Fix a span in `parse_ty_bare_fn`.

It currently goes one token too far.

Example: line 259 of `tests/ui/abi/compatibility.rs`:
```
test_abi_compatible!(fn_fn, fn(), fn(i32) -> i32);
```
This commit changes the span for the second element from `fn(),` to
`fn()`, i.e. removes the extraneous comma.

compiler/rustc_ast/src/ast.rs

@@ -2126,7 +2126,8 @@ pub struct BareFnTy {
     pub ext: Extern,
     pub generic_params: ThinVec<GenericParam>,
     pub decl: P<FnDecl>,
-    /// Span of the `fn(...) -> ...` part.
+    /// Span of the `[unsafe] [extern] fn(...) -> ...` part, i.e. everything
+    /// after the generic params (if there are any, e.g. `for<'a>`).
     pub decl_span: Span,
 }
 

compiler/rustc_parse/src/parser/ty.rs

@@ -608,7 +608,7 @@ impl<'a> Parser<'a> {
             self.dcx().emit_err(FnPointerCannotBeAsync { span: whole_span, qualifier: span });
         }
         // FIXME(gen_blocks): emit a similar error for `gen fn()`
-        let decl_span = span_start.to(self.token.span);
+        let decl_span = span_start.to(self.prev_token.span);
         Ok(TyKind::BareFn(P(BareFnTy { ext, safety, generic_params: params, decl, decl_span })))
     }
 

tests/ui/rust-2024/safe-outside-extern.gated.stderr

@@ -26,7 +26,7 @@ error: function pointers cannot be declared with `safe` safety qualifier
   --> $DIR/safe-outside-extern.rs:24:14
    |
 LL | type FnPtr = safe fn(i32, i32) -> i32;
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 5 previous errors
 

tests/ui/rust-2024/safe-outside-extern.ungated.stderr

@@ -26,7 +26,7 @@ error: function pointers cannot be declared with `safe` safety qualifier
   --> $DIR/safe-outside-extern.rs:24:14
    |
 LL | type FnPtr = safe fn(i32, i32) -> i32;
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0658]: `unsafe extern {}` blocks and `safe` keyword are experimental
   --> $DIR/safe-outside-extern.rs:4:1