rustc shouldn't say 'not in scope' when using a derive macro as a macro_rules macro

[jyn514]

Consider the following code (playground):

fn f() {
    Debug!()
}

#[derive(Debug)]
struct S;

error: cannot find macro `Debug` in this scope
 --> src/lib.rs:2:5
  |
2 |     Debug!()
  |     ^^^^^

This is not particularly helpful (in fact it's incorrect). Debug is in scope, it's just not a macro rules macro. Instead the error should say something like

error: `Debug` is a derive macro, not a macro_rules! macro
 --> src/lib.rs:2:5
  |
2 |     Debug!()
  |     ^^^^^ note: attempting to use a derive as a macro_rules! macro

[jyn514]

This would have caught the second 'library error' in https://jyn514.github.io/2020/09/05/Rust-in-2021.html.

[petrochenkov]

The desirable error was reported previously, but it changed with introduction of two sub-namespaces in macro namespace - #54069.

So, derive Debug is never found during regular name lookup now for names in fn-like macro positions (Debug!).
So it needs a second lookup for diagnostics, which is currently done for other namespaces, but not other sub-namespaces in the macro namespace.
Compare with:

type A = Debug; // ERROR: expected type, found derive macro `Debug`

[dario23]

Current playground correctly says

error: cannot find macro `Debug` in this scope
 --> src/lib.rs:2:5
  |
2 |     Debug!()
  |     ^^^^^
  |
  = note: `Debug` is in scope, but it is a derive macro: `#[derive(Debug)]`

so let's close this? @jyn514

[jyn514]

oh amazing, good find!