Fix ICE-133117: multiple never-pattern arm doesn't have false_edge_start_block

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

@@ -1940,6 +1940,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// in match tree lowering.
     fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
         assert!(!candidate.subcandidates.is_empty());
+        if candidate.false_edge_start_block.is_none() {
+            candidate.false_edge_start_block = candidate.subcandidates[0].false_edge_start_block;
+        }
+
         if candidate.has_guard {
             // FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
             return;
@@ -1958,10 +1962,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // This candidate is about to become a leaf, so unset `or_span`.
         let or_span = candidate.or_span.take().unwrap();
         let source_info = self.source_info(or_span);
-
-        if candidate.false_edge_start_block.is_none() {
-            candidate.false_edge_start_block = candidate.subcandidates[0].false_edge_start_block;
-        }
 
         // Remove the (known-trivial) subcandidates from the candidate tree,
         // so that they aren't visible after match tree lowering, and wire them

tests/ui/rfcs/rfc-0000-never_patterns/ICE-133117-duplicate-never-arm.rs

@@ -0,0 +1,12 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+fn main() {
+    match () {
+        (!|
+        //~^ ERROR: mismatched types
+        !) if true => {} //~ ERROR a never pattern is always unreachable
+        //~^ ERROR: mismatched types
+        (!|!) if true => {} //~ ERROR a never pattern is always unreachable
+    }
+}

tests/ui/rfcs/rfc-0000-never_patterns/ICE-133117-duplicate-never-arm.stderr

@@ -0,0 +1,36 @@
+error: a never pattern is always unreachable
+  --> $DIR/ICE-133117-duplicate-never-arm.rs:8:23
+   |
+LL |         !) if true => {}
+   |                       ^^
+   |                       |
+   |                       this will never be executed
+   |                       help: remove this expression
+
+error: a never pattern is always unreachable
+  --> $DIR/ICE-133117-duplicate-never-arm.rs:10:26
+   |
+LL |         (!|!) if true => {}
+   |                          ^^
+   |                          |
+   |                          this will never be executed
+   |                          help: remove this expression
+
+error: mismatched types
+  --> $DIR/ICE-133117-duplicate-never-arm.rs:6:10
+   |
+LL |         (!|
+   |          ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `()`
+
+error: mismatched types
+  --> $DIR/ICE-133117-duplicate-never-arm.rs:8:9
+   |
+LL |         !) if true => {}
+   |         ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `()`
+
+error: aborting due to 4 previous errors
+