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;
@@ -1959,10 +1963,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         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
         // all to join up at a single shared pre-binding block.
@@ -2000,8 +2000,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         });
         if candidate.subcandidates.is_empty() {
-            // If `candidate` has become a leaf candidate, ensure it has a `pre_binding_block`.
-            candidate.pre_binding_block = Some(self.cfg.start_new_block());
+            // If `candidate` has become a leaf candidate, ensure it has a `pre_binding_block` and `otherwise_block`.
+            let next_block = self.cfg.start_new_block();
+            candidate.pre_binding_block = Some(next_block);
+            candidate.otherwise_block = Some(next_block);
         }
     }
 

tests/ui/rfcs/rfc-0000-never_patterns/ICE-130779-never-arm-no-oatherwise-block.rs

@@ -0,0 +1,12 @@
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum E { A }
+
+fn main() {
+    match E::A {
+        ! | //~ ERROR: a trailing `|` is not allowed in an or-pattern
+        //~^ ERROR: mismatched types
+        if true => {} //~ ERROR: a never pattern is always unreachable
+    }
+}

tests/ui/rfcs/rfc-0000-never_patterns/ICE-130779-never-arm-no-oatherwise-block.stderr

@@ -0,0 +1,33 @@
+error: a trailing `|` is not allowed in an or-pattern
+  --> $DIR/ICE-130779-never-arm-no-oatherwise-block.rs:8:11
+   |
+LL |         ! |
+   |         - ^
+   |         |
+   |         while parsing this or-pattern starting here
+   |
+help: remove the `|`
+   |
+LL -         ! |
+LL +         !
+   |
+
+error: a never pattern is always unreachable
+  --> $DIR/ICE-130779-never-arm-no-oatherwise-block.rs:10:20
+   |
+LL |         if true => {}
+   |                    ^^
+   |                    |
+   |                    this will never be executed
+   |                    help: remove this expression
+
+error: mismatched types
+  --> $DIR/ICE-130779-never-arm-no-oatherwise-block.rs:8:9
+   |
+LL |         ! |
+   |         ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `E`
+
+error: aborting due to 3 previous errors
+

tests/ui/rfcs/rfc-0000-never_patterns/ICE-133063-never-arm-no-otherwise-block.rs

@@ -0,0 +1,21 @@
+#![feature(never_patterns)]
+#![feature(if_let_guard)]
+#![allow(incomplete_features)]
+
+fn split_last(_: &()) -> Option<(&i32, &i32)> {
+    None
+}
+
+fn assign_twice() {
+    loop {
+        match () {
+            (!| //~ ERROR: mismatched types
+            !) if let _ = split_last(&()) => {} //~ ERROR a never pattern is always unreachable
+            //~^ ERROR: mismatched types
+            //~^^ WARNING: irrefutable `if let` guard pattern [irrefutable_let_patterns]
+            _ => {}
+        }
+    }
+}
+
+fn main() {}

tests/ui/rfcs/rfc-0000-never_patterns/ICE-133063-never-arm-no-otherwise-block.stderr

@@ -0,0 +1,37 @@
+error: a never pattern is always unreachable
+  --> $DIR/ICE-133063-never-arm-no-otherwise-block.rs:13:46
+   |
+LL |             !) if let _ = split_last(&()) => {}
+   |                                              ^^
+   |                                              |
+   |                                              this will never be executed
+   |                                              help: remove this expression
+
+error: mismatched types
+  --> $DIR/ICE-133063-never-arm-no-otherwise-block.rs:12:14
+   |
+LL |             (!|
+   |              ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `()`
+
+error: mismatched types
+  --> $DIR/ICE-133063-never-arm-no-otherwise-block.rs:13:13
+   |
+LL |             !) if let _ = split_last(&()) => {}
+   |             ^ a never pattern must be used on an uninhabited type
+   |
+   = note: the matched value is of type `()`
+
+warning: irrefutable `if let` guard pattern
+  --> $DIR/ICE-133063-never-arm-no-otherwise-block.rs:13:19
+   |
+LL |             !) if let _ = split_last(&()) => {}
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this pattern will always match, so the guard is useless
+   = help: consider removing the guard and adding a `let` inside the match arm
+   = note: `#[warn(irrefutable_let_patterns)]` on by default
+
+error: aborting due to 3 previous errors; 1 warning emitted
+

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
+