Identify missing item category in `impl`s

Identify missing item category in `impl`s

```rust
struct S;
impl S {
    pub hello_method(&self) {
        println!("Hello");
    }
}
fn main() { S.hello_method(); }
```

```rust
error: can't qualify macro invocation with `pub`
 --> file.rs:3:4
  |
3 |     pub hello_method(&self) {
  |     ^^^-            - expected `!` here for a macro invocation
  |        |
  |        did you mean to write `fn` here for a method declaration?
  |
  = help: try adjusting the macro to put `pub` inside the invocation
```