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.

[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

[traviscross]

Thanks to @mu001999 for putting this forward and to @petrochenkov for supporting this. This makes sense to me. I propose that we do it, and I'll then file a concern we discussed in the meeting (thanks @Nadrieril) about documentation.

@rfcbot fcp merge lang

[rust-rfcbot]

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

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

Concerns:

• consistent-axioms (Support importing path-segment keyword with renaming #146972 (comment))
• documentation (Support importing path-segment keyword with renaming #146972 (comment))

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot concern documentation

The linked Reference PR (rust-lang/reference#2010) is a minor clarification. Let's please go ahead and also make a Reference PR documenting the rules described in this stabilization report.

[tmandry]

@rfcbot reviewed

[nikomatsakis]

@rfcbot concern consistent-axioms

I am a bit confused by why we would make errors in some of these cases and also the overall philosophical approach.

In particular I expect as an "axiom" that, for all $path, the following should EITHER both work OR both error:

use $path::{self};
use $path::self;

...but the PR description suggests that where use foo::{self} would work but use foo::self would not. I understand why the latter would be discouraged (with a lint) but I think it should be allowed. I can imagine code generators or macros wanting to add self or wanting to be able to simplify a {identifier} to identifier without having to special-case self.

My rule of thumb is that the language should accept things that have only one obvious meaning but, if they seem "redundant" or may suggest a broken mental model for users, issue a lint.

Thinking over my model for how this works, I think of it like this...

• an import path has two things: (a) the absolute path and (b) optionally, a name;
• something like use foo::bar has (a) the absolute path foo::bar with (b) the name bar
• $path::self is "identity", it preserves both
• self, super, crate, and $crat, when used as the prefix of a path, expand to (a) the absolute path you'd expect but (b) no name.
• it is an error to import something with no name without a use.

This implies that e.g. use super is an error but use super as X, use super::{self as X} and use super::self as X should all work.

[petrochenkov]

@nikomatsakis
Right now the model is that self in use prefix::{self as name}; is not a part of the path, but rather a part of the braced import {...} syntax.
This self is never resolved as a name, but gets rewritten into use prefix as name instead.
So it could e.g. use some non-keyword operator instead (use prefix::{<- as name} or something).
This PR continues to follow that model.

We could certainly extend the language to make it a part of the path, but then I'd rather support self and super in all positions in paths, like . and .. are supported everywhere in filesystem paths.
If this is a desirable direction, then this PR could bring back the support for use std::{io::self} (without extending it), and unbreak most of the regressions.