feat(compile): 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 serves a similar purpose as RUSTFLAGS=-Dwarnings / RUSTFLAGS=-Awarnings but without invalidation caches.

build.warnings = "deny" will

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

(this matches RUSTFLAGS=-Dwarnings)

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.

build.warnings = "allow" will

only hide lint warnings and not hard warnings
- Note: RUSTFLAGS=-Awarnings will suppress rustc hard warnings
hide non-local warnings for --verbose --verbose
hide the warning summary line (number of warnings per crate)

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.

rustbot · 2026-03-26T21:46:57Z

r? @weihanglo

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The reviewer was selected based on:

Owners of files modified in this PR: @ehuss, @epage, @weihanglo
@ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
Random selection from ehuss, weihanglo

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

src/doc/src/reference/config.md

epage · 2026-04-01T20:59:37Z

FYI I found a bug where build.warnings = "allow" would hide denied lints causing no message to be shown and an abnormal rustc exit message. I have a local fixed prepared, just waiting on #16821.

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

epage · 2026-04-02T20:33:49Z

I've updated the stabilization report to reflect #16827 being merged. I also found and fixed #16824.

kazini · 2026-04-03T18:44:06Z

I really hope this goes through.

src/doc/src/guide/continuous-integration.md

rustbot · 2026-04-07T18:32:26Z

This PR was rebased onto a different master 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.

orlp · 2026-04-10T09:52:44Z

Regarding the "hard warnings" discussion, I provided a suggestion here to add a deny-all option: #14802 (comment).

jyn514 · 2026-04-10T10:06:14Z

Please do not allow people to override rustc's decision to force-show a warning :/ we only do that for cases where it really matters.

0xPoe

Thanks!

View changes since this review

0xPoe · 2026-04-14T20:17:16Z

src/doc/src/reference/config.md

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


Your English is definitely better than mine 😆, but I'm not sure if using "are affected" twice makes sense here.

0xPoe · 2026-04-14T20:19:40Z

src/cargo/core/features.rs

            "package-workspace" => stabilized_warn(k, "1.89", STABILIZED_PACKAGE_WORKSPACE),
            "build-dir" => stabilized_warn(k, "1.91", STABILIZED_BUILD_DIR),
            "config-include" => stabilized_warn(k, "1.93", STABILIZED_CONFIG_INCLUDE),
+            "warnings" => stabilized_warn(k, "1.97", STABILIZED_WARNINGS),


What is the policy for selecting the version here?

rust-rfcbot · 2026-04-14T20:24:47Z

🔔 This is now entering its final comment period, as per the review above. 🔔

weihanglo · 2026-04-14T21:13:54Z

src/doc/src/guide/continuous-integration.md

+    env:
+      CARGO_BUILD_WARNINGS: deny
+    steps:
+      - uses: actions/checkout@v4


While we've talked about this (we are not sure whether our CI example is responsible for updating to newer versions), I think we can still use the latest version here.

Suggested change

- uses: actions/checkout@v4

- uses: actions/checkout@v6

View changes since the review

weihanglo · 2026-04-14T21:16:04Z

src/doc/src/reference/config.md

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


A bit clearer?

Suggested change

Adjust the effective level for local lint warnings.

Adjust the effective level for lint warnings from local packages.

View changes since the review

weihanglo · 2026-04-14T21:18:23Z

src/doc/src/reference/config.md

+* `"warn"`: continue to emit the lints as warnings (default).
+* `"allow"`: hide the lints.
+* `"deny"`: emit an error for a crate that has lint warnings.
+  Use `--keep-going` to see the lint warnings for all crates.


"for all crates" is a bit confusing. This is not going to show warning for dependencies, right?

View changes since the review

weihanglo · 2026-04-14T21:37:29Z

@rust-rfcbot reviewed

Though

only errors for lint warnings and not hard warnings

only hide lint warnings and not hard warnings

Could we have some definition of hard warnings written down somewhere? I don't think we need to make a formal/stable definition, but at least we should capture what we have today. I guess they are

cargo::warnings from Cargo build scripts
Compiler diagnostics that doesn't have a code field in their JSON diagonstic

The current implementation is somehow subtle and I am lost

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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arlosi reviewed Mar 27, 2026

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ehuss added this to Cargo status tracker Mar 31, 2026

ehuss moved this to FCP merge in Cargo status tracker Mar 31, 2026

epage changed the title ~~feat: Stabilize build.warnings~~ feat(compile): Stabilize build.warnings Apr 2, 2026

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shian15810 reviewed Apr 5, 2026

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shian15810 mentioned this pull request Apr 5, 2026

Why not to set rustflags empty by default? actions-rust-lang/setup-rust-toolchain#48

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0xPoe reviewed Apr 14, 2026

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	Adjust the effective level for local lint warnings.
	Adjust the effective level for lint warnings from local packages.

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