Add notes to macro-not-found diagnostics to point out how things with the same name were not a match.

[m-ou-se]

This adds notes like:

error: cannot find derive macro `Serialize` in this scope
  --> $DIR/issue-88206.rs:22:10
   |
LL | #[derive(Serialize)]
   |          ^^^^^^^^^
   |
note: `Serialize` is imported here, but it is not a derive macro
  --> $DIR/issue-88206.rs:17:11
   |
LL | use hey::{Serialize, Deserialize};
   |           ^^^^^^^^^

Fixes #88206

Includes #88229

r? @estebank

[estebank]

I love how these are looking 😃

I'm leaving some nitpicks, but we don't need to address them all to land this.

[estebank]

For the cases where we don't have a span, wouldn't it make sense to turn this into the primary span? Something like:

error: cannot find macro `Copy` in this scope
  --> $DIR/issue-88206.rs:56:5
   |
LL |     Copy!();
   |     ^^^^ this is not a macro, it is a derive macro: `#[derive(Copy)]`

[m-ou-se]

I'd avoid pointing the note at the identifier the user wrote, because the error already says cannot find `Thing`, and the note is about another Thing, not the Thing they wrote. (Even though they have the same name.)

So in the case we can point at an import, we do that to highlight Thing here is not found, but Thing there exists but is something else. Pointing both at the same identifier would make it less clear that we're talking about two different Things.

[estebank]

@bors r+

[bors]

📌 Commit 908ce2f has been approved by estebank