Auto merge of #118490 - Nadrieril:arena-alloc-matrix, r=<try>

[Experiment] Exhaustiveness: allocate memory better

Exhaustiveness is a recursive algorithm that allocates a bunch of slices at every step. Let's see if I can improve performance by improving allocations.

Already just using `Vec::with_capacity` is showing impressive improvements on my local measurements.

r? `@ghost`

Author: bors

Diff for: compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

@@ -599,7 +599,8 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     // an or-pattern. Panics if `self` is empty.
     fn expand_or_pat<'a>(&'a self) -> impl Iterator<Item = PatStack<'p, 'tcx>> + Captures<'a> {
         self.head().flatten_or_pat().into_iter().map(move |pat| {
-            let mut new_pats = smallvec![pat];
+            let mut new_pats = SmallVec::<[_; 2]>::with_capacity(self.len());
+            new_pats.push(pat);
             new_pats.extend_from_slice(&self.pats[1..]);
             PatStack { pats: new_pats }
         })
@@ -614,7 +615,9 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     ) -> PatStack<'p, 'tcx> {
         // 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 ctor_fields = self.head().specialize(pcx, ctor);
+        let mut new_pats = SmallVec::<[_; 2]>::with_capacity(self.len() + ctor_fields.len() - 1);
+        new_pats.extend_from_slice(&ctor_fields);
         new_pats.extend_from_slice(&self.pats[1..]);
         PatStack { pats: new_pats }
     }
@@ -732,18 +735,11 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
     }
 
     /// Build a new matrix from an iterator of `MatchArm`s.
-    fn new<'a>(
-        cx: &MatchCheckCtxt<'p, 'tcx>,
-        iter: impl Iterator<Item = &'a MatchArm<'p, 'tcx>>,
-        scrut_ty: Ty<'tcx>,
-    ) -> Self
-    where
-        'p: 'a,
-    {
+    fn new(cx: &MatchCheckCtxt<'p, 'tcx>, arms: &[MatchArm<'p, 'tcx>], scrut_ty: Ty<'tcx>) -> Self {
         let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty, DUMMY_SP));
         let wildcard_row = PatStack::from_pattern(wild_pattern);
-        let mut matrix = Matrix { rows: vec![], wildcard_row };
-        for (row_id, arm) in iter.enumerate() {
+        let mut matrix = Matrix { rows: Vec::with_capacity(arms.len()), wildcard_row };
+        for (row_id, arm) in arms.iter().enumerate() {
             let v = MatrixRow {
                 pats: PatStack::from_pattern(arm.pat),
                 parent_row: row_id, // dummy, we won't read it
@@ -806,7 +802,8 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         ctor: &Constructor<'tcx>,
     ) -> Matrix<'p, 'tcx> {
         let wildcard_row = self.wildcard_row.pop_head_constructor(pcx, ctor);
-        let mut matrix = Matrix { rows: vec![], wildcard_row };
+        let rows = Vec::with_capacity(self.rows.len()); // Better waste capacity than reallocate a lot.
+        let mut matrix = Matrix { rows, wildcard_row };
         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, i);
@@ -1386,7 +1383,7 @@ pub(crate) fn compute_match_usefulness<'p, 'tcx>(
     arms: &[MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
 ) -> UsefulnessReport<'p, 'tcx> {
-    let mut matrix = Matrix::new(cx, arms.iter(), scrut_ty);
+    let mut matrix = Matrix::new(cx, arms, scrut_ty);
     let non_exhaustiveness_witnesses =
         compute_exhaustiveness_and_reachability(cx, &mut matrix, true);