Auto merge of #119667 - Nadrieril:remove-wildcard-row, r=compiler-errors

Exhaustiveness: remove `Matrix.wildcard_row`

To compute exhaustiveness, we check whether an extra row with a wildcard added at the end of the match expression would be reachable. We used to store an actual such row of patterns in the `Matrix`, but it's a bit redundant since we know it only contains wildcards. It was kept because we used it to get the type of each column (and relevancy). With this PR, we keep track of the types (and relevancy) directly.

This is part of me splitting up #119581 for ease of review.

r? `@compiler-errors`

compiler/rustc_pattern_analysis/src/lints.rs

@@ -83,8 +83,9 @@ impl<'p, 'tcx> PatternColumn<'p, 'tcx> {
             (0..arity).map(|_| Self { patterns: Vec::new() }).collect();
         let relevant_patterns =
             self.patterns.iter().filter(|pat| ctor.is_covered_by(pcx, pat.ctor()));
+        let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
         for pat in relevant_patterns {
-            let specialized = pat.specialize(pcx, ctor);
+            let specialized = pat.specialize(pcx, ctor, ctor_sub_tys);
             for (subpat, column) in specialized.iter().zip(&mut specialized_columns) {
                 if subpat.is_or_pat() {
                     column.patterns.extend(subpat.flatten_or_pat())

compiler/rustc_pattern_analysis/src/pat.rs

@@ -81,10 +81,11 @@ impl<'p, Cx: TypeCx> DeconstructedPat<'p, Cx> {
         &self,
         pcx: &PlaceCtxt<'_, 'p, Cx>,
         other_ctor: &Constructor<Cx>,
+        ctor_sub_tys: &[Cx::Ty],
     ) -> SmallVec<[&'p DeconstructedPat<'p, Cx>; 2]> {
         let wildcard_sub_tys = || {
-            let tys = pcx.ctor_sub_tys(other_ctor);
-            tys.iter()
+            ctor_sub_tys
+                .iter()
                 .map(|ty| DeconstructedPat::wildcard(*ty))
                 .map(|pat| pcx.mcx.wildcard_arena.alloc(pat) as &_)
                 .collect()

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -717,7 +717,7 @@ use std::fmt;
 
 use crate::constructor::{Constructor, ConstructorSet};
 use crate::pat::{DeconstructedPat, WitnessPat};
-use crate::{Captures, MatchArm, MatchCtxt, TypeCx, TypedArena};
+use crate::{Captures, MatchArm, MatchCtxt, TypeCx};
 
 use self::ValidityConstraint::*;
 
@@ -874,11 +874,12 @@ impl<'p, Cx: TypeCx> PatStack<'p, Cx> {
         &self,
         pcx: &PlaceCtxt<'_, 'p, Cx>,
         ctor: &Constructor<Cx>,
+        ctor_sub_tys: &[Cx::Ty],
         ctor_is_relevant: bool,
     ) -> PatStack<'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);
+        let mut new_pats = self.head().specialize(pcx, ctor, ctor_sub_tys);
         new_pats.extend_from_slice(&self.pats[1..]);
         // `ctor` is relevant for this row if it is the actual constructor of this row, or if the
         // row has a wildcard and `ctor` is relevant for wildcards.
@@ -950,11 +951,12 @@ impl<'p, Cx: TypeCx> MatrixRow<'p, Cx> {
         &self,
         pcx: &PlaceCtxt<'_, 'p, Cx>,
         ctor: &Constructor<Cx>,
+        ctor_sub_tys: &[Cx::Ty],
         ctor_is_relevant: bool,
         parent_row: usize,
     ) -> MatrixRow<'p, Cx> {
         MatrixRow {
-            pats: self.pats.pop_head_constructor(pcx, ctor, ctor_is_relevant),
+            pats: self.pats.pop_head_constructor(pcx, ctor, ctor_sub_tys, ctor_is_relevant),
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
@@ -984,11 +986,13 @@ struct Matrix<'p, Cx: TypeCx> {
     /// each column must have the same type. Each column corresponds to a place within the
     /// scrutinee.
     rows: Vec<MatrixRow<'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, Cx>,
+    /// Track the type of each column/place.
+    place_ty: SmallVec<[Cx::Ty; 2]>,
     /// Track for each column/place whether it contains a known valid value.
     place_validity: SmallVec<[ValidityConstraint; 2]>,
+    /// Track whether the virtual wildcard row used to compute exhaustiveness is relevant. See top
+    /// of the file for details on relevancy.
+    wildcard_row_is_relevant: bool,
 }
 
 impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
@@ -1007,17 +1011,15 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
 
     /// Build a new matrix from an iterator of `MatchArm`s.
     fn new(
-        wildcard_arena: &'p TypedArena<DeconstructedPat<'p, Cx>>,
         arms: &[MatchArm<'p, Cx>],
         scrut_ty: Cx::Ty,
         scrut_validity: ValidityConstraint,
     ) -> Self {
-        let wild_pattern = wildcard_arena.alloc(DeconstructedPat::wildcard(scrut_ty));
-        let wildcard_row = PatStack::from_pattern(wild_pattern);
         let mut matrix = Matrix {
             rows: Vec::with_capacity(arms.len()),
-            wildcard_row,
+            place_ty: smallvec![scrut_ty],
             place_validity: smallvec![scrut_validity],
+            wildcard_row_is_relevant: true,
         };
         for (row_id, arm) in arms.iter().enumerate() {
             let v = MatrixRow {
@@ -1032,10 +1034,10 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     }
 
     fn head_ty(&self) -> Option<Cx::Ty> {
-        self.wildcard_row.head_opt().map(|pat| pat.ty())
+        self.place_ty.first().copied()
     }
     fn column_count(&self) -> usize {
-        self.wildcard_row.len()
+        self.place_ty.len()
     }
 
     fn rows(
@@ -1063,17 +1065,24 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
         ctor: &Constructor<Cx>,
         ctor_is_relevant: bool,
     ) -> Matrix<'p, Cx> {
-        let wildcard_row = self.wildcard_row.pop_head_constructor(pcx, ctor, ctor_is_relevant);
-        let new_validity = self.place_validity[0].specialize(ctor);
-        let new_place_validity = std::iter::repeat(new_validity)
+        let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
+        let specialized_place_ty =
+            ctor_sub_tys.iter().chain(self.place_ty[1..].iter()).copied().collect();
+        let ctor_sub_validity = self.place_validity[0].specialize(ctor);
+        let specialized_place_validity = std::iter::repeat(ctor_sub_validity)
             .take(ctor.arity(pcx))
             .chain(self.place_validity[1..].iter().copied())
             .collect();
-        let mut matrix =
-            Matrix { rows: Vec::new(), wildcard_row, place_validity: new_place_validity };
+        let mut matrix = Matrix {
+            rows: Vec::new(),
+            place_ty: specialized_place_ty,
+            place_validity: specialized_place_validity,
+            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()) {
-                let new_row = row.pop_head_constructor(pcx, ctor, ctor_is_relevant, i);
+                let new_row =
+                    row.pop_head_constructor(pcx, ctor, ctor_sub_tys, ctor_is_relevant, i);
                 matrix.expand_and_push(new_row);
             }
         }
@@ -1335,7 +1344,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
 ) -> WitnessMatrix<Cx> {
     debug_assert!(matrix.rows().all(|r| r.len() == matrix.column_count()));
 
-    if !matrix.wildcard_row.relevant && matrix.rows().all(|r| !r.pats.relevant) {
+    if !matrix.wildcard_row_is_relevant && matrix.rows().all(|r| !r.pats.relevant) {
         // Here we know that nothing will contribute further to exhaustiveness or usefulness. This
         // is purely an optimization: skipping this check doesn't affect correctness. See the top of
         // the file for details.
@@ -1356,7 +1365,7 @@ fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
         }
         // No (unguarded) rows, so the match is not exhaustive. We return a new witness unless
         // irrelevant.
-        return if matrix.wildcard_row.relevant {
+        return if matrix.wildcard_row_is_relevant {
             WitnessMatrix::unit_witness()
         } else {
             // We choose to not report anything here; see at the top for details.
@@ -1466,7 +1475,7 @@ pub fn compute_match_usefulness<'p, Cx: TypeCx>(
     scrut_ty: Cx::Ty,
     scrut_validity: ValidityConstraint,
 ) -> UsefulnessReport<'p, Cx> {
-    let mut matrix = Matrix::new(cx.wildcard_arena, arms, scrut_ty, scrut_validity);
+    let mut matrix = Matrix::new(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();