Don't allow flattened format_args in const. #139624

m-ou-se · 2025-04-10T09:50:28Z

We allow format_args!("a") in const, but don't allow any format_args with arguments in const, such as format_args!("{}", arg).

However, we accidentally allow format_args!("hello {}", "world") in const, as it gets flattened to format_args!("hello world").

This also applies to panic in const.

This wasn't supposed to happen. I added protection against this in the format args flattening code, ~~but I accidentally marked a function as const that shouldn't have been const~~ but this was removed in #135139.

This is a breaking change. The crater found no breakage, however.

This breaks things like:

const _: () = if false { panic!("a {}", "a") };

and

const F: std::fmt::Arguments<'static> = format_args!("a {}", "a");

m-ou-se · 2025-04-10T09:50:48Z

@bors try

Don't allow flattened format_args in const. Fixes rust-lang#139136 and rust-lang#139621 by breaking the 'flattened format_args' cases. This is a breaking change. Let's try a crater run.

bors · 2025-04-10T09:51:59Z

⌛ Trying commit cc791eb with merge 6619027...

bors · 2025-04-10T11:57:03Z

☀️ Try build successful - checks-actions
Build commit: 6619027 (6619027b13a87a3597115f2c8231f107bf3a7a5f)

m-ou-se · 2025-04-10T12:06:40Z

@craterbot check

craterbot · 2025-04-10T12:06:54Z

👌 Experiment pr-139624 created and queued.
🤖 Automatically detected try build 6619027
🔍 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

craterbot · 2025-04-10T12:07:07Z

🚧 Experiment pr-139624 is now running

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

craterbot · 2025-04-11T11:16:09Z

🎉 Experiment pr-139624 is completed!
📊 3 regressed and 5 fixed (612545 total)
📰 Open the full 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

RalfJung · 2025-04-11T11:54:28Z

These all seem spurious.

Landing this requires FCP, but I'm not sure which team -- lang and/or libs-api?

m-ou-se · 2025-04-11T12:12:30Z

@rfcbot merge

rfcbot · 2025-04-11T12:12:33Z

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

No concerns currently listed.

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.

m-ou-se · 2025-04-22T20:30:26Z

but I accidentally marked a function as const that shouldn't have been const

This wasn't actually the case. I did implement this correctly. The problem was caused by #135139, which added const to a bunch of functions without properly considering the effect on the format_args!() macro.

This means that this problem has only been around since 1.86.0, which explains why we didn't find any actual breakage.

It might make sense to backport this change to land it in 1.87.0.

library/core/src/fmt/rt.rs

rfcbot · 2025-04-26T15:15:52Z

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.

This will be merged soon.

jhpratt · 2025-04-26T16:14:50Z

r=me with a test added per Ralf's suggestion.

m-ou-se · 2025-04-30T11:23:09Z

@bors r=jhpratt

bors · 2025-04-30T11:23:13Z

📌 Commit 56426db has been approved by jhpratt

It is now in the queue for this repository.

…iaskrgr Rollup of 9 pull requests Successful merges: - rust-lang#134232 (Share the naked asm impl between cg_ssa and cg_clif) - rust-lang#139624 (Don't allow flattened format_args in const.) - rust-lang#140090 (Check bare function idents for non snake-case name) - rust-lang#140203 (Issue an error when using `no_mangle` on language items) - rust-lang#140450 (ast: Remove token visiting from AST visitor) - rust-lang#140498 (Misc tweaks to HIR typeck (mostly w.r.t. checking calls)) - rust-lang#140504 (transmutability: ensure_sufficient_stack when answering query) - rust-lang#140506 (unstable-book: fix capitalization) - rust-lang#140516 (Replace use of rustc_type_ir by rustc_middle) Failed merges: - rust-lang#140374 (Resolve instance for SymFn in global/naked asm) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#139624 - m-ou-se:unconst-format-args, r=jhpratt Don't allow flattened format_args in const. Fixes rust-lang#139136 Fixes rust-lang#139621 We allow `format_args!("a")` in const, but don't allow any format_args with arguments in const, such as `format_args!("{}", arg)`. However, we accidentally allow `format_args!("hello {}", "world")` in const, as it gets flattened to `format_args!("hello world")`. This also applies to panic in const. This wasn't supposed to happen. I added protection against this in the format args flattening code, ~~but I accidentally marked a function as const that shouldn't have been const~~ but this was removed in rust-lang#135139. This is a breaking change. The crater found no breakage, however. This breaks things like: ```rust const _: () = if false { panic!("a {}", "a") }; ``` and ```rust const F: std::fmt::Arguments<'static> = format_args!("a {}", "a"); ```

tests/ui/consts/const-eval/format.rs

[beta] backports - Don't allow flattened format_args in const. rust-lang#139624 - set subsections_via_symbols for ld64 helper sections rust-lang#139752 - Fix detection of `main` function if there are expressions around it rust-lang#140220 - rustdoc: Fix doctest heuristic for main fn wrapping rust-lang#140420 - extend the list of registered dylibs on `test::prepare_cargo_test` rust-lang#140563 r? cuviper

Don't allow flattened format_args in const.

cc791eb

rustbot assigned Mark-Simulacrum Apr 10, 2025

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Apr 10, 2025

m-ou-se assigned m-ou-se and unassigned Mark-Simulacrum Apr 10, 2025

m-ou-se mentioned this pull request Apr 10, 2025

Inlining/flattening of format_args!() accidentally exposed as stable through const #139136

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m-ou-se added S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 10, 2025

m-ou-se mentioned this pull request Apr 7, 2025

Tracking Issue for super let #139076

Open

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. labels Apr 10, 2025

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Apr 11, 2025

m-ou-se marked this pull request as ready for review April 11, 2025 12:05

m-ou-se mentioned this pull request Apr 22, 2025

[generic_assert] Constify methods used by the formatting system #135139

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m-ou-se added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Apr 22, 2025

RalfJung reviewed Apr 23, 2025

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m-ou-se added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Apr 23, 2025

rustbot mentioned this pull request Apr 26, 2025

Tracking issue for release notes of #139624: Don't allow flattened format_args in const. #140338

Open

Add test for format_args!("{}", 0) in const.

56426db

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Apr 30, 2025

matthiaskrgr mentioned this pull request Apr 30, 2025

Rollup of 9 pull requests #140520

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bors merged commit bc99a04 into rust-lang:master Apr 30, 2025
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rustbot added this to the 1.88.0 milestone Apr 30, 2025

m-ou-se deleted the unconst-format-args branch May 1, 2025 09:18

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theemathas mentioned this pull request May 2, 2025

Nightly rejects panic!(..) invocation which is accepted by stable with "cannot call non-const formatting macro in constants" #140585

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cuviper mentioned this pull request May 2, 2025

[beta] backports #140592

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cuviper modified the milestones: 1.88.0, 1.87.0 May 2, 2025

cuviper removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label May 2, 2025

jieyouxu removed needs-fcp This change is insta-stable, so needs a completed FCP to proceed. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels May 3, 2025

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Don't allow flattened format_args in const. #139624

Don't allow flattened format_args in const. #139624

m-ou-se commented Apr 10, 2025 •

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Don't allow flattened format_args in const. #139624

Don't allow flattened format_args in const. #139624

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