Rollup merge of #125049 - dtolnay:castbrace, r=compiler-errors

Disallow cast with trailing braced macro in let-else

This fixes an edge case I noticed while porting #118880 and #119062 to syn.

Previously, rustc incorrectly accepted code such as:

```rust
let foo = &std::ptr::null as &'static dyn std::ops::Fn() -> *const primitive! {
    8
} else {
    return;
};
```

even though a right curl brace `}` directly before `else` in a `let...else` statement is not supposed to be valid syntax.

Author: fmease