Prototype Cargo build analysis

[nikomatsakis]

Metadata
Point of contact	@weihanglo
Team(s)	cargo
Goal document	2025h2/cargo-build-analysis

Summary

Prototype support for build analysis in Cargo by recording build metadata across invocations.
This will introduce unstable subcommands in cargo report
to explain why crates were rebuilt and to surface timing data from past builds,
helping developers diagnose and improve their build performance.

Tasks and status

• Discussion and moral support (cargo )
• Implementation (@weihanglo)
• Standard reviews (cargo )
• Author call for testing blog post (@weihanglo)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[weihanglo]

Cargo tracking issue: rust-lang/cargo#15844.
The first implementation was rust-lang/cargo#15845 in August that added build.analysis.enabled = true to unconditionally generate timing HTML. Further implementations tasks is listed in rust-lang/cargo#15844 (comment).

Haven't yet got any progress in September.

[weihanglo]

Instead of using a full-fledged database like SQLite, we switched to a basic JSONL-based logging system to collect build metrics. A simple design doc can be found here: https://hackmd.io/K5-sGEJeR5mLGsJLXqsHrw.

Here are the recent pull requests:

• feat(build-analysis): JSONL-based logging infra cargo#16150
• feat(build-analysis): emit timing-info log cargo#16179

To enable it, set CARGO_BUILD_ANALYSIS_ENABLED=true or set the Cargo config file like this:

[build.analysis]
enabled = true

As of today (nightly-2025-11-03), it currently emits build-started and timing-info two log events to $CARGO_HOME/log/ (~/.cargo/log/ by default). The shape of timing-info JSON is basically the shape of the unstable --timing=json. I anticipate when this is stabilized we don't need --timing=json.

The build.analysis.enable is a non-blocking unstable feature. Unless bugs, should be able to set unconditionally even on stable toolchain. When not supported, it would just warn the unknown config merely.