Add check-pass test for &raw

[fmease]

&raw denotes a normal/non-raw borrow of the path raw, not the start of raw borrow expr since it's not followed by either const or mut. Ensure this (and variants) will never regress!

When I saw the open diagnostic issue #133231 (better parse error (recovery) on &raw <expr>), it made me think that we have to make sure that we will never commit too early/overzealously(†) when encountering the sequence &raw, even during parse error recovery!

Modifying the parser to eagerly treat &raw as the start of a raw borrow expr only lead to a single UI test failing, namely tests/ui/enum-discriminant/ptr_niche.rs. However, this is just coincidental — it doesn't intentionally test this edge case of the grammar.

---

†: With "eager" I mean something like:

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 0904a42d8a4..68d690fd602 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -873,11 +873,16 @@ fn error_remove_borrow_lifetime(&self, span: Span, lt_span: Span) {

     /// Parse `mut?` or `raw [ const | mut ]`.
     fn parse_borrow_modifiers(&mut self) -> (ast::BorrowKind, ast::Mutability) {
-        if self.check_keyword(kw::Raw) && self.look_ahead(1, Token::is_mutability) {
+        if self.eat_keyword(kw::Raw) {
             // `raw [ const | mut ]`.
-            let found_raw = self.eat_keyword(kw::Raw);
-            assert!(found_raw);
-            let mutability = self.parse_const_or_mut().unwrap();
+            let mutability = self.parse_const_or_mut().unwrap_or_else(|| {
+                let span = self.prev_token.span;
+                self.dcx().emit_err(ExpectedMutOrConstInRawBorrowExpr {
+                    span,
+                    after_ampersand: span.shrink_to_hi(),
+                });
+                ast::Mutability::Not
+            });
             (ast::BorrowKind::Raw, mutability)
         } else {
             // `mut?`

---

r? compiler

[Noratrieb]

heh, GitHub's syntax highlighting does not like it

@bors r+ rollup

[bors]

📌 Commit f1d2a6a has been approved by Noratrieb

It is now in the queue for this repository.