Stabilize extern_crate_item_prelude

[petrochenkov]

Closes#55599

[petrochenkov]

r? @Centril

[petrochenkov]

Stabilization report

Feature name: #![feature(extern_crate_item_prelude)]
Version target: 1.31 (2018-12-07) aka 2018 edition release

Implemented in #54658 and landed 23 days ago.
Tracking issue is #55599.
Documentation issue is rust-lang/reference#466.
Tests for the feature can be found in src/test/ui/imports/extern-prelude-extern-crate-*.rs and src/test/ui-fulldeps/proc-macro/extern-prelude-extern-crate-*.rs.

Why it's desirable to stabilize for the edition release

The feature provides the same usage model for "normal" crates (passed with --extern or core/std) and crates that cannot be passed with --extern on stable like proc_macro, alloc or rustc_* crates.

As a result this code "just works" on 2018 edition:

externcrate proc_macro;use proc_macro::TokenStream;

If this is not stabilized for the edition release, nothing too terrible happens, but users of the aforementioned crates will have to use them like something from the local crate for some time (e.g. until the next release).

externcrate proc_macro;usecrate::proc_macro::TokenStream;

What is being stabilized

If item extern crate orig_name as rename; is placed into the crate root, then it adds name rename into the "extern prelude".
If item extern crate orig_name; is placed into the crate root, then it adds name orig_name into the "extern prelude".

If the name foo is in extern prelude, then 1) it's in scope in the whole crate including inner modules and 2) absolute paths on 2018 edition can refer to it.

externcrate foo;mod m {use foo;// OKuse::foo;// OKlet x = foo::bar;// OKlet x = ::foo::bar;// OK}

Some details and corner cases

If extern crate is placed in non-root module, then it doesn't add anything into extern prelude.
This means there's opt-out from this feature:

mod inner {pub(crate)externcrate foo;}use inner::foo;

If name is first added to extern prelude with --extern name and then with extern crate other_totally_unrelated_crate as name;, then the second one wins and name will refer to the other totally unrelated crate.

Macro expanded extern crate items still add names to extern prelude, however such items cannot override names passes with --extern (there's a dedicated error). This is a special case of a more general rule.

Interactions of the feature with unstable macros 2.0 are were buggy (#55668), but the bug is already fixed in #55884.

[Centril]

I propose that we stabilize #![feature(extern_crate_item_prelude)] in 1.31 (-> beta branch) per the report given in #56032 (comment).

@rfcbot merge

[rfcbot]

Team member @Centril has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Centril
• @aturon
• @cramertj
• @eddyb
• @joshtriplett
• @nikomatsakis
• @nrc
• @pnkfelix
• @scottmcm
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[bors]

☔ The latest upstream changes (presumably #56042) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

@bors: p=1

beta nominated

[nikomatsakis]

(I checked @pnkfelix as they are on PTO)

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[petrochenkov]

@bors r=nikomatsakis

[bors]

📌 Commit d4934c7 has been approved by nikomatsakis

[bors]

⌛ Testing commit d4934c7 with merge 780658a...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 780658a to master...