Bad error message when using attributes proc_macro

[SimonSapin]

Upgrading Servo to today’s Nightly caused a confusing pair of error messages:

error[E0432]: unresolved import `dom_struct::dom_struct`
  --> components/script/dom/htmlulistelement.rs:10:5
   |
10 | use dom_struct::dom_struct;
   |     ^^^^^^^^^^^^^^^^^^^^^^ no `dom_struct` in the root

error[E0658]: The attribute `dom_struct` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
  --> components/script/dom/bluetoothpermissionresult.rs:26:1
   |
26 | #[dom_struct]
   | ^^^^^^^^^^^^^
   |
   = help: add #![feature(custom_attribute)] to the crate attributes to enable

Adding #[macro_use] changes the second error to:

error[E0658]: attribute procedural macros are experimental (see issue #38356)
  --> components/script/dom/attr.rs:28:1
   |
28 | #[dom_struct]
   | ^^^^^^^^^^^^^
   | 
  ::: components/script/lib.rs:35:1
   |
35 | #[macro_use] extern crate dom_struct;
   | ------------ procedural macro imported here
   |
   = help: add #![feature(proc_macro)] to the crate attributes to enable

All three messages are wrong:

• dom_struct::dom_struct does exist
• feature(custom_attribute) is not the correct fix
• feature(proc_macro) is not only not the correct fix, it was already present!

The correct fix is to add #![feature(use_extern_macros)], but the only way to figure that out is to read #52081.

CC @alexcrichton

[alexcrichton]

I believe this is largely fixed with #52458, and the remaining pieces will be fixed by #50911

[arcriley]

Is there a way to roll back to a previous nightly?

[SimonSapin]

@arcriley You can run rustup install nightly-2018-07-15 and cargo +nightly-2018-07-15 build, for example. Or use a directory override or a toolchain file: https://github.com/rust-lang-nursery/rustup.rs/#toolchain-specification

[dominikwerder]

The correct fix is to add #![feature(use_extern_macros)], but the only way to figure that out is to read #52081.

Thank you @SimonSapin

[steveklabnik]

Both PRs that were said to fix this have been closed; given that this issue is over a year old, and that it's going to be hard to reproduce, I'm going to give this a close. @SimonSapin if you have a way to reproduce this today, let me know and we can re-open!

[SimonSapin]

The correct fix is to add #![feature(use_extern_macros)]

I suspect that other things have been stabilized in the meantime and this is no longer relevant.