Implement unstable trait impl

[tiif]

This PR allows marking impls of stable trait with stable type as unstable.

The design and mentoring are done by @BoxyUwU

[bors]

☔ The latest upstream changes (presumably #142299) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #142483) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

gg gamer

[BoxyUwU]

Suggested change

	fn enabled(self, symbol: <TyCtxt<'tcx> as Interner>::Symbol) -> bool {
	self.enabled(symbol)
	}
	fn feature_bound_holds_in_crate(self, symbol: <TyCtxt<'tcx> as Interner>::Symbol) -> bool {
	// We don't consider feature bounds to hold in the crate if we `staged_api` feature is
	// even when the feature is enabled. This is to prevent accidentally leaking unstable APIs
	// to stable.
	!self.staged_api() && self.enabled(symbol)
	}

[tiif]

I tweaked the comment by a little bit

        // We don't consider feature bounds to hold in the crate when `staged_api` feature is
        // enabled, even if it is enabled through `#[feature]`. 
        // This is to prevent accidentally leaking unstable APIs to stable.

hope this captured what you intended to say :D

[BoxyUwU]

👍 I also meant to entirely remove the enabled function though, I don't think its an API we need to expose if we have feature_bound_holds_in_crate being used by the new solver

[tiif]

oh yea makes sense.

[tiif]

I tried to use feature_bound_holds_in_crate in the old solver, just like what is done in the new solver, but it requires rustc_type_ir::inherent::*, and has scary message

error: do not use `rustc_type_ir::inherent` unless you're inside of the trait solver
  --> compiler/rustc_trait_selection/src/traits/fulfill.rs:15:20
   |
15 | use rustc_type_ir::inherent::*;
   |                    ^^^^^^^^
   |
   = note: the method or struct you're looking for is likely defined somewhere else downstream in the compiler
   = note: `-D rustc::usage-of-type-ir-inherent` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(rustc::usage_of_type_ir_inherent)]`

so i decided to just write it out manually :)

[BoxyUwU]

Yeah only the new solver needs to use the fancy API, old solver can just do the logic manually

[tiif]

Do we want to change the error message to unstable_feature_bound may not be used outside of the standard library or it is acceptable to call it stability attribute?

[BoxyUwU]

i think its okay as-is

[tiif]

I am leaving the conflicts there because I don't want to rebase for now (the conflict also happens quite frequently), so it would be easier for boxy to review.

hi @jdonszelmann, it'd be nice to get a vibeck from you for the attribute change :)

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

changes to the core type system

cc @compiler-errors, @lcnr

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

changes to inspect_obligations.rs

cc @compiler-errors, @lcnr

Some changes occurred in compiler/rustc_attr_data_structures

cc @jdonszelmann

[jdonszelmann]

Hi tiff! Will review this week :)