Simplify or-patterns in MIR building

Author: varkor

src/librustc_mir/build/matches/simplify.rs

@@ -35,6 +35,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         changed = true;
                     }
                     Err(match_pair) => {
+                        // Re-add the `match_pair` we were unable to simplify.
                         candidate.match_pairs.push(match_pair);
                     }
                 }
@@ -198,7 +199,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Ok(())
             }
 
-            PatKind::Or { .. } => Err(match_pair),
+            PatKind::Or { ref pats } => {
+                candidate
+                    .match_pairs
+                    .extend(pats.iter().map(|pat| MatchPair::new(match_pair.place.clone(), pat)));
+                Ok(())
+            }
         }
     }
 }

src/test/ui/or-patterns/irrefutable-or-binding.rs

@@ -0,0 +1,8 @@
+// run-pass
+
+#![feature(or_patterns)]
+//~^ WARN the feature `or_patterns` is incomplete and may cause the compiler to crash
+
+fn main() {
+    let _ | _ = (); // ok
+}

src/test/ui/or-patterns/irrefutable-or-binding.stderr

@@ -0,0 +1,8 @@
+warning: the feature `or_patterns` is incomplete and may cause the compiler to crash
+  --> $DIR/irrefutable-or-binding.rs:3:12
+   |
+LL | #![feature(or_patterns)]
+   |            ^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+