Explicitly export core and std macros

[Voultapher]

Currently all core and std macros are automatically added to the prelude via #[macro_use]. However a situation arose where we want to add a new macro assert_matches but don't want to pull it into the standard prelude for compatibility reasons. By explicitly exporting the macros found in the core and std crates we get to decide on a per macro basis and can later add them via the rust_20xx preludes.

Closes #53977
Unlocks #137487

Reference PR:

• Reflect explicit macro import in reference reference#2077

Stabilization report lib

Everything N/A or already covered by lang report except, breaking changes: The unstable and never intended for public use format_args_nl macro is no longer publicly accessible as requested by @petrochenkov. Affects <10 crates including dependencies.

Stabilization report lang

Summary

Explicitly export core and std macros.

This change if merged would change the code injected into user crates to no longer include #[macro_use] on extern crate core and extern crate std. This change is motivated by a near term goal and a longer term goal. The near term goal is to allow a macro to be defined at the std or core crate root but not have it be part of the implicit prelude. Such macros can then be separately promoted to the prelude in a new edition. Specifically this is blocking the stabilization of assert_matches #137487. The longer term goal is to gradually deprecate #[macro_use]. By no longer requiring it for standard library usage, this serves as a step towards that goal. For more information see #53977.

PR link: #139493

Tracking:

• Tracking Issue for future-incompatibility lint ambiguous_panic_imports #147319

Reference PRs:

• Explicitly export core and std macros #139493

cc @rust-lang/lang @rust-lang/lang-advisors

What is stabilized

Stabilization:

* `#[macro_use]` is no longer automatically included in the crate root module. This allows the explicit import of macros in the `core` and `std` prelude e.g. `pub use crate::dbg;`. *No* code that was previously not accepted will be accepted now.

* `ambiguous_panic_imports` lint. *No* code that was previously not accepted will be accepted now. Code that previously passed without warnings, but included the following or equivalent - only pertaining to core vs std panic - will now receive a warning:

    ```rust
    #![no_std]

    extern crate std;
    use std::prelude::v1::*;

    fn xx() {
        panic!(); // resolves to core::panic
        //~^ WARNING `panic` is ambiguous
        //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    }
    ```

Design

N/A

Reference

What updates are needed to the Reference? Link to each PR. If the Reference is missing content needed for describing this feature, discuss that.

• Reflect explicit macro import in reference reference#2077

RFC history

What RFCs have been accepted for this feature?

N/A

Answers to unresolved questions

N/A

Post-RFC changes

What other user-visible changes have occurred since the RFC was accepted? Describe both changes that the lang team accepted (and link to those decisions) as well as changes that are being presented to the team for the first time in this stabilization report.

N/A

Key points

What decisions have been most difficult and what behaviors to be stabilized have proved most contentious? Summarize the major arguments on all sides and link to earlier documents and discussions.

• Nothing was really contentious.

Nightly extensions

Are there extensions to this feature that remain unstable? How do we know that we are not accidentally committing to those?

N/A

Doors closed

What doors does this stabilization close for later changes to the language? E.g., does this stabilization make any other RFCs, lang experiments, or known in-flight proposals more difficult or impossible to do later?

No known doors are closed.

Feedback

Call for testing

Has a "call for testing" been done? If so, what feedback was received?

No.

Nightly use

Do any known nightly users use this feature? Counting instances of #![feature(FEATURE_NAME)] on GitHub with grep might be informative.

N/A

Implementation

Major parts

Summarize the major parts of the implementation and provide links into the code and to relevant PRs.

See, e.g., this breakdown of the major parts of async closures:

• https://rustc-dev-guide.rust-lang.org/coroutine-closures.html

The key change is compiler/rustc_builtin_macros/src/standard_library_imports.rs removing the macro_use inject and the v1.rs preludes now explicitly pub useing the macros https://github.com/rust-lang/rust/pull/139493/files#diff-a6f9f476d41575b19b399c6d236197355556958218fd035549db6d584dbdea1d + https://github.com/rust-lang/rust/pull/139493/files#diff-49849ff961ebc978f98448c8990cf7aae8e94cb03db44f016011aa8400170587.

Coverage

Summarize the test coverage of this feature.

Consider what the "edges" of this feature are. We're particularly interested in seeing tests that assure us about exactly what nearby things we're not stabilizing. Tests should of course comprehensively demonstrate that the feature works. Think too about demonstrating the diagnostics seen when common mistakes are made and the feature is used incorrectly.

Within each test, include a comment at the top describing the purpose of the test and what set of invariants it intends to demonstrate. This is a great help to our review.

Describe any known or intentional gaps in test coverage.

Contextualize and link to test folders and individual tests.

A variety of UI tests including edge cases have been added.

Outstanding bugs

What outstanding bugs involve this feature? List them. Should any block the stabilization? Discuss why or why not.

An old bug is made more noticeable by this change #145577 but it was recommended to not block on it #139493 (comment).

Outstanding FIXMEs

What FIXMEs are still in the code for that feature and why is it OK to leave them there?

// Turn ambiguity errors for core vs std panic into warnings.
// FIXME: Remove with lang team approval.

https://github.com/rust-lang/rust/pull/139493/files#diff-c046507afdba3b0705638f53fffa156cbad72ed17aa01d96d7bd1cc10b8d9bce

Tool changes

What changes must be made to our other tools to support this feature. Has this work been done? Link to any relevant PRs and issues.

• rustfmt
• rust-analyzer
• rustdoc (both JSON and HTML)
• cargo
• clippy
• rustup
• docs.rs

No known changes needed or expected.

Breaking changes

If this stabilization represents a known breaking change, link to the crater report, the analysis of the crater report, and to all PRs we've made to ecosystem projects affected by this breakage. Discuss any limitations of what we're able to know about or to fix.

Breaking changes:

* It's possible for user code to invoke an ambiguity by defining their own macros with standard library names and glob importing them, e.g. `use nom::*` importing `nom::dbg`. In practice this happens rarely based on crater data. The 3 public crates where this was an issue, have been fixed. The ambiguous panic import is more common and affects a non-trivial amount of the public - and likely private - crate ecosystem. To avoid a breaking change, a new future incompatible lint was added ambiguous_panic_imports see https://github.com/rust-lang/rust/issues/147319. This allows current code to continue compiling, albeit with a new warning. Future editions of Rust make this an error and future versions of Rust can choose to make this error. Technically this is a breaking change, but crater gives us the confidence that the impact will be at worst a new warning for 99+% of public and private crates.

    ```rust
    #![no_std]

    extern crate std;
    use std::prelude::v1::*;

    fn xx() {
        panic!(); // resolves to core::panic
        //~^ WARNING `panic` is ambiguous
        //~| WARNING this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    }
    ```

* Code using `#![no_implicit_prelude]` *and* Rust edition 2015 will no longer automatically have access to the prelude macros. The following works on nightly by would stop working with this change:

    ```rust
    #![no_implicit_prelude]

    // Uncomment to fix error.
    // use std::{panic, vec};

    fn main() {
        let _ = vec![3, 6];
        panic!();
    }
    ```

Crater found no instance of this pattern in use. Affected code can fix the issue by directly importing the macros. The new behavior matches the behavior of `#![no_implicit_prelude]` in Rust editions 2018 and beyond and it's intuitive meaning.

Crater report:

• https://crater-reports.s3.amazonaws.com/pr-139493-2/index.html (latest run, but partial run)
• https://crater-reports.s3.amazonaws.com/pr-139493-1/index.html (previous full run, one fix missing)

Crater analysis:

• Discussed in breaking changes.

PRs to affected crates:

• Fix ambiguous assert_eq uses Michael-F-Bryan/gcode-rs#57
• Fix uses of assert_eq stbuehler/rust-ipcrypt#1
• Fix ambiguous assert_eq imports jcreekmore/dmidecode#55

Type system, opsem

Compile-time checks

What compilation-time checks are done that are needed to prevent undefined behavior?

Link to tests demonstrating that these checks are being done.

N/A

Type system rules

What type system rules are enforced for this feature and what is the purpose of each?

N/A

Sound by default?

Does the feature's implementation need specific checks to prevent UB, or is it sound by default and need specific opt-in to perform the dangerous/unsafe operations? If it is not sound by default, what is the rationale?

N/A

Breaks the AM?

Can users use this feature to introduce undefined behavior, or use this feature to break the abstraction of Rust and expose the underlying assembly-level implementation? Describe this if so.

N/A

Common interactions

Temporaries

Does this feature introduce new expressions that can produce temporaries? What are the scopes of those temporaries?

N/A

Drop order

Does this feature raise questions about the order in which we should drop values? Talk about the decisions made here and how they're consistent with our earlier decisions.

N/A

Pre-expansion / post-expansion

Does this feature raise questions about what should be accepted pre-expansion (e.g. in code covered by #[cfg(false)]) versus what should be accepted post-expansion? What decisions were made about this?

N/A

Edition hygiene

If this feature is gated on an edition, how do we decide, in the context of the edition hygiene of tokens, whether to accept or reject code. E.g., what token do we use to decide?

N/A

SemVer implications

Does this feature create any new ways in which library authors must take care to prevent breaking downstreams when making minor-version releases? Describe these. Are these new hazards "major" or "minor" according to RFC 1105?

No.

Exposing other features

Are there any other unstable features whose behavior may be exposed by this feature in any way? What features present the highest risk of that?

No.

History

List issues and PRs that are important for understanding how we got here.

• This change was asked for here Stabilize assert_matches and move it to core::macros #137487 (comment)

Acknowledgments

Summarize contributors to the feature by name for recognition and so that those people are notified about the stabilization. Does anyone who worked on this not think it should be stabilized right now? We'd like to hear about that if so.

More or less solo developed by @Voultapher with some help from @petrochenkov.

Open items

List any known items that have not yet been completed and that should be before this is stabilized.

None.

[rustbot]

r? @ChrisDenton

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Voultapher]

r? @Amanieu

[Voultapher]

@Amanieu the tidy issue highlights an annoying and unforeseen side-effect of this change. The vec module is now part of the prelude. In effect this means that for example this code:

fn xx(i: vec::IntoIter<i32>) {
    let _ = i.as_slice();
}

fn main() {}

that currently doesn't compile on stable would now compile. Initially I thought this would cause name collisions if users define their own vec module but so far I wasn't able to produce those, it seems to always prefer the local module. But regardless, I think we don't want to allow access to a standard library namespace without going through std, alloc or core. AFAIK there is no way to pub use only the macro and not the module namespace without modifications. I have two ideas how to tackle this, maybe we can rename vec to vec_xx internally and have separate use expressions or we have to add another crate that we can #[macro_use] inject into the prelude that only contains the vec macro. Thoughts?

[traviscross]

See also:

• 2024 edition prelude item docs are not inlined, and cause some duplication and confusion #136102

[petrochenkov]

There's an issue for this change - #53977.

[dtolnay]

@Voultapher, avoiding the vec module re-export can be done like this:

#[macro_export]
macro_rules! myvec {
    () => {};
}

pub mod myvec {
    pub struct Vec;
}

pub mod prelude {
    // Bad: re-exports both macro and type namespace
    // pub use crate::myvec;
    
    mod vec_macro_only {
        #[allow(hidden_glob_reexports)]
        mod myvec {}
        pub use crate::*;
    }
    pub use self::vec_macro_only::myvec;
}

fn main() {
    prelude::myvec!();
    let _: prelude::myvec::Vec; // error
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=5e50828c593e04ba0e98f48c9d8696b4

[Voultapher]

I've applied the suggestion by @dtolnay local tests seem promising. @Kobzol could we please do a timer run to see if this PR impacts compile-times.

[petrochenkov]

env and panic (and maybe something else now?) need to be treated in the same way as vec.

[Kobzol]

@Voultapher Based on the CI failure I think that a try build would fail now.

[Voultapher]

Ok, I'll try to get the CI passing first.

[Voultapher]

@petrochenkov I went through all macros and searched the docs and env and panic seem to be the only other ones affected.

[Voultapher]

@Amanieu this program previously worked:

use std::*;

fn main() {
    panic!("panic works")
}

and now runs into:

error[E0659]: `panic` is ambiguous
   --> src/main.rs:4:5
    |
4   |     panic!("panic works")
    |     ^^^^^ ambiguous name
    |
    = note: ambiguous because of a conflict between a name from a glob import and an outer scope during import or macro resolution
note: `panic` could refer to the macro imported here
   --> src/main.rs:1:5
    |
1   | use std::*;
    |     ^^^^^^
    = help: consider adding an explicit import of `panic` to disambiguate
    = help: or use `crate::panic` to refer to this macro unambiguously
note: `panic` could also refer to the macro defined here
   --> rust/library/std/src/prelude/mod.rs:157:13
    |
157 |     pub use super::v1::*;
    |             ^^^^^^^^^

I don't see how we can resolve that without changing language import rules and or special casing the prelude import.

[Amanieu]

@petrochenkov Do you have any ideas about that?

[petrochenkov]

Could you add a test making sure that the modules vec, env and panic are not in prelude?

[petrochenkov]

@petrochenkov Do you have any ideas about that?

The ambiguity wouldn't happen if it was the same panic in std root and in the stdlib prelude.
However, std and core have two different panic macros.

Previously #[macro_use] extern crate std; would add the std's panic to macro_use prelude, and #[macro_use] extern crate core; would add the core's panic.
This PR always adds the core's panic.

[Voultapher]

One thing we talked about is that, as we understood it during the call, this PR adds a special case to name resolution for panic. If that's the case, please be sure to discuss that in the revised stabilization report and in the revised Reference PR (we don't have a merged section on name resolution yet, but this is at least worth an admonition somewhere).

I'm not sure that's the case any more than previously discussed in that it turns an error into a warning. The content implications are the exact same as discussed with the lint, since it is the lint. So not sure how to mention that without it becoming confusing.

[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.

[rustbot]

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[Voultapher]

I've updated the PR description and changed to report_in_deps: false.