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.

compiler/rustc_privacy/src/lib.rs

@@ -2064,7 +2064,11 @@ impl<'tcx> Visitor<'tcx> for PrivateItemsInPublicInterfacesVisitor<'tcx> {
             // Subitems of trait impls have inherited publicity.
             hir::ItemKind::Impl(ref impl_) => {
                 let impl_vis = ty::Visibility::of_impl(item.def_id, tcx, &Default::default());
-                self.check(item.def_id, impl_vis).generics().predicates();
+                // check that private components do not appear in the generics or predicates of inherent impls
+                // this check is intentionally NOT performed for impls of traits, per #90586
+                if impl_.of_trait.is_none() {
+                    self.check(item.def_id, impl_vis).generics().predicates();
+                }
                 for impl_item_ref in impl_.items {
                     let impl_item_vis = if impl_.of_trait.is_none() {
                         min(tcx.visibility(impl_item_ref.id.def_id), impl_vis, tcx)