Identify missing item category in impls

```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
```

src/test/ui/did_you_mean/issue-40006.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct S;
+
+impl S {
+    pub hello_method(&self) {
+        println!("Hello");
+    }
+}
+
+fn main() {
+    S.hello_method();
+}

src/test/ui/did_you_mean/issue-40006.stderr

@@ -0,0 +1,12 @@
+error: can't qualify macro invocation with `pub`
+  --> $DIR/issue-40006.rs:14:5
+   |
+14 |     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
+
+error: aborting due to previous error
+