Understanding and Resolving Rust Error E0657

Introduction to Rust Error E0657

Rust Error E0657 occurs when a lifetime bound on a trait implementation is captured at an incorrect place. In this article, we will explore this error in detail and understand how to fix it using an example.

Understanding the E0657 Error

To comprehend the E0657 error, first, consider the following erroneous code example:

#![allow(unused)]
fn main() {
trait Id<T> {}
trait Lt<'a> {}

impl<'a> Lt<'a> for () {}
impl<T> Id<T> for T {}

fn free_fn_capture_hrtb_in_impl_trait()
    -> Box<for<'a> Id<impl Lt<'a>>> // error!
{
    Box::new(())
}

struct Foo;
impl Foo {
    fn impl_fn_capture_hrtb_in_impl_trait()
        -> Box<for<'a> Id<impl Lt<'a>>> // error!
    {
        Box::new(())
    }
}
}

In the code above, you have used an inappropriate lifetime in the impl Trait. The impl Trait can only capture lifetimes bound at the fn or impl level.

Resolving the E0657 Error

To fix Rust Error E0657, you need to define the lifetime at the function or impl level and use that lifetime in the impl Trait. Let's see how to do this using the previous example.

Here is a corrected version of the code:

#![allow(unused)]
fn main() {
trait Id<T> {}
trait Lt<'a> {}

impl<'a> Lt<'a> for () {}
impl<T> Id<T> for T {}

fn free_fn_capture_hrtb_in_impl_trait<'b>()
    -> Box<for<'a> Id<impl Lt<'b>>> // ok!
{
    Box::new(())
}

struct Foo;
impl Foo {
    fn impl_fn_capture_hrtb_in_impl_trait<'b>()
        -> Box<for<'a> Id<impl Lt<'b>>> // ok!
    {
        Box::new(())
    }
}
}

In the corrected example, we have defined the lifetime 'b at the function level, which allows us to use it in the impl Trait. This change resolves the E0657 error.

Conclusion

In this article, we have explored Rust Error E0657, which occurs when a lifetime bound on a trait implementation is captured at an incorrect place. By understanding the error and learning how to correctly define and use lifetimes in impl Traits, you can effectively resolve this error in your Rust code.