Tracking issue for feature extern_crate_item_prelude

[petrochenkov]

With this feature extern crate foo as bar; item placed into the crate root puts the name bar into extern prelude. This has the next effects:

• The name is in scope in the whole crate, including inner modules, like other prelude names.
• On 2018 edition imports can refer to that name use bar::zzz; or use ::bar::zzz;, plus non-import absolute paths can refer to it as well let z = ::bar::zzz;.

More details are available on the implementation PR - #54658.

[Nemo157]

Is this intended to be stable for the initial 2018 release? It seems important for a nice experience with proc-macros (otherwise you must use crate::proc_macro everywhere).

If not, this appears to have no forward-compat warning when building under 2015, which causes a failure to migrate with cargo fix:

error[E0658]: use of extern prelude names introduced with `extern crate` items is unstable (see issue #54658)
  --> interp-macros/src/lib.rs:12:5
   |
12 | use proc_macro::TokenStream;
   |     ^^^^^^^^^^
   |
   = help: add #![feature(extern_crate_item_prelude)] to the crate attributes to enable

[SimonSapin]

this appears to have no forward-compat warning

I’ve also hit this and filed #55592

[steveklabnik]

Knowing if this will ship in 1.31 or not is important for the book, for the reasons that @Nemo157 mentioned. Could we maybe get an answer here?

[SimonSapin]

@petrochenkov, @rust-lang/lang Are there any blockers for stabilizing this feature? How do you feel about backporting its stabilization to 1.31 beta? Should FCP be proposed now?

[SimonSapin]

In servo/servo#22086 I’ve migrated a number of Servo crates to the 2018 editions, but I left out proc-macro and plugin crates in part because of this issue (or lack of other good way to deal with crate dependencies not known to Cargo).

[petrochenkov]

@aturon and @steveklabnik wanted to stabilize this for the edition release as well (on #edition-planning discord channel).

This should be ready for stabilization and backport to 1.31 beta once #55884 lands (or some fixes for this feature are extracted from that PR).
I'll prepare a stabilization report.