Add initial integration for `--json=timings` behing `-Zsection-timings` #15780

Kobzol · 2025-07-28T15:47:46Z

What does this PR try to resolve?

This PR adds initial support into Cargo for JSON timing sections, implemented in rustc in rust-lang/rust#142123. This allows Cargo to read frontend/codegen/linking time from rustc, and thus reporting slightly more detailed data in the cargo build --timings output.

The PR modifies Cargo to tell rustc to emit the section messages (--json=...,timings), and it adds the section timings data to the HTML table output and the JSON output. It does not yet integration different sections in the HTML unit chart (I want to do that as a follow-up).

Note that the JSON timings are currently only supported on the nightly compiler (they are not stabilized). The new behavior is thus gated behing an unstable Cargo flag (-Zsection-timings). When the flag is unused, the HTML table should look more or less the same as before, just that the code now supports both options.

How to test and review this PR?

You can run e.g. this to generate the timing report with a nightly compiler:

export RUSTC=`rustup +nightly which rustc`
target/debug/cargo build -Zsection-timings --timings

on some crate, e.g. ripgrep.

Tracking issue: #15817

rustbot · 2025-07-28T15:47:52Z

r? @weihanglo

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src/cargo/core/compiler/timings.rs

src/cargo/core/compiler/mod.rs

src/cargo/core/compiler/timings.rs

epage · 2025-07-29T20:47:54Z

src/cargo/core/compiler/timings.rs

@@ -64,8 +64,9 @@ pub struct Timings<'gctx> {
    cpu_usage: Vec<(f64, f64)>,
 }

-/// Section of compilation.
-struct TimingSection {


Mind moving the rename into the commit that added it so this commit can be focused just on json output?

Why was this marked as resolved? I'm not seeing it resolved as requested nor an explanation for why it shouldn't happen

src/cargo/core/compiler/timings.rs

src/cargo/util/machine_message.rs

Kobzol · 2025-07-30T05:28:31Z

Ok, so based on feedback I'm fine with moving this to a fully dynamic approach.

Before I do that though, do you have any suggestions on how to detect whether the timings flag should be passed to Cargo? Is there any precedent for doing something in Cargo only on the nightly channel, or more precisely only with nightly rustc? Or would you prefer that we add an unstable Cargo flag for this?

epage · 2025-08-05T18:39:55Z

Before I do that though, do you have any suggestions on how to detect whether the timings flag should be passed to Cargo? Is there any precedent for doing something in Cargo only on the nightly channel, or more precisely only with nightly rustc? Or would you prefer that we add an unstable Cargo flag for this?

We discussed this in today's cargo team meeting. We should not enable an unstable rustc feature implicitly but require an explicit opt-in.

People using nightly for other functionality (e.g. miri or a sanitizer) shouldn't pick up unstable features they aren't using
We'd want more than to assume that rustc outputs it in a compatible way / the flag exists and instead have feature detection which is difficult to get right.

Kobzol · 2025-08-05T18:41:49Z

Fair enough, I'll add an explicit feature flag then. It's true that if it was implicit, if the rustc format changes in the future, it could break nightly cargo users of cargo build --timings. That would be solvable if Cargo was a subtree, but with a submodule it would be quite difficult to ensure that both rustc and cargo change at once.

Kobzol · 2025-08-08T09:32:04Z

Ok, I rewrote the PR and updated the PR description. It is now gated behind an unstable -Zsection-timings flag, and it renders the sections into the HTML table in a fully dynamic manner.

src/cargo/core/compiler/timings.rs

…s` is enabled

Kobzol · 2025-08-12T15:44:38Z

Restructured the commits

epage

Thanks!

Update cargo 23 commits in 840b83a10fb0e039a83f4d70ad032892c287570a..9a0712751ca9444775c7352d3c942a155b656437 2025-07-30 13:59:19 +0000 to 2025-08-15 20:30:47 +0000 - Fixes error while running the cargo clippy --all-targets -- -D warning (rust-lang/cargo#15843) - Clarify that `cargo doc --no-deps` is cumulative and won’t delete prev (rust-lang/cargo#15800) - docs: Formatting and cross-linking to build-dir/target-dir docs (rust-lang/cargo#15840) - Stabilize `build.build-dir` (rust-lang/cargo#15833) - make resolve features public for cargo-as-a-library (rust-lang/cargo#15835) - chore(deps): bump slab from 0.4.10 to 0.4.11 (rust-lang/cargo#15832) - chore: remove x86_64-apple-darwin from CI and tests (rust-lang/cargo#15831) - chore(deps): update msrv (3 versions) to v1.87 (rust-lang/cargo#15819) - perf(package): Always reuse the workspace's target-dir (rust-lang/cargo#15783) - More helpful error for invalid cargo-features = [] (rust-lang/cargo#15781) - Add initial integration for `--json=timings` behing `-Zsection-timings` (rust-lang/cargo#15780) - add is_inherited methods to InheritableDependency and InheritableField (rust-lang/cargo#15828) - chore(deps): update compatible (rust-lang/cargo#15804) - docs(unstable): Link out to the Plumbing commands effort (rust-lang/cargo#15821) - chore(deps): update cargo-semver-checks to v0.43.0 (rust-lang/cargo#15825) - test(build-std): relax the thread name assertion (rust-lang/cargo#15822) - chore(deps): update msrv (1 version) to v1.89 (rust-lang/cargo#15815) - Update semver tests for 1.89 (rust-lang/cargo#15816) - Accessing each build script's `OUT_DIR` and in the correct order (rust-lang/cargo#15776) - chore: bump to 0.92.0; update changelog (rust-lang/cargo#15807) - docs: `-Zpackage-workspace` has been stabilized (rust-lang/cargo#15808) - chore(deps): update rust crate cargo_metadata to 0.21.0 (rust-lang/cargo#15795) - docs(build-rs): Fix broken intra-doc links (rust-lang/cargo#15810) r? ghost

rustbot assigned weihanglo Jul 28, 2025

rustbot added A-build-execution Area: anything dealing with executing the compiler A-timings Area: timings S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 28, 2025

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Kobzol changed the title ~~Add initial integration for --json=timings~~ Add initial integration for --json=timings behing -Zsection-timings Aug 8, 2025

Kobzol mentioned this pull request Aug 8, 2025

Tracking Issue for -Zsection-timings #15817

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Kobzol force-pushed the timings-linker branch from cda2cd1 to 362a796 Compare August 8, 2025 09:26

rustbot added A-documenting-cargo-itself Area: Cargo's documentation A-unstable Area: nightly unstable support labels Aug 8, 2025

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Kobzol force-pushed the timings-linker branch from 78e1edd to 71a0df9 Compare August 9, 2025 08:18

Kobzol force-pushed the timings-linker branch from 71a0df9 to 2a98709 Compare August 12, 2025 06:58

Kobzol added 6 commits August 12, 2025 17:42

Add unstable -Zsection-timings flag

399dff8

Pass --json=timings to rustc when --timings and `-Zsection-timing…

62e4054

…s` is enabled

Parse and store section timing events

dbd298a

Add section timing data to JSON --timings output

037cf46

Make headers of the HTML --timings unit table dynamic

5224b47

Add compilation sections to the HTML unit table

199e9e7

Kobzol force-pushed the timings-linker branch from 9a53d87 to 199e9e7 Compare August 12, 2025 15:44

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Merged via the queue into rust-lang:master with commit 1e52a8c Aug 12, 2025
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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 12, 2025

Kobzol deleted the timings-linker branch August 12, 2025 18:27

weihanglo mentioned this pull request Aug 16, 2025

Update cargo rust-lang/rust#145478

Open

Add initial integration for --json=timings behing -Zsection-timings #15780

Add initial integration for --json=timings behing -Zsection-timings #15780

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Add initial integration for `--json=timings` behing `-Zsection-timings` #15780

Add initial integration for `--json=timings` behing `-Zsection-timings` #15780

Kobzol commented Jul 28, 2025 •

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