Support importing path-segment keyword with renaming

[mu001999]

Reference PR:

• Clarify use $crate restriction reference#2010
• Document importing path-segment keyword reference#2136

Description:

• Make imports consistent, self/super/crate/$crate can be imported with renaming now
  • Allow use self/super/crate/$crate as name; and also use super/crate/$crate::{self as name};
  • Deny use self/super/crate/$crate; and also use super/crate/$crate::{self};
  • Deny use ::self;, use ::{self};, use ::self as name; and use ::{self as name};
• Proper error messages, and E0430 and E0431 are no longer emitted now
  • [E0430]: self import can only appear once in an import list was emitted before when std::fmt::{self, self}, but the emitted [E0252]: the name fmt is defined multiple times is good enough
  • [E0431]: self import can only appear in an import list with a non-empty prefix was emiited before when use {self as name};, now use {self as name}; is allowed; for use {self};, it's denied because it lacks renaming and will emit imports need to be explicitly named with suggesting renaming.

Fixes #29036
Fixes #35612
Fixes #37156
Fixes #146967
Fixes #149811

r? petrochenkov

[rustbot]

Failed to set assignee to [petrochenkov]: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[mu001999]

Previous summary: #146972 (comment)

Update:

1. Recover previous logic making things like use std::{io::self} work, and unbreak most of the regressions
2. Draft a reference update for this change: Document importing path-segment keyword reference#2136

The language extension (supporting self at the end of paths, and lints for cases like redundant use of ::self) will be landed separately, and keep this PR doing what it does (i.e. bugfixing), as @petrochenkov suggested in #146972 (comment)

[mu001999]

@petrochenkov Could you review the new commit? If it's okay, I will re-nominate then. Thanks ;)

[traviscross]

@bors try

TODO: @craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146972/retry-regressed-list.txt

[joshtriplett]

Based on the summary
#146972 (comment) (and the previously linked summary in #146972 (comment) ), this seems reasonable, so I'm starting an FCP.

@rfcbot merge lang

[nikomatsakis]

Thank you all for the discussion and iteration, the current state seems good to me. I understand it to be:

This PR unifies and extends the behavior of importing path-segment keywords (crate/$crate/super/self), resolving several long-standing inconsistencies.

Previously, Rust only allowed use crate as name; without renaming support for other path keywords. This PR enables importing these keywords with explicit renaming. And it also denies importing the crate/$crate/super keywords without renaming. (<-- this is the key change we made during discussion)

What's now allowed

For crate and $crate:

* `use crate as name;`

* `use crate::{self as name};`

* `use $crate as name;`

* `use $crate::{self as name};`

For super (including chained super::super):

* `use super as name;`

* `use super::{self as name};`

* `use super::super as name;`

* `use super::super::{self as name};`

For self:

* `use self as name;`

* `use self::{self as name};`

* `use a::{b::self};`

Removed error codes

Two error codes are no longer emitted:

* **E0430**: Previously emitted for duplicate `self` imports like `std::fmt::{self, self}`. The existing E0252 ("name defined multiple times") provides sufficient guidance.

* **E0431**: Previously emitted for `use {self as name};`. This pattern is now explicitly allowed. For `use {self};` (without renaming), a clearer error suggests adding an explicit rename.

Impact

This change makes the language more consistent and predictable when working with path keywords in imports.

Previous crater run shows 2 groups of regressions, 18 in total. 17 of these no longer apply as they were due to the stricter checking of self imports in nested positions. 1 regression still applies due to denying importing $crate without renaming, i.e., use $crate::{self}; - #146972 (comment).

[nikomatsakis]

@rfcbot reviewed

[joshtriplett]

Trying again:

@rfcbot merge lang

[rust-bors]

☀️ Try build successful (CI)
Build commit: 310c4dc (310c4dc96b5921ed757c4cd00c4b33924ee903c6, parent: 625b63f9e148d511e187c71e5f70643ee62c77fb)

[traviscross]

@craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146972/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-146972-1 created and queued.
🤖 Automatically detected try build 310c4dc
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[traviscross]

The FCP situation here is that we have an open proposed FCP from December with all boxes checked and two concerns.

One was mine, about the absence of a corresponding Reference PR. There now is one:

• Document importing path-segment keyword reference#2136

It'll need to be approved by @rust-lang/lang-docs before we merge this PR:

@rustbot labels +S-waiting-on-documentation

But we can resolve the concern:

@rfcbot resolve documentation

The other concern, consistent-axioms, was filed by @nikomatsakis. That's probably resolved now, but he'll have to resolve that.

[traviscross]

cc @rust-lang/fls

[nikomatsakis]

@rfcbot resolve consistent-axioms

[rust-rfcbot]

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