UNCOVERED_PARAM_IN_PROJECTION in rustc_lint_defs::builtin - Rust
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.

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## UNCOVERED_PARAM_IN_PROJECTION

# Static UNCOVERED_PARAM_IN_PROJECTION

Source

    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.