feat: Stabilize `build.warnings` by epage · Pull Request #16796 · rust-lang/cargo

epage · 2026-03-26T21:46:53Z

What does this PR try to resolve?

This allows users to either

hide warnings (silence warnings in the terminal #14258)
error on warnings (Add --deny-warnings functionality for all commands. #8424)

build.warnings is setup to mirror the behavior of RUSTFLAGS=-Dwarnings, including

only errors for lint warnings and not hard warnings
only errors for local warnings and not non-local warnings visible with --verbose --verbose
stop the build without --keep-going

These conditions were not originally met and also came as feedback from rust-lang/rust which has been dogfooding this since the merge of rust-lang/rust#148332.

Things are a bit different with RUSTFLAGS=-Awarnings:

Hard warnings are hidden for rustc but not cargo (rustc seems in the wrong imo)
- In particular, we shouldn't mask the edition warning for Cargo Script
both hide the warning summary line (number of warnings per crate)
both hide non-local warnings for --verbose --verbose

One design constraint we are operating on with this is that changing build.warnings should not cause a rebuild, unlike a RUSTFLAGS solution.

Closes #14802

How to test and review this PR?

My main concern over this was how the naming scheme would extend to rust-lang/rfcs#3730 but that RFC has not gained much interest

build seems as good of a home as any.

This allows users to either - hide warnings (rust-lang#14258) - error on warnings (rust-lang#8424) `build.warnings` is setup to mirror the behavior of `RUSTFLAGS=-Dwarnings`, including - only errors for lint warnings and not hard warnings - only errors for local warnings and not non-local warnings visible with `--verbose --verbose` - stop the build without `--keep-going` These conditions were not originally met and also came as feedback from rust-lang/rust which has been dogfooding this since the merge of rust-lang/rust#148332. Things are a bit different with `RUSTFLAGS=-Awarnings`: - Hard warnings are hidden for rustc but not cargo (rustc seems in the wrong imo) - In particular, we shouldn't mask the edition warning for Cargo Script - both hide the warning summary line (number of warnings per crate) - both hide non-local warnings for `--verbose --verbose` One design constraint we are operating on with this is that changing `build.warnings` should not cause a rebuild, unlike a `RUSTFLAGS` solution. Closes rust-lang#14802

rustbot · 2026-03-26T21:46:57Z

r? @weihanglo

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Owners of files modified in this PR: @ehuss, @epage, @weihanglo
@ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
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epage · 2026-03-26T21:47:06Z

@rfcbot fcp merge cargo

rust-rfcbot · 2026-03-26T21:47:24Z

Team member @epage 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!

See this document for info about what commands tagged team members can give me.

epage · 2026-03-26T21:48:54Z

Right now, we exactly mirror RUSTFLAGS=[AD]warnings behavior.

My one concern is with build.warnings = "allow" with hard warnings.

Right now, rustc warnings are hidden and cargo warnings are shown. We likely want these to be consistent
We shouldn't be hiding the Cargo Script warning
Hard warnings should be rare
deny ignores hard warnings

From this, I would lean towards making allow show hard warnings always.

arlosi

I agree with stabilizing the existing behavior, but I think we need to be clear about exactly what this option impacts.

"Hard warnings" don't seem to be defined as term in our documentation.

View changes since this review

arlosi · 2026-03-27T15:49:38Z

src/doc/src/reference/config.md

+Only warnings within the user's control to resolve or adjust the level of are affected,
+e.g. leaving as-is non-lint warnings or warnings from dependencies visible through `--verbose --verbose`.


Suggested change

Only warnings within the user's control to resolve or adjust the level of are affected,

e.g. leaving as-is non-lint warnings or warnings from dependencies visible through `--verbose --verbose`.

Only warnings within the user's control to resolve or adjust the level of are affected.

Non-lint warnings or warnings from dependencies visible through `--verbose --verbose` are not affected.

I was going for this to be descriptive, rather than prescriptive, and felt listing the cases as examples would help set that tone.

arlosi · 2026-03-27T15:58:25Z

src/doc/src/reference/config.md

+* Default: `warn`
+* Environment: `CARGO_BUILD_WARNINGS`
+
+Adjust the effective lint level for warnings.


This section seems confusing between the terms lint and warning.

The following sounds like all warnings have a lint level that we're adjusting.

Adjust the effective lint level for warnings.

This sounds like all lints can have an adjustable lint level that we're changing here.

warn: continue to emit the lints as warnings (default).

This is now emitting errors for warnings not lints.

deny: emit an error for a crate that has warnings

It is tricky to talk about this precisely enough while not adding so many qualifiers that it makes it hard to follow. My intent with the last paragraph ("Only warnings") was to clarify this.

arlosi · 2026-03-27T16:01:22Z

src/doc/src/reference/config.md

+* `allow`: hide the lints.
+* `deny`: emit an error for a crate that has warnings. Use `--keep-going` to see see the warnings for all crates.
+
+Only warnings within the user's control to resolve or adjust the level of are affected,


Should we be more precise about what "user's control to resolve" means?

That is what I was seeking to clarify by providing examples.

epage added the T-cargo Team: Cargo label Mar 26, 2026

rustbot assigned weihanglo Mar 26, 2026

rustbot added A-configuration Area: cargo config files and env vars A-documenting-cargo-itself Area: Cargo's documentation labels Mar 26, 2026

rustbot added A-unstable Area: nightly unstable support S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 26, 2026

rust-rfcbot added proposed-final-comment-period An FCP proposal has started, but not yet signed off. disposition-merge FCP with intent to merge labels Mar 26, 2026

epage mentioned this pull request Mar 26, 2026

RFC: Custom lint profiles rust-lang/rfcs#3926

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feat: Stabilize `build.warnings`#16796

feat: Stabilize `build.warnings`#16796
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		Only warnings within the user's control to resolve or adjust the level of are affected,
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