Rework Cargo Build Dir Layout

[nikomatsakis]

Metadata
Point of contact	@ranger-ross
Team(s)	cargo
Goal document	2025h2/cargo-build-dir-layout

Summary

Rework the Cargo build directory layout to have smaller self contained "units"

Tasks and status

• Standard reviews (cargo )
• Implementation (@ranger-ross)

[nikomatsakis]

This issue is intended for status updates only.

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

[ranger-ross]

Status update October 6, 2025

The build-dir was split out of target-dir as part of rust-lang/cargo#14125 and scheduled for stabilization in Rust 1.91.0. 🎉

Before re-organizing the build-dir layout we wanted to improve the existing layout tests to make sure we do not make any unexpected changes. This testing harness improvement was merged in rust-lang/cargo#15874.

The initial build-dir layout reorganization PR has been posted rust-lang/cargo#15947 and discussion/reviews are under way.