Support importing path-segment keyword with renaming

[mu001999]

Reference PR:

• Clarify use $crate restriction reference#2010

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.

[petrochenkov]

Thanks!
I'll start crater next.
@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 18c1d18 (18c1d185ac7a6a3bf399296b35e4b2736809a756, parent: 9050733395f7c10560d49f549e022f411fa28cea)

[petrochenkov]

@craterbot check

[craterbot]

👌 Experiment pr-146972 created and queued.
🤖 Automatically detected try build 18c1d18
🔍 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

[mu001999]

@petrochenkov Thanks a ton! I appreciate you taking the time to review this.

[bors]

☔ The latest upstream changes (presumably #149462) made this pull request unmergeable. Please resolve the merge conflicts.

[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.

[craterbot]

🚧 Experiment pr-146972 is now running

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

[craterbot]

🎉 Experiment pr-146972 is completed!
📊 21 regressed and 2 fixed (746877 total)
📊 2104 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-146972/retry-regressed-list.txt ↩

[petrochenkov]

There are two groups of regressions.

One (17 cases, self without braces at the end):

• csv2svg-0.1.12 use {std::{io::self}}
• ChangedUser.sabi.ebc01e2200acd6d16c243015faf05a3355566158 - use std::{io::self}
• GoldsteinE.uuis.56228af2c5f7bdc31734d1ab3e9bb192f5675102 - use crate::{ui::self}
• JRF63.Rust-Practice-Rosalind.1f97206e167b276c936edf5702e97f005ce9cb08 - use std::{env::self}
• Schaudge.MisMatchFinder.145cf4c02ff7dfc1240ae78f41e239126299468c - use mismatchfinder::{bamreader::self}
• SebastianHollizeck.rust-mismatchfinder.1101a81b439e6cd431882cb22af8ef505214fb1a - use mismatchfinder::{bamreader::self}
• ZGUNNERL1.caesar-cipher.046d699719fa1253b002c261dc59c2b42509679c - use std::{process::self}
• anders94.postgres-evm.a8df860f976845082f26d00da0b78a700eac13e2 - use tokio::{time::self}
• brassardfelix-ASTAR.MisMatchFinder_revised.6447066f25651b3b9c7f3ada94560d6e794306f2 - use mismatchfinder::{bamreader::self}
• eugenesvk.helpFuturesSignals.78b11808fa4d2874fad5e1b0e5c274b59b1efb0d - use futures::{future::self}
• kai-thanner.emoji-picker.8610e7c1669e26defc58793cb1b5ff18acaee123 - use std::{fs::self}
• nataliescottdavidson.walkbuilder-example.8624c9bb7373b44ee1e126c64a4a260898f44a96 - use std::{fs::self}
• solanamonk.lwus-gumdrop.65531329a2f48a5f0a9d008926a281e2fee7534b - use solana_program::{system_instruction::self}
• xinyu-zheng.umalloc.feeb120703fea7a81fb52c0a92a1d1c68c7b7442 - use core::{cmp::self}
• extreme-666.666.666666 - use std::{thread::self}
• patoka-0.1.0 - use crate::{worker::{task_assistant::self}}
• volga-0.7.2 - use tokio::{io::self}

And another (1 case, import without a name):

• rutie-serde-0.3.0 use $crate::{self}

[petrochenkov]

@mu001999
Could you write the change description for lang team?
#146978 (comment) or #147984 (comment) or #149596 (comment) could be used as examples.
@rustbot author

[mu001999]

Could you write the change description for lang team?

Sure! I'll do this later

[mu001999]

Change description for lang team

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};

Tighter error checking for self imports

Non-keyword path segments using self continue to work as before, but we now more strictly enforce the rule that self imports must appear within braces. Previously, nested use-trees like use a::{b::self}; were accepted. These are now rejected.

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.

Crater run shows 2 groups of regressions, 18 in total: 17 of these are due to the stricter checking of self imports in nested positions, 1 of these is due to denying importing $crate without renaming, i.e., use $crate::{self}; - #146972 (comment).

[mu001999]

@rustbot ready