"Private trait in public interface warning" changes to error when an unrelated pub(crate) is present

[simonadameit]

Implementing a public trait for all types that implement a private trait emits a warning. However the presence of pub(crate) in an unrelated piece of code turns that warning into an error.

The warning is: warning: private trait `PrivateTrait` in public interface (error E0445)
The error is: error[E0445]: private trait `PrivateTrait` in public interface

The behaviour can be reproduced by commenting out pub(crate) struct Foo in this code:

trait PrivateTrait {}

pub trait PublicTrait {}

impl<T> PublicTrait for T where T: PrivateTrait {}


/*
Comment this out and the error is gone and the code compiles with a warning
*/
pub(crate) struct Foo;

Meta

I confirmed the presence of this behaviour on the rust playground for:

• Stable channel
  Build using the Stable version: 1.55.0

• Beta channel
  Build using the Beta version: 1.56.0-beta.4
  (2021-10-04 e6e620e)

• Nightly channel
  Build using the Nightly version: 1.57.0-nightly
  (2021-10-12 d7c97a0)

References

• This issue might be related: Spurious private-in-public error for impl block #59090
• @vickenty found this code in the compiler that might be the cause

[trentj]

I believe #48054 means this should one day be accepted without any diagnostic, although the current implementation status is that it still triggers the legacy public-in-private lint. Even so, that lint ought to be a warning, not an error.

[SNCPlay42]

This comment looks relevant:

rust/compiler/rustc_privacy/src/lib.rs

Lines 303 to 308 in eeb16a2

	////////////////////////////////////////////////////////////////////////////////
	/// Visitor used to determine if pub(restricted) is used anywhere in the crate.
	///
	/// This is done so that `private_in_public` warnings can be turned into hard errors
	/// in crates that have been updated to use pub(restricted).
	////////////////////////////////////////////////////////////////////////////////

I think the intent is that since newer crates can use pub(crate) and similar visibilities, they no longer need to use private-in-public items to achieve the same effect, and so the compiler can emit a hard error instead of accepting the behavior for compatibility reasons when the new visibilities are being used.

[PatchMixolydic]

That seems like pretty surprising behaviour, especially given Rust's avoidance of action at a distance. Granted, the warning is a future compat lint, but I'd expect it to always warn or always error in a given release using a given edition.

[Dylan-DPC]

This code now compiles fine without the error.