Support importing path-segment keyword with renaming

[mu001999]

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Reference PR

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

Description

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 these keywords without renaming.

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};

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]}; and use ::{self [as name]};. These patterns are now allowed or denied but with new clearer errors.
  • For use {self as name}; and use ::{self as name}; (in edition 2015), they are allowed now and equivalent to use crate as name;
  • For use {self}; and use ::{self}; (in edition 2015) without renaming, they are equivalent to use crate;, the new clearer error suggests adding an explicit rename.
  • For use ::{self [as name]}; after edition 2015, it is equivalent to use ${extern-prelude} [as name];, it is denied with new errors.

Future

We plan to remove error E0429 and support self at the end of paths (#146972 (comment)). This language extension and lint for redundant ::self instead of hard error E0429 will be landed separately in the future.

---

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

r? petrochenkov

[rustbot]

Failed to set assignee to [petrochenkov]: invalid assignee

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[rust-rfcbot]

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

[mu001999]

Spurious failure

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[traviscross]

@mu001999: Please be sure to see the comment in #146972 (comment). Let us know what you'd propose in rust-lang/reference#2136 for that, then mark that PR ready for review. When the Reference PR is approved, this PR can land.

[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.

[ehuss]

The documentation should be ready now.

@petrochenkov Do you want to give this a final review?

[petrochenkov]

Yay, finally.
@bors r+

[rust-bors]

📌 Commit 6609e4b has been approved by petrochenkov

It is now in the queue for this repository.