Fix #71977

src/librustc_mir_build/hair/pattern/_match.rs

@@ -276,6 +276,7 @@ use self::Usefulness::*;
 use self::WitnessPreference::*;
 
 use rustc_data_structures::captures::Captures;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_index::vec::Idx;
 
 use super::{compare_const_vals, PatternFoldable, PatternFolder};
@@ -1852,16 +1853,35 @@ crate fn is_useful<'p, 'tcx>(
         // We need to push the already-seen patterns into the matrix in order to detect redundant
         // branches like `Some(_) | Some(0)`. We also keep track of the unreachable subpatterns.
         let mut matrix = matrix.clone();
-        let mut unreachable_pats = Vec::new();
+        // `Vec` of all the unreachable branches of the current or-pattern.
+        let mut unreachable_branches = Vec::new();
+        // Subpatterns that are unreachable from all branches. E.g. in the following case, the last
+        // `true` is unreachable only from one branch, so it is overall reachable.
+        // ```
+        // match (true, true) {
+        //     (true, true) => {}
+        //     (false | true, false | true) => {}
+        // }
+        // ```
+        let mut unreachable_subpats = FxHashSet::default();
+        // Whether any branch at all is useful.
         let mut any_is_useful = false;
+
         for v in vs {
             let res = is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
             match res {
                 Useful(pats) => {
-                    any_is_useful = true;
-                    unreachable_pats.extend(pats);
+                    if !any_is_useful {
+                        any_is_useful = true;
+                        // Initialize with the first set of unreachable subpatterns encountered.
+                        unreachable_subpats = pats.into_iter().collect();
+                    } else {
+                        // Keep the patterns unreachable from both this and previous branches.
+                        unreachable_subpats =
+                            pats.into_iter().filter(|p| unreachable_subpats.contains(p)).collect();
+                    }
                 }
-                NotUseful => unreachable_pats.push(v.head().span),
+                NotUseful => unreachable_branches.push(v.head().span),
                 UsefulWithWitness(_) => {
                     bug!("Encountered or-pat in `v` during exhaustiveness checking")
                 }
@@ -1871,7 +1891,13 @@ crate fn is_useful<'p, 'tcx>(
                 matrix.push(v);
             }
         }
-        return if any_is_useful { Useful(unreachable_pats) } else { NotUseful };
+        if any_is_useful {
+            // Collect all the unreachable patterns.
+            unreachable_branches.extend(unreachable_subpats);
+            return Useful(unreachable_branches);
+        } else {
+            return NotUseful;
+        }
     }
 
     // FIXME(Nadrieril): Hack to work around type normalization issues (see #72476).

src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.rs

@@ -29,6 +29,9 @@ fn main() {
         (1, 4 | 5) => {} //~ ERROR unreachable pattern
         _ => {}
     }
+    match (true, true) {
+        (false | true, false | true) => (),
+    }
     match (Some(0u8),) {
         (None | Some(1 | 2),) => {}
         (Some(1),) => {} //~ ERROR unreachable pattern
@@ -67,4 +70,29 @@ fn main() {
              | 1) => {}
         _ => {}
     }
+
+    // A subpattern that is only unreachable in one branch is overall reachable.
+    match (true, true) {
+        (true, true) => {}
+        (false | true, false | true) => {}
+    }
+    match (true, true) {
+        (true, false) => {}
+        (false, true) => {}
+        (false | true, false | true) => {}
+    }
+    // A subpattern that is unreachable in all branches is overall unreachable.
+    match (true, true) {
+        (false, true) => {}
+        (true, true) => {}
+        (false | true, false
+            | true) => {} //~ ERROR unreachable
+    }
+    match (true, true) {
+        (true, false) => {}
+        (true, true) => {}
+        (false
+            | true, //~ ERROR unreachable
+            false | true) => {}
+    }
 }

src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.stderr

@@ -53,52 +53,64 @@ LL |         (1, 4 | 5) => {}
    |         ^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:34:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:37:9
    |
 LL |         (Some(1),) => {}
    |         ^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:35:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:38:9
    |
 LL |         (None,) => {}
    |         ^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:40:9
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:43:9
    |
 LL |         ((1..=4,),) => {}
    |         ^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:45:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:48:14
    |
 LL |         (1 | 1,) => {}
    |              ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:52:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:53:15
    |
-LL |             | 0] => {}
+LL |             | 0
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:50:15
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:55:15
    |
-LL |             | 0
+LL |             | 0] => {}
    |               ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:60:10
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:63:10
    |
 LL |         [1
    |          ^
 
 error: unreachable pattern
-  --> $DIR/exhaustiveness-unreachable-pattern.rs:66:14
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:69:14
    |
 LL |         Some(0
    |              ^
 
-error: aborting due to 16 previous errors
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:89:15
+   |
+LL |             | true) => {}
+   |               ^^^^
+
+error: unreachable pattern
+  --> $DIR/exhaustiveness-unreachable-pattern.rs:95:15
+   |
+LL |             | true,
+   |               ^^^^
+
+error: aborting due to 18 previous errors
 

src/test/ui/or-patterns/search-via-bindings.rs

@@ -3,7 +3,6 @@
 // run-pass
 
 #![feature(or_patterns)]
-#![allow(unreachable_patterns)] // FIXME(or-patterns) this shouldn't trigger
 
 fn search(target: (bool, bool, bool)) -> u32 {
     let x = ((false, true), (false, true), (false, true));