E0271 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0271

A type mismatched an associated type of a trait.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
    //                    ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
    //                        |            |
    //         This says `foo` can         |
    //           only be used with         |
    //              some type that         |
    //         implements `Trait`.         |
    //                                     |
    //                             This says not only must
    //                             `T` be an impl of `Trait`
    //                             but also that the impl
    //                             must assign the type `u32`
    //                             to the associated type.
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = &'static str; }
    //~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    //      |                             |
    // `i8` does have                     |
    // implementation                     |
    // of `Trait`...                      |
    //                     ... but it is an implementation
    //                     that assigns `&'static str` to
    //                     the associated type.

    foo(3_i8);
    // Here, we invoke `foo` with an `i8`, which does not satisfy
    // the constraint `<i8 as Trait>::AssociatedType=u32`, and
    // therefore the type-checker complains with this error code.
    }

The issue can be resolved by changing the associated type:

1. in the `foo` implementation:

    #![allow(unused)]
    fn main() {
    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = &'static str; }

    foo(3_i8);
    }

2. in the `Trait` implementation for `i8`:

    #![allow(unused)]
    fn main() {
    trait Trait { type AssociatedType; }

    fn foo<T>(t: T) where T: Trait<AssociatedType = u32> {
        println!("in foo");
    }

    impl Trait for i8 { type AssociatedType = u32; }

    foo(3_i8);
    }