`private_interfaces` not linting RPITIT

[mladedav]

Code

trait Bar {}

pub trait Foo {
    fn foo() -> impl Bar;
}

fn main() {}

Current output

Empty output with no warnings.

Desired output

warning: trait `Bar` is more private than the item `Foo::foo`
 --> foo.rs:4:1
  |
4 |     fn foo() -> impl Bar;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ associated function `Foo::foo` is reachable at visibility `pub`
  |
note: but trait `Bar` is only usable at visibility `pub(crate)`
 --> foo.rs:1:1
  |
1 | trait Bar {}
  | ^^^^^^^^^
  = note: `#[warn(private_interfaces)]` on by default

warning: 1 warning emitted

Rationale and extra context

No response

Other cases

Both using a concrete type or using RPIT in a method outside traits produce correct lints:

struct X;

pub trait Foo {
    fn foo() -> X;
}

produces

warning: type `X` is more private than the item `Foo::foo`

trait Bar {}
impl Bar for () {}

fn foo() -> impl Bar {}

Produces

warning: trait `Bar` is more private than the item `foo`

Rust Version

rustc 1.90.0-nightly (5adb489a8 2025-07-05)
binary: rustc
commit-hash: 5adb489a8034f7b56b29f3b28af0813c866f679c
commit-date: 2025-07-05
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

Anything else?

No response

[mladedav]

After merging #143357 the original code now outputs the following error:

warning: trait `Bar` is more private than the item `Foo::foo::{anon_assoc#0}`
 --> src/main.rs:4:17
  |
4 |     fn foo() -> impl Bar;
  |                 ^^^^^^^^ opaque type `Foo::foo::{anon_assoc#0}` is reachable at visibility `pub`
  |
note: but trait `Bar` is only usable at visibility `pub(crate)`
 --> src/main.rs:1:1
  |
1 | trait Bar {}
  | ^^^^^^^^^
  = note: `#[warn(private_bounds)]` on by default

So now the lint could just have an improved message but it's no longer a false negative as of nightly-2025-07-15.