<[Self; 1]>
     where
@@ -811,7 +811,7 @@ impl<'tcx> Constructor<'tcx> {
     /// this checks for inclusion.
     // We inline because this has a single call site in `Matrix::specialize_constructor`.
     #[inline]
-    pub(super) fn is_covered_by<'p>(&self, pcx: PatCtxt<'_, 'p, 'tcx>, other: &Self) -> bool {
+    pub(super) fn is_covered_by<'p>(&self, pcx: &PatCtxt<'_, 'p, 'tcx>, other: &Self) -> bool {
         // This must be kept in sync with `is_covered_by_any`.
         match (self, other) {
             // Wildcards cover anything
@@ -865,7 +865,7 @@ impl<'tcx> Constructor<'tcx> {
     /// assumed to have been split from a wildcard.
     fn is_covered_by_any<'p>(
         &self,
-        pcx: PatCtxt<'_, 'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         used_ctors: &[Constructor<'tcx>],
     ) -> bool {
         if used_ctors.is_empty() {
@@ -918,7 +918,7 @@ pub(super) struct SplitWildcard<'tcx> {
 }
 
 impl<'tcx> SplitWildcard<'tcx> {
-    pub(super) fn new<'p>(pcx: PatCtxt<'_, 'p, 'tcx>) -> Self {
+    pub(super) fn new<'p>(pcx: &PatCtxt<'_, 'p, 'tcx>) -> Self {
         debug!("SplitWildcard::new({:?})", pcx.ty);
         let cx = pcx.cx;
         let make_range = |start, end| {
@@ -1044,7 +1044,7 @@ impl<'tcx> SplitWildcard<'tcx> {
     /// do what you want.
     pub(super) fn split<'a>(
         &mut self,
-        pcx: PatCtxt<'_, '_, 'tcx>,
+        pcx: &PatCtxt<'_, '_, 'tcx>,
         ctors: impl Iterator<Item = &'a Constructor<'tcx>> + Clone,
     ) where
         'tcx: 'a,
@@ -1056,21 +1056,21 @@ impl<'tcx> SplitWildcard<'tcx> {
     }
 
     /// Whether there are any value constructors for this type that are not present in the matrix.
-    fn any_missing(&self, pcx: PatCtxt<'_, '_, 'tcx>) -> bool {
+    fn any_missing(&self, pcx: &PatCtxt<'_, '_, 'tcx>) -> bool {
         self.iter_missing(pcx).next().is_some()
     }
 
     /// Iterate over the constructors for this type that are not present in the matrix.
     pub(super) fn iter_missing<'a, 'p>(
         &'a self,
-        pcx: PatCtxt<'a, 'p, 'tcx>,
+        pcx: &'a PatCtxt<'a, 'p, 'tcx>,
     ) -> impl Iterator<Item = &'a Constructor<'tcx>> + Captures<'p> {
         self.all_ctors.iter().filter(move |ctor| !ctor.is_covered_by_any(pcx, &self.matrix_ctors))
     }
 
     /// Return the set of constructors resulting from splitting the wildcard. As explained at the
     /// top of the file, if any constructors are missing we can ignore the present ones.
-    fn into_ctors(self, pcx: PatCtxt<'_, '_, 'tcx>) -> SmallVec<[Constructor<'tcx>; 1]> {
+    fn into_ctors(self, pcx: &PatCtxt<'_, '_, 'tcx>) -> SmallVec<[Constructor<'tcx>; 1]> {
         if self.any_missing(pcx) {
             // Some constructors are missing, thus we can specialize with the special `Missing`
             // constructor, which stands for those constructors that are not seen in the matrix,
@@ -1202,35 +1202,32 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
 
     /// Creates a new list of wildcard fields for a given constructor. The result must have a
     /// length of `constructor.arity()`.
-    pub(super) fn wildcards(
-        cx: &MatchCheckCtxt<'p, 'tcx>,
-        ty: Ty<'tcx>,
-        constructor: &Constructor<'tcx>,
-    ) -> Self {
+    #[instrument(level = "trace")]
+    pub(super) fn wildcards(pcx: &PatCtxt<'_, 'p, 'tcx>, constructor: &Constructor<'tcx>) -> Self {
         let ret = match constructor {
-            Single | Variant(_) => match ty.kind() {
-                ty::Tuple(fs) => Fields::wildcards_from_tys(cx, fs.iter()),
-                ty::Ref(_, rty, _) => Fields::wildcards_from_tys(cx, once(*rty)),
+            Single | Variant(_) => match pcx.ty.kind() {
+                ty::Tuple(fs) => Fields::wildcards_from_tys(pcx.cx, fs.iter()),
+                ty::Ref(_, rty, _) => Fields::wildcards_from_tys(pcx.cx, once(*rty)),
                 ty::Adt(adt, substs) => {
                     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.
-                        Fields::wildcards_from_tys(cx, once(substs.type_at(0)))
+                        Fields::wildcards_from_tys(pcx.cx, once(substs.type_at(0)))
                     } else {
                         let variant = &adt.variant(constructor.variant_index_for_adt(*adt));
-                        let tys = Fields::list_variant_nonhidden_fields(cx, ty, variant)
+                        let tys = Fields::list_variant_nonhidden_fields(pcx.cx, pcx.ty, variant)
                             .map(|(_, ty)| ty);
-                        Fields::wildcards_from_tys(cx, tys)
+                        Fields::wildcards_from_tys(pcx.cx, tys)
                     }
                 }
-                _ => bug!("Unexpected type for `Single` constructor: {:?}", ty),
+                _ => bug!("Unexpected type for `Single` constructor: {:?}", pcx),
             },
-            Slice(slice) => match *ty.kind() {
+            Slice(slice) => match *pcx.ty.kind() {
                 ty::Slice(ty) | ty::Array(ty, _) => {
                     let arity = slice.arity();
-                    Fields::wildcards_from_tys(cx, (0..arity).map(|_| ty))
+                    Fields::wildcards_from_tys(pcx.cx, (0..arity).map(|_| ty))
                 }
-                _ => bug!("bad slice pattern {:?} {:?}", constructor, ty),
+                _ => bug!("bad slice pattern {:?} {:?}", constructor, pcx),
             },
             Str(..)
             | FloatRange(..)
@@ -1243,7 +1240,7 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
         };
-        debug!("Fields::wildcards({:?}, {:?}) = {:#?}", constructor, ty, ret);
+        debug!(?ret);
         ret
     }
 
@@ -1285,8 +1282,8 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
     /// Construct a pattern that matches everything that starts with this constructor.
     /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
     /// `Some(_)`.
-    pub(super) fn wild_from_ctor(pcx: PatCtxt<'_, 'p, 'tcx>, ctor: Constructor<'tcx>) -> Self {
-        let fields = Fields::wildcards(pcx.cx, pcx.ty, &ctor);
+    pub(super) fn wild_from_ctor(pcx: &PatCtxt<'_, 'p, 'tcx>, ctor: Constructor<'tcx>) -> Self {
+        let fields = Fields::wildcards(pcx, &ctor);
         DeconstructedPat::new(ctor, fields, pcx.ty, DUMMY_SP)
     }
 
@@ -1553,13 +1550,13 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
     /// `other_ctor` can be different from `self.ctor`, but must be covered by it.
     pub(super) fn specialize<'a>(
         &'a self,
-        cx: &MatchCheckCtxt<'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         other_ctor: &Constructor<'tcx>,
     ) -> SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]> {
         match (&self.ctor, other_ctor) {
             (Wildcard, _) => {
                 // We return a wildcard for each field of `other_ctor`.
-                Fields::wildcards(cx, self.ty, other_ctor).iter_patterns().collect()
+                Fields::wildcards(pcx, other_ctor).iter_patterns().collect()
             }
             (Slice(self_slice), Slice(other_slice))
                 if self_slice.arity() != other_slice.arity() =>
@@ -1578,7 +1575,7 @@ impl<'p, 'tcx> DeconstructedPat<'p, 'tcx> {
                         let prefix = &self.fields.fields[..prefix];
                         let suffix = &self.fields.fields[self_slice.arity() - suffix..];
                         let wildcard: &_ =
-                            cx.pattern_arena.alloc(DeconstructedPat::wildcard(inner_ty));
+                            pcx.cx.pattern_arena.alloc(DeconstructedPat::wildcard(inner_ty));
                         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()
diff --git a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
index 9e7a267ecbd..0a660ef30c8 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
@@ -411,12 +411,12 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     /// This is roughly the inverse of `Constructor::apply`.
     fn pop_head_constructor(
         &self,
-        cx: &MatchCheckCtxt<'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         ctor: &Constructor<'tcx>,
     ) -> PatStack<'p, 'tcx> {
         // We pop the head pattern and push the new fields extracted from the arguments of
         // `self.head()`.
-        let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(cx, ctor);
+        let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(pcx, ctor);
         new_fields.extend_from_slice(&self.pats[1..]);
         PatStack::from_vec(new_fields)
     }
@@ -469,13 +469,13 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
     /// This computes `S(constructor, self)`. See top of the file for explanations.
     fn specialize_constructor(
         &self,
-        pcx: PatCtxt<'_, 'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         ctor: &Constructor<'tcx>,
     ) -> Matrix<'p, 'tcx> {
         let mut matrix = Matrix::empty();
         for row in &self.patterns {
             if ctor.is_covered_by(pcx, row.head().ctor()) {
-                let new_row = row.pop_head_constructor(pcx.cx, ctor);
+                let new_row = row.pop_head_constructor(pcx, ctor);
                 matrix.push(new_row);
             }
         }
@@ -575,7 +575,7 @@ impl<'p, 'tcx> Usefulness<'p, 'tcx> {
     /// with the results of specializing with the other constructors.
     fn apply_constructor(
         self,
-        pcx: PatCtxt<'_, 'p, 'tcx>,
+        pcx: &PatCtxt<'_, 'p, 'tcx>,
         matrix: &Matrix<'p, 'tcx>, // used to compute missing ctors
         ctor: &Constructor<'tcx>,
     ) -> Self {
@@ -713,7 +713,7 @@ impl<'p, 'tcx> Witness<'p, 'tcx> {
     ///
     /// left_ty: struct X { a: (bool, &'static str), b: usize}
     /// pats: [(false, "foo"), 42]  => X { a: (false, "foo"), b: 42 }
-    fn apply_constructor(mut self, pcx: PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Self {
+    fn apply_constructor(mut self, pcx: &PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Self {
         let pat = {
             let len = self.0.len();
             let arity = ctor.arity(pcx);
@@ -786,7 +786,7 @@ fn is_useful<'p, 'tcx>(
     is_under_guard: bool,
     is_top_level: bool,
 ) -> Usefulness<'p, 'tcx> {
-    debug!("matrix,v={:?}{:?}", matrix, v);
+    debug!(?matrix, ?v);
     let Matrix { patterns: rows, .. } = matrix;
 
     // The base case. We are pattern-matching on () and the return value is
@@ -806,11 +806,6 @@ fn is_useful<'p, 'tcx>(
 
     debug_assert!(rows.iter().all(|r| r.len() == v.len()));
 
-    let ty = v.head().ty();
-    let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
-    debug!("v.head: {:?}, v.span: {:?}", v.head(), v.head().span());
-    let pcx = PatCtxt { cx, ty, span: v.head().span(), is_top_level, is_non_exhaustive };
-
     // If the first pattern is an or-pattern, expand it.
     let mut ret = Usefulness::new_not_useful(witness_preference);
     if v.head().is_or_pat() {
@@ -832,6 +827,11 @@ fn is_useful<'p, 'tcx>(
             }
         }
     } else {
+        let ty = v.head().ty();
+        let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
+        debug!("v.head: {:?}, v.span: {:?}", v.head(), v.head().span());
+        let pcx = &PatCtxt { cx, ty, span: v.head().span(), is_top_level, is_non_exhaustive };
+
         let v_ctor = v.head().ctor();
         debug!(?v_ctor);
         if let Constructor::IntRange(ctor_range) = &v_ctor {
@@ -853,7 +853,7 @@ fn is_useful<'p, 'tcx>(
             debug!("specialize({:?})", ctor);
             // We cache the result of `Fields::wildcards` because it is used a lot.
             let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor);
-            let v = v.pop_head_constructor(cx, &ctor);
+            let v = v.pop_head_constructor(pcx, &ctor);
             let usefulness = ensure_sufficient_stack(|| {
                 is_useful(cx, &spec_matrix, &v, witness_preference, hir_id, is_under_guard, false)
             });