Rollup merge of #126553 - Nadrieril:expand-or-pat-into-above, r=matthewjasper

match lowering: expand or-candidates mixed with candidates above

This PR tweaks match lowering of or-patterns. Consider this:
```rust
match (x, y) {
    (1, true) => 1,
    (2, false) => 2,
    (1 | 2, true | false) => 3,
    (3 | 4, true | false) => 4,
    _ => 5,
}
```
One might hope that this can be compiled to a single `SwitchInt` on `x` followed by some boolean checks. Before this PR, we compile this to 3 `SwitchInt`s on `x`, because an arm that contains more than one or-pattern was compiled on its own. This PR groups branch `3` with the two branches above, getting us down to 2 `SwitchInt`s on `x`.

We can't in general expand or-patterns freely, because this interacts poorly with another optimization we do: or-pattern simplification. When an or-pattern doesn't involve bindings, we branch the success paths of all its alternatives to the same block. The drawback is that in a case like:
```rust
match (1, true) {
    (1 | 2, false) => unreachable!(),
    (2, _) => unreachable!(),
    _ => {}
}
```
if we used a single `SwitchInt`, by the time we test `false` we don't know whether we came from the `1` case or the `2` case, so we don't know where to go if `false` doesn't match.

Hence the limitation: we can process or-pattern alternatives alongside candidates that precede it, but not candidates that follow it. (Unless the or-pattern is the only remaining match pair of its candidate, in which case we can process it alongside whatever).

This PR allows the processing of or-pattern alternatives alongside candidates that precede it. One benefit is that we now process or-patterns in a single place in `mod.rs`.

r? ``@matthewjasper``

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

@@ -1074,12 +1074,9 @@ struct Candidate<'pat, 'tcx> {
     // because that would break binding consistency.
     subcandidates: Vec<Candidate<'pat, 'tcx>>,
 
-    /// ...and the guard must be evaluated if there is one.
+    /// ...and if there is a guard it must be evaluated; if it's `false` then branch to `otherwise_block`.
     has_guard: bool,
 
-    /// If the guard is `false` then branch to `otherwise_block`.
-    otherwise_block: Option<BasicBlock>,
-
     /// If the candidate matches, bindings and ascriptions must be established.
     extra_data: PatternExtraData<'tcx>,
 
@@ -1090,6 +1087,9 @@ struct Candidate<'pat, 'tcx> {
     /// The block before the `bindings` have been established.
     pre_binding_block: Option<BasicBlock>,
 
+    /// The block to branch to if the guard or a nested candidate fails to match.
+    otherwise_block: Option<BasicBlock>,
+
     /// The earliest block that has only candidates >= this one as descendents. Used for false
     /// edges, see the doc for [`Builder::match_expr`].
     false_edge_start_block: Option<BasicBlock>,
@@ -1364,56 +1364,105 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         otherwise_block: BasicBlock,
         candidates: &mut [&mut Candidate<'pat, 'tcx>],
     ) {
-        let mut split_or_candidate = false;
+        // We process or-patterns here. If any candidate starts with an or-pattern, we have to
-        for candidate in &mut *candidates {
+        // expand the or-pattern before we can proceed further.
-            if let [MatchPair { test_case: TestCase::Or { .. }, .. }] = &*candidate.match_pairs {
+        //
-                // Split a candidate in which the only match-pair is an or-pattern into multiple
+        // We can't expand them freely however. The rule is: if the candidate has an or-pattern as
-                // candidates. This is so that
+        // its only remaining match pair, we can expand it freely. If it has other match pairs, we
-                //
+        // can expand it but we can't process more candidates after it.
-                // match x {
+        //
-                //     0 | 1 => { ... },
+        // If we didn't stop, the `otherwise` cases could get mixed up. E.g. in the following,
-                //     2 | 3 => { ... },
+        // or-pattern simplification (in `merge_trivial_subcandidates`) makes it so the `1` and `2`
-                // }
+        // cases branch to a same block (which then tests `false`). If we took `(2, _)` in the same
-                //
+        // set of candidates, when we reach the block that tests `false` we don't know whether we
-                // only generates a single switch.
+        // came from `1` or `2`, hence we can't know where to branch on failure.
-                let match_pair = candidate.match_pairs.pop().unwrap();
+        // ```ignore(illustrative)
-                self.create_or_subcandidates(candidate, match_pair);
+        // match (1, true) {
-                split_or_candidate = true;
+        //     (1 | 2, false) => {},
+        //     (2, _) => {},
+        //     _ => {}
+        // }
+        // ```
+        //
+        // We therefore split the `candidates` slice in two, expand or-patterns in the first half,
+        // and process both halves separately.
+        let mut expand_until = 0;
+        for (i, candidate) in candidates.iter().enumerate() {
+            if matches!(
+                &*candidate.match_pairs,
+                [MatchPair { test_case: TestCase::Or { .. }, .. }, ..]
+            ) {
+                expand_until = i + 1;
+                if candidate.match_pairs.len() > 1 {
+                    break;
+                }
             }
         }
+        let (candidates_to_expand, remaining_candidates) = candidates.split_at_mut(expand_until);
 
         ensure_sufficient_stack(|| {
-            if split_or_candidate {
+            if candidates_to_expand.is_empty() {
-                // At least one of the candidates has been split into subcandidates.
+                // No candidates start with an or-pattern, we can continue.
-                // We need to change the candidate list to include those.
+                self.match_expanded_candidates(
-                let mut new_candidates = Vec::new();
+                    span,
-                for candidate in candidates.iter_mut() {
+                    scrutinee_span,
-                    candidate.visit_leaves(|leaf_candidate| new_candidates.push(leaf_candidate));
+                    start_block,
+                    otherwise_block,
+                    remaining_candidates,
+                );
+            } else {
+                // Expand one level of or-patterns for each candidate in `candidates_to_expand`.
+                let mut expanded_candidates = Vec::new();
+                for candidate in candidates_to_expand.iter_mut() {
+                    if let [MatchPair { test_case: TestCase::Or { .. }, .. }, ..] =
+                        &*candidate.match_pairs
+                    {
+                        let or_match_pair = candidate.match_pairs.remove(0);
+                        // Expand the or-pattern into subcandidates.
+                        self.create_or_subcandidates(candidate, or_match_pair);
+                        // Collect the newly created subcandidates.
+                        for subcandidate in candidate.subcandidates.iter_mut() {
+                            expanded_candidates.push(subcandidate);
+                        }
+                    } else {
+                        expanded_candidates.push(candidate);
+                    }
                 }
+
+                // Process the expanded candidates.
+                let remainder_start = self.cfg.start_new_block();
+                // There might be new or-patterns obtained from expanding the old ones, so we call
+                // `match_candidates` again.
                 self.match_candidates(
                     span,
                     scrutinee_span,
                     start_block,
-                    otherwise_block,
+                    remainder_start,
-                    &mut *new_candidates,
+                    expanded_candidates.as_mut_slice(),
                 );
 
-                for candidate in candidates {
+                // Simplify subcandidates and process any leftover match pairs.
-                    self.merge_trivial_subcandidates(candidate);
+                for candidate in candidates_to_expand {
+                    if !candidate.subcandidates.is_empty() {
+                        self.finalize_or_candidate(span, scrutinee_span, candidate);
+                    }
                 }
-            } else {
+
-                self.match_simplified_candidates(
+                // Process the remaining candidates.
+                self.match_candidates(
                     span,
                     scrutinee_span,
-                    start_block,
+                    remainder_start,
                     otherwise_block,
-                    candidates,
+                    remaining_candidates,
                 );
             }
         });
     }
 
-    fn match_simplified_candidates(
+    /// Construct the decision tree for `candidates`. Caller must ensure that no candidate in
+    /// `candidates` starts with an or-pattern.
+    fn match_expanded_candidates(
         &mut self,
         span: Span,
         scrutinee_span: Span,
@@ -1438,7 +1487,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // The first candidate has satisfied all its match pairs; we link it up and continue
                 // with the remaining candidates.
                 start_block = self.select_matched_candidate(first, start_block);
-                self.match_simplified_candidates(
+                self.match_expanded_candidates(
                     span,
                     scrutinee_span,
                     start_block,
@@ -1448,7 +1497,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             candidates => {
                 // The first candidate has some unsatisfied match pairs; we proceed to do more tests.
-                self.test_candidates_with_or(
+                self.test_candidates(
                     span,
                     scrutinee_span,
                     candidates,
@@ -1495,16 +1544,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
         candidate.pre_binding_block = Some(start_block);
         let otherwise_block = self.cfg.start_new_block();
-        if candidate.has_guard {
+        // Create the otherwise block for this candidate, which is the
-            // Create the otherwise block for this candidate, which is the
+        // pre-binding block for the next candidate.
-            // pre-binding block for the next candidate.
+        candidate.otherwise_block = Some(otherwise_block);
-            candidate.otherwise_block = Some(otherwise_block);
-        }
         otherwise_block
     }
 
-    /// Tests a candidate where there are only or-patterns left to test, or
+    /// Simplify subcandidates and process any leftover match pairs. The candidate should have been
-    /// forwards to [Builder::test_candidates].
+    /// expanded with `create_or_subcandidates`.
     ///
     /// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
     /// so:
@@ -1556,84 +1603,56 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ///      |
     ///     ...
     /// ```
-    fn test_candidates_with_or(
+    fn finalize_or_candidate(
         &mut self,
         span: Span,
         scrutinee_span: Span,
-        candidates: &mut [&mut Candidate<'_, 'tcx>],
+        candidate: &mut Candidate<'_, 'tcx>,
-        start_block: BasicBlock,
-        otherwise_block: BasicBlock,
     ) {
-        let (first_candidate, remaining_candidates) = candidates.split_first_mut().unwrap();
+        if candidate.subcandidates.is_empty() {
-        assert!(first_candidate.subcandidates.is_empty());
-        if !matches!(first_candidate.match_pairs[0].test_case, TestCase::Or { .. }) {
-            self.test_candidates(span, scrutinee_span, candidates, start_block, otherwise_block);
             return;
         }
 
-        let first_match_pair = first_candidate.match_pairs.remove(0);
+        self.merge_trivial_subcandidates(candidate);
-        let remaining_match_pairs = mem::take(&mut first_candidate.match_pairs);
-        let remainder_start = self.cfg.start_new_block();
-        // Test the alternatives of this or-pattern.
-        self.test_or_pattern(first_candidate, start_block, remainder_start, first_match_pair);
 
-        if !remaining_match_pairs.is_empty() {
+        if !candidate.match_pairs.is_empty() {
             // If more match pairs remain, test them after each subcandidate.
             // We could add them to the or-candidates before the call to `test_or_pattern` but this
             // would make it impossible to detect simplifiable or-patterns. That would guarantee
             // exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`.
-            first_candidate.visit_leaves(|leaf_candidate| {
+            let mut last_otherwise = None;
+            candidate.visit_leaves(|leaf_candidate| {
+                last_otherwise = leaf_candidate.otherwise_block;
+            });
+            let remaining_match_pairs = mem::take(&mut candidate.match_pairs);
+            candidate.visit_leaves(|leaf_candidate| {
                 assert!(leaf_candidate.match_pairs.is_empty());
                 leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned());
                 let or_start = leaf_candidate.pre_binding_block.unwrap();
-                // In a case like `(a | b, c | d)`, if `a` succeeds and `c | d` fails, we know `(b,
+                // In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q,
-                // c | d)` will fail too. If there is no guard, we skip testing of `b` by branching
+                // R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching
-                // directly to `remainder_start`. If there is a guard, we have to try `(b, c | d)`.
+                // directly to `last_otherwise`. If there is a guard,
-                let or_otherwise = leaf_candidate.otherwise_block.unwrap_or(remainder_start);
+                // `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we
-                self.test_candidates_with_or(
+                // can't skip `Q`.
+                let or_otherwise = if leaf_candidate.has_guard {
+                    leaf_candidate.otherwise_block.unwrap()
+                } else {
+                    last_otherwise.unwrap()
+                };
+                self.match_candidates(
                     span,
                     scrutinee_span,
-                    &mut [leaf_candidate],
                     or_start,
                     or_otherwise,
+                    &mut [leaf_candidate],
                 );
             });
         }
-
-        // Test the remaining candidates.
-        self.match_candidates(
-            span,
-            scrutinee_span,
-            remainder_start,
-            otherwise_block,
-            remaining_candidates,
-        );
-    }
-
-    #[instrument(skip(self, start_block, otherwise_block, candidate, match_pair), level = "debug")]
-    fn test_or_pattern<'pat>(
-        &mut self,
-        candidate: &mut Candidate<'pat, 'tcx>,
-        start_block: BasicBlock,
-        otherwise_block: BasicBlock,
-        match_pair: MatchPair<'pat, 'tcx>,
-    ) {
-        let or_span = match_pair.pattern.span;
-        self.create_or_subcandidates(candidate, match_pair);
-        let mut or_candidate_refs: Vec<_> = candidate.subcandidates.iter_mut().collect();
-        self.match_candidates(
-            or_span,
-            or_span,
-            start_block,
-            otherwise_block,
-            &mut or_candidate_refs,
-        );
-        self.merge_trivial_subcandidates(candidate);
     }
 
     /// Given a match-pair that corresponds to an or-pattern, expand each subpattern into a new
     /// subcandidate. Any candidate that has been expanded that way should be passed to
-    /// `merge_trivial_subcandidates` after its subcandidates have been processed.
+    /// `finalize_or_candidate` after its subcandidates have been processed.
     fn create_or_subcandidates<'pat>(
         &mut self,
         candidate: &mut Candidate<'pat, 'tcx>,
@@ -1651,8 +1670,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     }
 
     /// Try to merge all of the subcandidates of the given candidate into one. This avoids
-    /// exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. The or-pattern should have
+    /// exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. The candidate should have been
-    /// been expanded with `create_or_subcandidates`.
+    /// expanded with `create_or_subcandidates`.
     fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
         if candidate.subcandidates.is_empty() || candidate.has_guard {
             // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
@@ -1664,6 +1683,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             subcandidate.subcandidates.is_empty() && subcandidate.extra_data.is_empty()
         });
         if can_merge {
+            let mut last_otherwise = None;
             let any_matches = self.cfg.start_new_block();
             let or_span = candidate.or_span.take().unwrap();
             let source_info = self.source_info(or_span);
@@ -1674,8 +1694,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             for subcandidate in mem::take(&mut candidate.subcandidates) {
                 let or_block = subcandidate.pre_binding_block.unwrap();
                 self.cfg.goto(or_block, source_info, any_matches);
+                last_otherwise = subcandidate.otherwise_block;
             }
             candidate.pre_binding_block = Some(any_matches);
+            assert!(last_otherwise.is_some());
+            candidate.otherwise_block = last_otherwise;
         } else {
             // Never subcandidates may have a set of bindings inconsistent with their siblings,
             // which would break later code. So we filter them out. Note that we can't filter out

tests/mir-opt/or_pattern.rs

@@ -0,0 +1,24 @@
+// skip-filecheck
+
+// EMIT_MIR or_pattern.shortcut_second_or.SimplifyCfg-initial.after.mir
+fn shortcut_second_or() {
+    // Check that after matching `0`, failing to match `2 | 3` skips trying to match `(1, 2 | 3)`.
+    match ((0, 0), 0) {
+        (x @ (0, _) | x @ (_, 1), y @ 2 | y @ 3) => {}
+        _ => {}
+    }
+}
+
+// EMIT_MIR or_pattern.single_switchint.SimplifyCfg-initial.after.mir
+fn single_switchint() {
+    // Check how many `SwitchInt`s we do. In theory a single one is necessary.
+    match (1, true) {
+        (1, true) => 1,
+        (2, false) => 2,
+        (1 | 2, true | false) => 3,
+        (3 | 4, true | false) => 4,
+        _ => 5,
+    };
+}
+
+fn main() {}

tests/mir-opt/or_pattern.shortcut_second_or.SimplifyCfg-initial.after.mir

@@ -0,0 +1,100 @@
+// MIR for `shortcut_second_or` after SimplifyCfg-initial
+
+fn shortcut_second_or() -> () {
+    let mut _0: ();
+    let mut _1: ((i32, i32), i32);
+    let mut _2: (i32, i32);
+    let _3: (i32, i32);
+    let _4: i32;
+    scope 1 {
+        debug x => _3;
+        debug y => _4;
+    }
+
+    bb0: {
+        StorageLive(_1);
+        StorageLive(_2);
+        _2 = (const 0_i32, const 0_i32);
+        _1 = (move _2, const 0_i32);
+        StorageDead(_2);
+        PlaceMention(_1);
+        switchInt(((_1.0: (i32, i32)).0: i32)) -> [0: bb4, otherwise: bb2];
+    }
+
+    bb1: {
+        _0 = const ();
+        goto -> bb14;
+    }
+
+    bb2: {
+        switchInt(((_1.0: (i32, i32)).1: i32)) -> [1: bb3, otherwise: bb1];
+    }
+
+    bb3: {
+        switchInt((_1.1: i32)) -> [2: bb7, 3: bb8, otherwise: bb1];
+    }
+
+    bb4: {
+        switchInt((_1.1: i32)) -> [2: bb5, 3: bb6, otherwise: bb1];
+    }
+
+    bb5: {
+        falseEdge -> [real: bb10, imaginary: bb6];
+    }
+
+    bb6: {
+        falseEdge -> [real: bb11, imaginary: bb2];
+    }
+
+    bb7: {
+        falseEdge -> [real: bb12, imaginary: bb8];
+    }
+
+    bb8: {
+        falseEdge -> [real: bb13, imaginary: bb1];
+    }
+
+    bb9: {
+        _0 = const ();
+        StorageDead(_4);
+        StorageDead(_3);
+        goto -> bb14;
+    }
+
+    bb10: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb11: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb12: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb13: {
+        StorageLive(_3);
+        _3 = (_1.0: (i32, i32));
+        StorageLive(_4);
+        _4 = (_1.1: i32);
+        goto -> bb9;
+    }
+
+    bb14: {
+        StorageDead(_1);
+        return;
+    }
+}

tests/mir-opt/or_pattern.single_switchint.SimplifyCfg-initial.after.mir

@@ -0,0 +1,75 @@
+// MIR for `single_switchint` after SimplifyCfg-initial
+
+fn single_switchint() -> () {
+    let mut _0: ();
+    let _1: i32;
+    let mut _2: (i32, bool);
+
+    bb0: {
+        StorageLive(_1);
+        StorageLive(_2);
+        _2 = (const 1_i32, const true);
+        PlaceMention(_2);
+        switchInt((_2.0: i32)) -> [1: bb2, 2: bb4, otherwise: bb1];
+    }
+
+    bb1: {
+        switchInt((_2.0: i32)) -> [3: bb8, 4: bb8, otherwise: bb7];
+    }
+
+    bb2: {
+        switchInt((_2.1: bool)) -> [0: bb6, otherwise: bb3];
+    }
+
+    bb3: {
+        falseEdge -> [real: bb9, imaginary: bb4];
+    }
+
+    bb4: {
+        switchInt((_2.1: bool)) -> [0: bb5, otherwise: bb6];
+    }
+
+    bb5: {
+        falseEdge -> [real: bb10, imaginary: bb6];
+    }
+
+    bb6: {
+        falseEdge -> [real: bb11, imaginary: bb1];
+    }
+
+    bb7: {
+        _1 = const 5_i32;
+        goto -> bb13;
+    }
+
+    bb8: {
+        falseEdge -> [real: bb12, imaginary: bb7];
+    }
+
+    bb9: {
+        _1 = const 1_i32;
+        goto -> bb13;
+    }
+
+    bb10: {
+        _1 = const 2_i32;
+        goto -> bb13;
+    }
+
+    bb11: {
+        _1 = const 3_i32;
+        goto -> bb13;
+    }
+
+    bb12: {
+        _1 = const 4_i32;
+        goto -> bb13;
+    }
+
+    bb13: {
+        StorageDead(_2);
+        StorageDead(_1);
+        _0 = const ();
+        return;
+    }
+}

tests/ui/or-patterns/bindings-runpass-2.rs

@@ -26,5 +26,6 @@ fn main() {
     assert_eq!(or_at(Err(7)), 207);
     assert_eq!(or_at(Err(8)), 8);
     assert_eq!(or_at(Err(20)), 220);
+    assert_eq!(or_at(Err(34)), 134);
     assert_eq!(or_at(Err(50)), 500);
 }

tests/ui/or-patterns/inner-or-pat.or3.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/inner-or-pat.rs:38:54
+  --> $DIR/inner-or-pat.rs:36:54
    |
 LL |     match x {
    |           - this expression has type `&str`

tests/ui/or-patterns/inner-or-pat.or4.stderr

@@ -1,5 +1,5 @@
 error[E0408]: variable `x` is not bound in all patterns
-  --> $DIR/inner-or-pat.rs:53:37
+  --> $DIR/inner-or-pat.rs:51:37
    |
 LL |         (x @ "red" | (x @ "blue" |  "red")) => {
    |                       -             ^^^^^ pattern doesn't bind `x`

tests/ui/or-patterns/inner-or-pat.rs

@@ -1,7 +1,5 @@
-//@ revisions: or1 or2 or3 or4 or5
+//@ revisions: or1 or3 or4
 //@ [or1] run-pass
-//@ [or2] run-pass
-//@ [or5] run-pass
 
 #![allow(unreachable_patterns)]
 #![allow(unused_variables)]

tests/ui/or-patterns/issue-70413-no-unreachable-pat-and-guard.rs

@@ -1,21 +1,20 @@
-//@ check-pass
+//@ run-pass
 
 #![deny(unreachable_patterns)]
 
 fn main() {
-    match (3,42) {
+    match (3, 42) {
-        (a,_) | (_,a) if a > 10 => {println!("{}", a)}
+        (a, _) | (_, a) if a > 10 => {}
-        _ => ()
+        _ => unreachable!(),
     }
 
-    match Some((3,42)) {
+    match Some((3, 42)) {
-        Some((a, _)) | Some((_, a)) if a > 10 => {println!("{}", a)}
+        Some((a, _)) | Some((_, a)) if a > 10 => {}
-        _ => ()
+        _ => unreachable!(),
-
     }
 
-    match Some((3,42)) {
+    match Some((3, 42)) {
-        Some((a, _) | (_, a)) if a > 10 => {println!("{}", a)}
+        Some((a, _) | (_, a)) if a > 10 => {}
-        _ => ()
+        _ => unreachable!(),
     }
 }

tests/ui/or-patterns/search-via-bindings.rs

@@ -42,6 +42,23 @@ fn search_old_style(target: (bool, bool, bool)) -> u32 {
     }
 }
 
+// Check that a dummy or-pattern also leads to running the guard multiple times.
+fn search_with_dummy(target: (bool, bool)) -> u32 {
+    let x = ((false, true), (false, true), ());
+    let mut guard_count = 0;
+    match x {
+        ((a, _) | (_, a), (b, _) | (_, b), _ | _)
+            if {
+                guard_count += 1;
+                (a, b) == target
+            } =>
+        {
+            guard_count
+        }
+        _ => unreachable!(),
+    }
+}
+
 fn main() {
     assert_eq!(search((false, false, false)), 1);
     assert_eq!(search((false, false, true)), 2);
@@ -60,4 +77,9 @@ fn main() {
     assert_eq!(search_old_style((true, false, true)), 6);
     assert_eq!(search_old_style((true, true, false)), 7);
     assert_eq!(search_old_style((true, true, true)), 8);
+
+    assert_eq!(search_with_dummy((false, false)), 1);
+    assert_eq!(search_with_dummy((false, true)), 3);
+    assert_eq!(search_with_dummy((true, false)), 5);
+    assert_eq!(search_with_dummy((true, true)), 7);
 }

tests/ui/or-patterns/simplification_subtleties.rs

@@ -0,0 +1,11 @@
+//@ run-pass
+
+#[allow(unreachable_patterns)]
+fn main() {
+    // Test that we don't naively sort the two `2`s together and confuse the failure paths.
+    match (1, true) {
+        (1 | 2, false | false) => unreachable!(),
+        (2, _) => unreachable!(),
+        _ => {}
+    }
+}