fix(timing)!: remove `--timings=<FMT>` optional format values #16420

weihanglo · 2025-12-19T20:18:06Z

What does this PR try to resolve?

Arguments to remove it

The --timings=json is obsolete as -Zbuild-analysis logging is
a more approachable option, which doesn't need passing
--timings=json ahead of time.
There is no support infra built around --timings=json yet,
while for -Zbuild-analysis we have cargo report timings already.
--timings=json is a UI feature inherently unstable, and has no tests.

Counterargument:

--timings=json outputs to stdout, but there is no alternative yet
also outputs to stdout.
feat(log): add output control #16418 was an attempt to add that back,
but we then decide to punt until seeing requests or needs.

How to test and review this PR?

cargo help build and check the manpage
cargo build --help and cehck the help text
cargo build --timings and it works

rustbot · 2025-12-19T20:18:11Z

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weihanglo · 2025-12-19T20:19:42Z

src/cargo/core/compiler/build_runner/mod.rs

Does it require an FCP?

Might be good to get a vibe check in case there is interest in build analysis having X amount of functionality before it can replace --timings=json

This has entered FCP: #16420 (comment).

I personally don't think we need to wait for the extra 10 days.

People may not notice the removal until at least it hits nightly.

People care about the evolution of timing report may have already followed the goal update in https://blog.rust-lang.org/2026/01/05/project-goals-2025-december-update/

This is a nightly feature.

I am fine if we still want to wait for the full FCP.

Arguments to remove it * The `--timings=json` is obsolete as `-Zbuild-analysis` logging is a more approachable option, which doesn't need passing `--timings=json` ahead of time. * There is no support infra built around `--timings=json` yet, while for `-Zbuild-analysis` we have `cargo report timings` already. * `--timings=json` is a UI feature inherently unstable, and has no tests. Counterargument: * `--timings=json` outputs to stdout, but there is no alternative yet also outputs to stdout.

weihanglo · 2025-12-19T20:28:13Z

@rfcbot fcp merge T-cargo

This removes support for --timings=json, making --timings a plain boolean flag after JSON timing output was removed. See the PR descriptions for more.

rust-rfcbot · 2025-12-19T20:28:16Z

Team member @weihanglo 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.

rust-rfcbot · 2026-01-06T22:17:49Z

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

Update cargo submodule 24 commits in 94c368ad2b9db0f0da5bdd8421cea13786ce4412..fc4c92b64d1e0046b66cbdc747cc1c17af8b35a0 2025-12-26 19:39:15 +0000 to 2026-01-08 06:54:56 +0000 - Fixed incorrect version comparision during build script dependency selection (rust-lang/cargo#16486) - refactor: new type for unit index (rust-lang/cargo#16485) - feat(test): Make CARGO_BIN_EXE_ available at runtime (rust-lang/cargo#16421) - fix(package): detect dirty files when run from workspace member (rust-lang/cargo#16479) - fix(timing)!: remove `--timings=<FMT>` optional format values (rust-lang/cargo#16420) - docs(unstable): expand docs for `-Zbuild-analysis` (rust-lang/cargo#16476) - test: add `-Zunstable-options` with custom targets (rust-lang/cargo#16467) - feat(report): add cargo report rebuilds (rust-lang/cargo#16456) - feat(test-support): Use test name for dir when running tests (rust-lang/cargo#16121) - refactor: Migrate some cases to expect/reason (rust-lang/cargo#16461) - docs(build-script): clarify OUT_DIR is not cleaned between builds (rust-lang/cargo#16437) - chore: Update dependencies (rust-lang/cargo#16460) - Update handlebars to 6.4.0 (rust-lang/cargo#16457) - chore(deps): update alpine docker tag to v3.23 (rust-lang/cargo#16454) - Any build scripts can now use cargo::metadata=KEY=VALUE (rust-lang/cargo#16436) - fix(log): add `dependencies` field to `UnitRegistered` (rust-lang/cargo#16448) - Implement fine grain locking for `build-dir` (rust-lang/cargo#16155) - feat(resolver): List features when no close match (rust-lang/cargo#16445) - feat(report): new command `cargo report sessions` (rust-lang/cargo#16428) - feat (patch): Display where the patch was defined in patch-related error messages (rust-lang/cargo#16407) - test(build-rs): Reduce from 'build' to 'check' where possible (rust-lang/cargo#16444) - feat(toml): TOML 1.1 parse support (rust-lang/cargo#16415) - feat(report): support --manifest-path in `cargo report timings` (rust-lang/cargo#16441) - fix(vendor): recursively filter git files in subdirectories (rust-lang/cargo#16439)

Update cargo submodule 27 commits in 94c368ad2b9db0f0da5bdd8421cea13786ce4412..8c133afcd5e0d69932fe11f5907683723f8d361d 2025-12-26 19:39:15 +0000 to 2026-01-09 03:50:15 +0000 - Isolate build script metadata progation between std and non-std crates (rust-lang/cargo#16489) - Add Clippy like lint groups (rust-lang/cargo#16464) - feat: in-memory only `Manifest` (rust-lang/cargo#16409) - Fixed incorrect version comparision during build script dependency selection (rust-lang/cargo#16486) - refactor: new type for unit index (rust-lang/cargo#16485) - feat(test): Make CARGO_BIN_EXE_ available at runtime (rust-lang/cargo#16421) - fix(package): detect dirty files when run from workspace member (rust-lang/cargo#16479) - fix(timing)!: remove `--timings=<FMT>` optional format values (rust-lang/cargo#16420) - docs(unstable): expand docs for `-Zbuild-analysis` (rust-lang/cargo#16476) - test: add `-Zunstable-options` with custom targets (rust-lang/cargo#16467) - feat(report): add cargo report rebuilds (rust-lang/cargo#16456) - feat(test-support): Use test name for dir when running tests (rust-lang/cargo#16121) - refactor: Migrate some cases to expect/reason (rust-lang/cargo#16461) - docs(build-script): clarify OUT_DIR is not cleaned between builds (rust-lang/cargo#16437) - chore: Update dependencies (rust-lang/cargo#16460) - Update handlebars to 6.4.0 (rust-lang/cargo#16457) - chore(deps): update alpine docker tag to v3.23 (rust-lang/cargo#16454) - Any build scripts can now use cargo::metadata=KEY=VALUE (rust-lang/cargo#16436) - fix(log): add `dependencies` field to `UnitRegistered` (rust-lang/cargo#16448) - Implement fine grain locking for `build-dir` (rust-lang/cargo#16155) - feat(resolver): List features when no close match (rust-lang/cargo#16445) - feat(report): new command `cargo report sessions` (rust-lang/cargo#16428) - feat (patch): Display where the patch was defined in patch-related error messages (rust-lang/cargo#16407) - test(build-rs): Reduce from 'build' to 'check' where possible (rust-lang/cargo#16444) - feat(toml): TOML 1.1 parse support (rust-lang/cargo#16415) - feat(report): support --manifest-path in `cargo report timings` (rust-lang/cargo#16441) - fix(vendor): recursively filter git files in subdirectories (rust-lang/cargo#16439)

rustbot assigned ehuss Dec 19, 2025

weihanglo commented Dec 19, 2025

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weihanglo added the T-cargo Team: Cargo label Dec 19, 2025

weihanglo force-pushed the timing branch from acc1b50 to f851173 Compare December 19, 2025 20:27

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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 Dec 19, 2025

weihanglo mentioned this pull request Dec 26, 2025

Prototype Cargo build analysis rust-lang/rust-project-goals#398

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weihanglo mentioned this pull request Jan 5, 2026

feat(report): add cargo report rebuilds #16456

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ehuss added this to Cargo status tracker Jan 6, 2026

ehuss moved this to FCP merge in Cargo status tracker Jan 6, 2026

weihanglo mentioned this pull request Jan 6, 2026

Reuse timing metric collection logic between --timings and -Zbuild-analysis #16474

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rust-rfcbot added the final-comment-period FCP — a period for last comments before action is taken label Jan 6, 2026

rust-rfcbot removed the proposed-final-comment-period An FCP proposal has started, but not yet signed off. label Jan 6, 2026

epage approved these changes Jan 7, 2026

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epage added this pull request to the merge queue Jan 7, 2026

Merged via the queue into rust-lang:master with commit 1436cf9 Jan 7, 2026
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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 7, 2026

weihanglo mentioned this pull request Jan 8, 2026

Update cargo submodule rust-lang/rust#150739

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rustbot added this to the 1.94.0 milestone Jan 9, 2026

weihanglo deleted the timing branch January 12, 2026 01:23

rust-rfcbot added finished-final-comment-period FCP complete to-announce and removed final-comment-period FCP — a period for last comments before action is taken labels Jan 16, 2026

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fix(timing)!: remove `--timings=<FMT>` optional format values #16420

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fix(timing)!: remove `--timings=<FMT>` optional format values #16420

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