Confusing diagnostics on #[derive(X)] when X is imported only as trait

[m-ou-se]

Reported by @Hoverbear here: https://twitter.com/a_hoverbear/status/1428763621572501507

use serde::{Deserialize, Serialize};

#[derive(Serialize)]
struct A;

Output:

error: cannot find derive macro `Serialize` in this scope
 --> src/main.rs:3:10
  |
3 | #[derive(Serialize)]
  |          ^^^^^^^^^

warning: unused import: `serde::Serialize`
 --> src/main.rs:1:5
  |
1 | use serde::Serialize;
  |     ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

The diagnostics shows that Serialize is not found, and then complains that Serialize was imported and unused. Confusing.

(Only the trait Serialize is imported, not the Serialize derive macro, as the derive feature wasn't enabled on serde.)

[m-ou-se]

I have a patch that improves things a bit by adding a note to the error:

error: cannot find derive macro `Serialize` in this scope
 --> src/main.rs:3:10
  |
3 | #[derive(Serialize)]
  |          ^^^^^^^^^
  |
note: `Serialize` is imported here, but it is not a derive macro
 --> src/main.rs:1:5
  |
1 | use serde::Serialize;
  |     ^^^^^^^^^^^^^^^^

Will clean it up, add some tests, and send the PR later.

[m-ou-se]

This adds notes to all 'macro not found' diagnostics where the name is in scope in some other namespace, or as another type of macro: #88232

[patientplatypus6]

I might suggest that you include something along the lines of

"are you sure that you imported the derive package of Derive in Cargo.toml?"

https://serde.rs/derive.html