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.