pub static UNCOVERED_PARAM_IN_PROJECTION: &Lint
Expand description
The uncovered_param_in_projection
lint detects a violation of one of Rust’s orphan rules for
foreign trait implementations that concerns the use of type parameters inside trait associated
type paths (“projections”) whose output may not be a local type that is mistakenly considered
to “cover” said parameters which is unsound and which may be rejected by a future version
of the compiler.
Originally reported in #99554.
§Example
ⓘ
// dependency.rs
#![crate_type = "lib"]
pub trait Trait<T, U> {}
ⓘ
// dependent.rs
trait Identity {
type Output;
}
impl<T> Identity for T {
type Output = T;
}
struct Local;
impl<T> dependency::Trait<Local, T> for <T as Identity>::Output {}
fn main() {}
This will produce:
warning[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
--> dependent.rs:11:6
|
11 | impl<T> dependency::Trait<Local, T> for <T as Identity>::Output {}
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #124559 <https://github.com/rust-lang/rust/issues/124559>
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
= note: `#[warn(uncovered_param_in_projection)]` on by default
§Explanation
FIXME(fmease): Write explainer.