"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...

[ChrisJefferson]

Just as another practical example, I wrote (having a C++ brain hiccup) pub typedef mytype = usize;, which produced the same message about macros and confused me until I found typedef isn't a macro, or in fact anything.

[gihrig]

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

But the error goes well beyond existential:

A missing const will trigger this error:

pub mod DEFAULT_BBOX {
  pub mod _northEast {
    pub lat: f64 = 48.82099347817258;
    pub lng: f64 = 9.299583435058596;
  }

error: can't qualify macro invocation with `pub`
  --> src/constants/Map.rs:11:5
   |
11 |     pub lat: f64 = 48.82099347817258;
   |     ^^^
   |
   = help: try adjusting the macro to put `pub` inside the invocation

The problem of course, is that the correct code is:

    pub const lat: f64 = ...

$ rustc -V
rustc 1.43.0-nightly (58b8343 2020-02-05)

macOS 10.14.6

[Centril]

Today you get:

error: visibility `pub` is not followed by an item
 --> src/lib.rs:1:1
  |
1 | pub existential type T: MotorCtrl;
  | ^^^ the visibility
  |
  = help: you likely meant to define an item, e.g., `pub fn foo() {}`

error: expected item, found `existential`
 --> src/lib.rs:1:5
  |
1 | pub existential type T: MotorCtrl;
  |     ^^^^^^^^^^^ expected item

...which seems good enough.