WIP: Lint unused features

[mu001999]

Fixes #44232
Fixes #151752

[jyn514]

this is neat! in the future, it would be easier to review if you split the rename into a separate commit from the change in behavior.

could you add some tests for this please?

View changes since this review

[mu001999]

Initial implementation completed. I plan to add more tests and check whether features in rustc are reported as unused correctly or not.

[jyn514]

this is confusing to me — do you mean it's only checked downstream by a later invocation of the compiler? like, when std is no longer the local crate?

[mu001999]

This library feature is declared in

rust/library/std/src/lib.rs

Lines 750 to 754 in f21b4c0

	// This is required to avoid an unstable error when `restricted-std` is not
	// enabled. The use of #![feature(restricted_std)] in rustc-std-workspace-std
	// is unconditional, so the unstable feature needs to be defined somewhere.
	#[unstable(feature = "restricted_std", issue = "none")]
	mod __restricted_std_workaround {}

It is only on the private mod, so that it cannot be recorded as used in downstream crates, and will always be an unused feature.

[mu001999]

But I'm not sure it's the best way to solve this. Because we can define such features arbitrarily. And as we know they will be always unused, maybe users who use such features should allow unused_features manually or export a pub thing like pub mod workaround {} to make them used? 🤔

[mu001999]

Nice, CI has passes (finally :)), although I allowed unused_features somewhere (I try to only allow when there are complex platform-specific features enabled). But I think the next could be to merge the removal of unused features separately.