Rollup merge of #40815 - estebank:issue-40006, r=GuillaumeGomez

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: missing `fn` for method declaration
 --> file.rs:3:4
  |
3 |     pub hello_method(&self) {
  |        ^ missing `fn`
```

Fix #40006. r? @pnkfelix CC @jonathandturner @GuillaumeGomez

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax_pos/lib.rs
Simple merge