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Rust Error E0377: Implementing CoerceUnsized trait for different structures

Understanding Rust Error E0377

Rust Error E0377 occurs when the CoerceUnsized trait is implemented for two different structures. This error is common when attempting to coerce between structures with different definitions, even if they have the same structure. The CoerceUnsized trait is used for dynamically sized types (DST) coerce and it's crucial for the types to have the same definition.

An Erroneous Example

Here's an example that triggers Rust Error E0377:

#![allow(unused)]
#![feature(coerce_unsized)]
fn main() {
use std::ops::CoerceUnsized;

pub struct Foo<T: ?Sized> {
    field_with_unsized_type: T,
}

pub struct Bar<T: ?Sized> {
    field_with_unsized_type: T,
}

// error: the trait `CoerceUnsized` may only be implemented for a coercion
//        between structures with the same definition
impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {}
}

In this example, even though the Foo and Bar structures are structurally identical, they are not considered the same type, which results in the E0377 error.

The Solution

To fix Rust Error E0377, you should ensure that you only implement the CoerceUnsized trait for types with the same definition. Your impl signature should look like:

impl CoerceUnsized<Type<U>> for Type<T> where T: CoerceUnsized<U>;

Here's the corrected example:

#![allow(unused)]
#![feature(coerce_unsized)]
fn main() {
use std::ops::CoerceUnsized;

pub struct Foo<T: ?Sized> {
    field_with_unsized_type: T,
}

// Implementation of CoerceUnsized for Foo
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
}

In this modification, the CoerceUnsized trait is implemented for the same type (Foo), resolving the E0377 error.

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