Expand description

The refining_impl_trait_internal lint detects impl Trait return types in method signatures that are refined by a trait implementation, meaning the implementation adds information about the return type that is not present in the trait.



use std::fmt::Display;

trait AsDisplay {
    fn as_display(&self) -> impl Display;

impl<'s> AsDisplay for &'s str {
    fn as_display(&self) -> Self {

fn main() {
    // users can observe that the return type of
    // `<&str as AsDisplay>::as_display()` is `&str`.
    let _x: &str = "".as_display();



Callers of methods for types where the implementation is known are able to observe the types written in the impl signature. This may be intended behavior, but may also lead to implementation details being revealed unintentionally. In particular, it may pose a semver hazard for authors of libraries who do not wish to make stronger guarantees about the types than what is written in the trait signature.

refining_impl_trait is a lint group composed of two lints:

  • refining_impl_trait_reachable, for refinements that are publically reachable outside a crate, and
  • refining_impl_trait_internal, for refinements that are only visible within a crate.

We are seeking feedback on each of these lints; see issue #121718 for more information.