feat(log): emit build-started JSON message with run_id

[weihanglo]

What does this PR try to resolve?

When both -Zbuild-analysis and --message-format=json are enabled,
emit a build-started JSON message to stdout containing the run_id.

This allows external tools to associate the JSON output stream
with the corresponding log file in ~/.cargo/log/{run_id}.jsonl.

While a better design of JSON message and structured logging is needed,
this provides a temporary workaround for those needing the old --timings=json flag.

Closes #16576

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[weihanglo]

cc @Kobzol @soqb

Until a better solution is out, I think this is sufficient for your use case (with some extra probe to ~/.cargo/log/ directory). Let me know if this is suffcient, or you have some other wild idea. I should create a separate issue discussing the interaction between logging and JSON message.

[Kobzol]

So, for my use-case with rustwide, it doesn't help :( Because rustwide (used e.g. by crater) runs Rust crates in a Docker container where CARGO_HOME is mounted as read-only:

Error: Experiment run failed

Caused by:
    command failed: exit status: 101
    
    error: failed to create directory `/opt/rustwide/cargo-home/log`
    
    Caused by:
      Read-only file system (os error 30)

If the writing into CARGO_HOME ever became performed default, I guess this would also currently break crater 😆

[weihanglo]

I think we have a few options here:

• Redirect log to stdout
• A configurable log directory path

The second one is less ambiguous and straightforward to implement, though I am not sure if that meets your use case

[Kobzol]

It would work for the Docker/container use-case, and making the log dir be configurable sounds like a good idea regardless. (streaming to stdout would also be appreciated, but I guess that can wait until there's more design work done on -Zbuild-analysis)

[soqb]

i am in roughly the same boat as @Kobzol here. a configurable log dir is necessary for my use case.

[oriongonza]

Useful for rust-lang/crater#825
Zulip

[weihanglo]

Created #16696 for specifying log file paths.

[rustbot]

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.