Tracking Issue for build-std-always

[ehuss]

This is a tracking issue for the RFC "3874" (rust-lang/rfcs#3874).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• N/A Style updates for any new syntax (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• What should the build-std.when configuration in .cargo/config.toml be named?
• What should the "always" and "never" values of build-std be named?
• What should build-std.crates be named?
• Should the standard library inherit RUSTFLAGS?
• Should rust-src be a default component?
• Should cargo clean delete builds of the standard library?

Implementation history

• Custom JSON target destabilization (see Tracking Issue for target-spec-json (flexible target specifications) #151528)