Rollup merge of #135409 - Shunpoco:issue-133117-ICE-never-false-edge-start-block, r=Nadrieril

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

Fixes #133117 , and close fixes #133063 , fixes #130779

In order to fix ICE-133117, at first I needed to tackle to ICE-133063 (this fixed 130779 as well).

### ICE-133063 and ICE-130779
This ICE is caused by those steps:
1. An arm has or-pattern, and all of the sub-candidates are never-pattern
2. In that case, all sub-candidates are removed in remove_never_subcandidates(). So the arm (candidate) has no sub-candidate.
3. In the current implementation, if there is no sub-candidate, the function assigns `pre_binding_block` into the candidate ([here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir_build/src/builder/matches/mod.rs#L2002-L2004)). However, otherwise_block should be assigned to the candidate as well, because the otherwise_block is unwrapped in multiple place (like in lower_match_tree()). As a result, it causes the panic.

I simply added the same block as pre_binding_block into otherwise_block, but I'm wondering if there is a better block to assign to otherwise_block (is it ok to assign the same block into pre_binding and otherwise?)

### ICE-133117
This is caused by those steps:
1. There are two arms, both are or-pattern and each has one match-pair (in the test code, both are `(!|!)`), and the second arm has a guard.
2. In match_candidate() for the first arm, it expands the second arm’s sub-candidates as well ([here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir_build/src/builder/matches/mod.rs#L1800-L1805)). As a result, the root candidate of the second arm is not evaluated/modified in match_candidate(). So a false_edge_start_block is not assigned to the candidate.
3. merge_trivial_subcandidates() is called against the candidate for the second arm. It just returns immediately because the candidate has a guard. So a flase_edge_start_block is not assigned to the candidate also in this function.
4. remove_never_subcandidates() is called against the candidate. Since all sub-candidates are never-pattern. they are removed.
5. In lower_match_tree(), since there is no sub-candidate for the candidate, the candidate itself is evaluated in visit_leave_rev ([here](https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir_build/src/builder/matches/mod.rs#L1532)). Because the candidate has no false_edge_start_block, it causes the panic.

So I modified the order of if blocks in merge_trivial_subcandidates() to assign a false_edge_start_block if the candidate doesn't have.

Author: matthiaskrgr

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

@@ -1986,6 +1986,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             return;
         }
 
+        let false_edge_start_block = candidate.subcandidates[0].false_edge_start_block;
         candidate.subcandidates.retain_mut(|candidate| {
             if candidate.extra_data.is_never {
                 candidate.visit_leaves(|subcandidate| {
@@ -2000,8 +2001,14 @@ 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);
+            // In addition, if `candidate` doesn't have `false_edge_start_block`, it should be assigned here.
+            if candidate.false_edge_start_block.is_none() {
+                candidate.false_edge_start_block = false_edge_start_block;
+            }
         }
     }
 

tests/crashes/130779.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/crashes/133063.rs

@@ -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/crashes/133117.rs

@@ -0,0 +1,16 @@
+#![feature(never_type)]
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum Void {}
+
+fn foo(x: Void) {
+    loop {
+        match x {
+            (!|!) if false => {} //~ ERROR a never pattern is always unreachable
+            _ => {}
+        }
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,11 @@
+error: a never pattern is always unreachable
+  --> $DIR/ICE-133063-never-arm-no-otherwise-block.rs:10:31
+   |
+LL |             (!|!) if false => {}
+   |                               ^^
+   |                               |
+   |                               this will never be executed
+   |                               help: remove this expression
+
+error: aborting due to 1 previous error
+

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

@@ -0,0 +1,14 @@
+#![feature(never_type)]
+#![feature(never_patterns)]
+#![allow(incomplete_features)]
+
+enum Void {}
+
+fn foo(x: Void) {
+    match x {
+        (!|!) if true => {} //~ ERROR a never pattern is always unreachable
+        (!|!) if true => {} //~ ERROR a never pattern is always unreachable
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,20 @@
+error: a never pattern is always unreachable
+  --> $DIR/ICE-133117-duplicate-never-arm.rs:9:26
+   |
+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: aborting due to 2 previous errors
+