CODEWITHSUNDEEP
rust
Robert Lord
Robert Lord

Reputation: 63

Getting "cannot extract an associated type from a higher-ranked trait bound in this context" error in Rust

When I run this code that uses higher-ranked trait bounds:

pub trait MyTrait<'a> {
    type Output: 'a;
    fn gimme_value(&self) -> Self::Output;
}

pub fn meow<T: for<'a> MyTrait<'a> + 'static>(val: &T) -> T::Output {
    val.gimme_value()
}

I'm seeing this error:

error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context

How can I make my function meow return this associated type, while still allowing T to be a higher ranked trait bound?

Upvotes: 0

Views: 107

Answers (1)

Robert Lord
Robert Lord

Reputation: 63

Just add a new generic to the meow function — let's call it R. When adding the trait restriction for T, define that T::Output is equal to R. Then, have the function return R instead of T::Output:

pub trait MyTrait<'a> {
    type Output: 'a;
    fn gimme_value(&self) -> Self::Output;
}

pub fn meow<R, T: for<'a> MyTrait<'a, Output=R> + 'static>(val: &T) -> R {
    val.gimme_value()
}

Upvotes: 1

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