Don't repeatedly simplify already-simplified match pairs

Author: Nadrieril

compiler/rustc_mir_build/src/build/matches/simplify.rs

@@ -59,22 +59,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         //
         // final bindings: [6, 7, 4, 5, 1, 2, 3]
         let mut accumulated_bindings = mem::take(candidate_bindings);
-        // Repeatedly simplify match pairs until fixed point is reached
+        let mut simplified_match_pairs = Vec::new();
+        // Repeatedly simplify match pairs until we're left with only unsimplifiable ones.
         loop {
-            let mut changed = false;
             for match_pair in mem::take(match_pairs) {
-                match self.simplify_match_pair(
+                if let Err(match_pair) = self.simplify_match_pair(
                     match_pair,
                     candidate_bindings,
                     candidate_ascriptions,
                     match_pairs,
                 ) {
-                    Ok(()) => {
-                        changed = true;
-                    }
-                    Err(match_pair) => {
-                        match_pairs.push(match_pair);
-                    }
+                    simplified_match_pairs.push(match_pair);
                 }
             }
 
@@ -83,14 +78,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             mem::swap(candidate_bindings, &mut accumulated_bindings);
             candidate_bindings.clear();
 
-            if !changed {
-                // If we were not able to simplify anymore, done.
+            if match_pairs.is_empty() {
                 break;
             }
         }
 
         // Store computed bindings back in `candidate_bindings`.
         mem::swap(candidate_bindings, &mut accumulated_bindings);
+        // Store simplified match pairs back in `match_pairs`.
+        mem::swap(match_pairs, &mut simplified_match_pairs);
 
         // Move or-patterns to the end, because they can result in us
         // creating additional candidates, so we want to test them as

tests/mir-opt/exponential_or.match_tuple.SimplifyCfg-initial.after.mir

@@ -19,7 +19,8 @@ fn match_tuple(_1: (u32, bool, Option<i32>, u32)) -> u32 {
 
     bb0: {
         PlaceMention(_1);
-        switchInt((_1.0: u32)) -> [1: bb2, 4: bb2, otherwise: bb1];
+        _2 = discriminant((_1.2: std::option::Option<i32>));
+        switchInt(move _2) -> [0: bb3, 1: bb2, otherwise: bb1];
     }
 
     bb1: {
@@ -28,12 +29,11 @@ fn match_tuple(_1: (u32, bool, Option<i32>, u32)) -> u32 {
     }
 
     bb2: {
-        _2 = discriminant((_1.2: std::option::Option<i32>));
-        switchInt(move _2) -> [0: bb4, 1: bb3, otherwise: bb1];
+        switchInt((((_1.2: std::option::Option<i32>) as Some).0: i32)) -> [1: bb3, 8: bb3, otherwise: bb1];
     }
 
     bb3: {
-        switchInt((((_1.2: std::option::Option<i32>) as Some).0: i32)) -> [1: bb4, 8: bb4, otherwise: bb1];
+        switchInt((_1.0: u32)) -> [1: bb4, 4: bb4, otherwise: bb1];
     }
 
     bb4: {