feat(log): add output control #16418

weihanglo · 2025-12-19T15:36:27Z

What does this PR try to resolve?

Add output control for build analysis logs.
The motivation of this are:

User may want to pipe build log to stdout and process them in a stream manner in real time, rather than in a log file.
Output to stdout so we can have a replacement for the unstable --timings=json option.

See doc update for more: 3df1405.

Example config:

[build.analysis]
enabled = true
console = true
file = false

How to test and review this PR?

cargo run -- build -p rustfix -Zbuild-analysis --config build.analysis.enabled=true --config build.analysis.console=true --config 'build.build-dir="target/mytmp"' --message-format=json

Open questions:

Console output is only enabled when --message-format has a JSON option, so it won't interleave with status messages if people don't want JSON messages at all.
Logging is best-effort for now. If there is any broken pipe, no error or warning will show
Alternative schemas
- A destination field
```
[build.analysis]
destination = ["file", "console"]
```
- Remove build.analysis.enabled. Each output option enablement control whether log is generated
```
[build.analysis]
console = true
file = true
```
- No build.analysis.file. console and file output are mutually exclusive
```
[build.analysis]
write-to-console = true # No log file output
```

So this preserve whether the config value is actually set.

This is for upcoming logging-to-shell feature

Tests are prepared to show the change of output destination control

rustbot · 2025-12-19T15:36:33Z

r? @ehuss

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epage · 2025-12-19T15:44:05Z

src/cargo/ops/cargo_compile/mod.rs

            root_units.iter().map(|unit| unit_to_index[&unit]).collect();

-        for (index, unit) in units.into_iter().enumerate() {
+        logger.log_batch(units.into_iter().enumerate().map(|(index, unit)| {


I'm not usually a fan of big map functions and tend to prefer explicit loops for them

shell holds a mutex lock and stdout lock, which are a bit expensive to work with. Other alternatives are having a buffer for shell print, or just eating the cost

epage · 2025-12-19T15:46:17Z

src/cargo/util/context/schema.rs

+    pub console: Option<bool>,
+    pub file: Option<bool>,


The PR describes what this does but doesn't provide the motivation.

I find this

confusing: it isn't very clear what these states mean and why you would use them

odd to have build analysis and to be able to turn off file

unclear on what purpose this is meant to serve, especially at this point

There is config doc update 3df1405.

Updated the PR description. Thanks for the nudge.

User may want to pipe build log to stdout and process them in a stream manner in real time, rather than in a log file.

As I've said before, I want us to unify the build log and json message format, so how do decide what is a "build log" and a "regular message" in that future? I'd rather just wait, see how stabilization goes on this, and evaluate always putting these messages out.

Output to stdout so we can have a replacement for the unstable --timings=json option.

Do we need to have feature parity with that?

I'd rather just wait, see how stabilization goes on this, and evaluate always putting these messages out.

That is fair. We should wait and see.

Add output control for build analysis logs. Example config: ```toml [build.analysis] enabled = true console = true file = false ``` Console output is only enabled when `--message-format` has a JSON option. Note that logging is best-effort for now. If there is any broken pipe, no error or warning will show

weihanglo · 2025-12-19T18:52:47Z

Closed as we want to wait and see how we build logs and JSON message should work with each other in the future.

Also, the original motivation of this was finding a good replacement for --timings=json but that is unstable, and people have workaround (with a bit more works) with -Zbuild-analysis, so the motivation was gone.

Thanks for the review!

### 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. <#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

weihanglo added 3 commits December 19, 2025 10:26

refactor(config): use Option to wrap bool

be4f335

So this preserve whether the config value is actually set.

refactor(log): batch log messages

0cbb155

This is for upcoming logging-to-shell feature

test(log): output to file only

9377e9e

Tests are prepared to show the change of output destination control

weihanglo added A-timings Area: timings Z-build-analysis Nightly: build-analysis labels Dec 19, 2025

rustbot assigned ehuss Dec 19, 2025

rustbot added A-documenting-cargo-itself Area: Cargo's documentation S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 19, 2025

epage reviewed Dec 19, 2025

View reviewed changes

weihanglo added 3 commits December 19, 2025 11:05

fix(log): use annotate-snippets style warning

8ad051a

docs(log): mention new output control

f8f660e

weihanglo force-pushed the timing branch from 3df1405 to f8f660e Compare December 19, 2025 16:07

weihanglo closed this Dec 19, 2025

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 19, 2025

weihanglo mentioned this pull request Dec 19, 2025

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

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