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`

compiler/rustc_expand/src/build.rs

@@ -505,7 +505,7 @@ impl<'a> ExtCtxt<'a> {
             attrs: AttrVec::new(),
             pat,
             guard: None,
-            body: expr,
+            body: Some(expr),
             span,
             id: ast::DUMMY_NODE_ID,
             is_placeholder: false,

compiler/rustc_expand/src/errors.rs

@@ -304,6 +304,8 @@ pub(crate) struct IncompleteParse<'a> {
     pub label_span: Span,
     pub macro_path: &'a ast::Path,
     pub kind_name: &'a str,
+    #[note(expand_macro_expands_to_match_arm)]
+    pub expands_to_match_arm: Option<()>,
 
     #[suggestion(
         expand_suggestion_add_semi,

compiler/rustc_expand/src/expand.rs

@@ -955,12 +955,15 @@ pub fn ensure_complete_parse<'a>(
             _ => None,
         };
 
+        let expands_to_match_arm = kind_name == "pattern" && parser.token == token::FatArrow;
+
         parser.sess.emit_err(IncompleteParse {
             span: def_site_span,
             token,
             label_span: span,
             macro_path,
             kind_name,
+            expands_to_match_arm: expands_to_match_arm.then_some(()),
             add_semicolon,
         });
     }

compiler/rustc_expand/src/placeholders.rs

@@ -119,7 +119,7 @@ pub fn placeholder(
         }]),
         AstFragmentKind::Arms => AstFragment::Arms(smallvec![ast::Arm {
             attrs: Default::default(),
-            body: expr_placeholder(),
+            body: Some(expr_placeholder()),
             guard: None,
             id,
             pat: pat(),