"can't qualify macro invocation with `pub`" error can fire without any macro invocations

[little-arhat]

With #63180 in effect, code using existential type produces confusing error:

error: can't qualify macro invocation with `pub`
 --> src/mixer.rs:6:1
  |
6 | pub existential type T: MotorCtrl;
  | ^^^
  |
  = help: try adjusting the macro to put `pub` inside the invocation

The actual fix is to change

pub existential type T: MotorCtrl; to pub type T = impl MotorCtrl; and to enable type_alias_impl_trait feature, instead of existential_type.

[Centril]

I don't think it is worth having special cased diagnostics for existential type.

[little-arhat]

@Centril true, it wasn't stabilized anyway. But I think it's good to have this issue here to help people googling an error.

[jonas-schievink]

The same issue occurs when using pub auto unsafe trait instead of the correct pub unsafe auto trait:

pub auto unsafe trait SingleCoreSend {}

error: can't qualify macro invocation with `pub`
 --> src/lib.rs:1:1
  |
1 | pub auto unsafe trait SingleCoreSend {}
  | ^^^
  |
  = help: try adjusting the macro to put `pub` inside the invocation

error: expected one of `!` or `::`, found `unsafe`
 --> src/lib.rs:1:10
  |
1 | pub auto unsafe trait SingleCoreSend {}
  |          ^^^^^^ expected one of `!` or `::` here

error: aborting due to 2 previous errors

Note that this happens even when the feature is turned off.

[birkenfeld]

Also seen with functions and a simple typo:

pub asyn fn test() {}

Error:

error: can't qualify macro invocation with `pub`
 --> src/main.rs:1:1
  |
1 | pub asyn fn test() {}
  | ^^^
  |
  = help: try adjusting the macro to put `pub` inside the invocation

error: expected one of `!` or `::`, found keyword `fn`
 --> src/main.rs:1:10
  |
1 | pub asyn fn test() {}
  |          ^^ expected one of `!` or `::`

I'm not sure how the parser gets the idea that a macro was coming there...