Rollup merge of rust-lang#134274 - fmease:amp-raw-is-a-normal-borrow, r=Noratrieb

Add check-pass test for `&raw`

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

When I saw the open diagnostic issue rust-lang#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](https://github.com/rust-lang/rust/blob/4847d6a9d07d4be9ba3196f6ad444af2d7bdde72/tests/ui/enum-discriminant/ptr_niche.rs). However, this is just coincidental — it didn't *intentionally* test this edge case of the grammar.

---

†: With "eager" I mean something like:

```patch
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

Author: matthiaskrgr

tests/ui/raw-ref-op/amp-raw-without-mut-const-is-a-normal-borrow.rs

@@ -0,0 +1,30 @@
+//! The token sequence `&raw` *only* starts a raw borrow expr if it's immediately
+//! followed by either `const` or `mut`. If that's not the case, the `&` denotes
+//! the start of a normal borrow expr where `raw` is interpreted as a regular
+//! identifier and thus denotes the start of a path expr.
+//!
+//! This test ensures that we never commit too early/overzealously in the parser
+//! when encountering the sequence `&raw` (even during parse error recovery) so
+//! as not to regress preexisting code.
+
+//@ check-pass
+
+fn main() { // the odd formatting in here is intentional
+    let raw = 0;
+    let _ = &raw;
+
+    let raw = 0;
+    let local = 1;
+    let _: i32 = &raw *local;
+
+    let raw = |_| ();
+    let local = [0];
+    let () = &raw (local[0]);
+}
+
+macro_rules! check {
+    ($e:expr) => { compile_error!("expr"); };
+    (&raw $e:expr) => {};
+}
+
+check!(&raw local);