Auto merge of #118527 - Nadrieril:never_patterns_parse, r=compiler-errors

never_patterns: Parse match arms with no body

Never patterns are meant to signal unreachable cases, and thus don't take bodies:
```rust
let ptr: *const Option<!> = ...;
match *ptr {
    None => { foo(); }
    Some(!),
}
```
This PR makes rustc accept the above, and enforces that an arm has a body xor is a never pattern. This affects parsing of match arms even with the feature off, so this is delicate. (Plus this is my first non-trivial change to the parser).

~~The last commit is optional; it introduces a bit of churn to allow the new suggestions to be machine-applicable. There may be a better solution? I'm not sure.~~ EDIT: I removed that commit

r? `@compiler-errors`

clippy_utils/src/ast_utils.rs

@@ -236,7 +236,7 @@ pub fn eq_field(l: &ExprField, r: &ExprField) -> bool {
 pub fn eq_arm(l: &Arm, r: &Arm) -> bool {
     l.is_placeholder == r.is_placeholder
         && eq_pat(&l.pat, &r.pat)
-        && eq_expr(&l.body, &r.body)
+        && eq_expr_opt(&l.body, &r.body)
         && eq_expr_opt(&l.guard, &r.guard)
         && over(&l.attrs, &r.attrs, eq_attr)
 }