Rollup merge of #90586 - jswrenn:relax-privacy-lints, r=petrochenkov
Relax priv-in-pub lint on generic bounds and where clauses of trait impls. The priv-in-pub lint is a legacy mechanism of the compiler, supplanted by a reachability-based [type privacy](https://github.com/rust-lang/rfcs/blob/master/text/2145-type-privacy.md) analysis. This PR does **not** relax type privacy; it only relaxes the lint (as proposed by the type privacy RFC) in the case of trait impls. ## Current Behavior On public trait impls, it's currently an **error** to have a `where` bound constraining a private type with a trait: ```rust pub trait Trait {} pub struct Type {} struct Priv {} impl Trait for Priv {} impl Trait for Type where Priv: Trait // ERROR {} ``` ...and it's a **warning** to have have a public type constrained by a private trait: ```rust pub trait Trait {} pub struct Type {} pub struct Pub {} trait Priv {} impl Priv for Pub {} impl Trait for Type where Pub: Priv // WARNING {} ``` This lint applies to `where` clauses in other contexts, too; e.g. on free functions: ```rust struct Priv<T>(T); pub trait Pub {} impl<T: Pub> Pub for Priv<T> {} pub fn function<T>() where Priv<T>: Pub // WARNING {} ``` **These constraints could be relaxed without issue.** ## New Behavior This lint is relaxed for `where` clauses on trait impls, such that it's okay to have a `where` bound constraining a private type with a trait: ```rust pub trait Trait {} pub struct Type {} struct Priv {} impl Trait for Priv {} impl Trait for Type where Priv: Trait // OK {} ``` ...and it's okay to have a public type constrained by a private trait: ```rust pub trait Trait {} pub struct Type {} pub struct Pub {} trait Priv {} impl Priv for Pub {} impl Trait for Type where Pub: Priv // OK {} ``` ## Rationale While the priv-in-pub lint is not essential for soundness, it *can* help programmers avoid pitfalls that would make their libraries difficult to use by others. For instance, such a lint *is* useful for free functions; e.g. if a downstream crate tries to call the `function` in the previous snippet in a generic context: ```rust fn callsite<T>() where Priv<T>: Pub // ERROR: omitting this bound is a compile error, but including it is too { function::<T>() } ``` ...it cannot do so without repeating `function`'s `where` bound, which we cannot do because `Priv` is out-of-scope. A lint for this case is arguably helpful. However, this same reasoning **doesn't** hold for trait impls. To call an unconstrained method on a public trait impl with private bounds, you don't need to forward those private bounds, you can forward the public trait: ```rust mod upstream { pub trait Trait { fn method(&self) {} } pub struct Type<T>(T); pub struct Pub<T>(T); trait Priv {} impl<T: Priv> Priv for Pub<T> {} impl<T> Trait for Type<T> where Pub<T>: Priv // WARNING {} } mod downstream { use super::upstream::*; fn function<T>(value: Type<T>) where Type<T>: Trait // <- no private deets! { value.method(); } } ``` **This PR only eliminates the lint on trait impls.** It leaves it intact for all other contexts, including trait definitions, inherent impls, and function definitions. It doesn't need to exist in those cases either, but I figured I'd first target a case where it's mostly pointless. ## Other Notes - See discussion [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/relax.20priv-in-pub.20lint.20for.20trait.20impl.20.60where.60.20bounds/near/222458397). - This PR effectively reverts #79291.
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