Use SmallVec for Matrix

compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

@@ -721,7 +721,7 @@ impl<'p, 'tcx> fmt::Debug for MatrixRow<'p, 'tcx> {
 /// the matrix will correspond to `scrutinee.0.Some.0` and the second column to `scrutinee.1`.
 #[derive(Clone)]
 struct Matrix<'p, 'tcx> {
-    rows: Vec<MatrixRow<'p, 'tcx>>,
+    rows: SmallVec<[MatrixRow<'p, 'tcx>; 8]>,
     /// 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>,
@@ -745,7 +745,7 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
     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(cx, wild_pattern);
-        let mut matrix = Matrix { rows: Vec::with_capacity(arms.len()), wildcard_row };
+        let mut matrix = Matrix { rows: SmallVec::with_capacity(arms.len()), wildcard_row };
         for (row_id, arm) in arms.iter().enumerate() {
             let v = MatrixRow {
                 pats: PatStack::from_pattern(cx, arm.pat),
@@ -809,7 +809,7 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
         ctor: &Constructor<'tcx>,
     ) -> Matrix<'p, 'tcx> {
         let wildcard_row = self.wildcard_row.pop_head_constructor(pcx, ctor);
-        let rows = Vec::with_capacity(self.rows.len()); // Better waste capacity than reallocate a lot.
+        let rows = SmallVec::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()) {