Understanding Rust Error E0376: CoerceUnsized Implementation on Non-struct Types

Rust Error E0376: CoerceUnsized Implementation on Non-struct Types

This error occurs when the CoerceUnsized trait is implemented for a type that is not a struct. To resolve this issue, ensure that the type provided to CoerceUnsized is a struct with only the last field containing an unsized type.

Understanding Unsized Types

Unsized types are types whose length and alignment are unknown at compile time. Any struct containing an unsized type is also unsized. In Rust, structs can only contain an unsized type if the field containing the unsized type is the last and only unsized type field in the struct.

Erroneous Code Example

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

struct Foo<T: ?Sized> {
    a: T,
}

// error: The type `U` is not a struct
impl<T, U> CoerceUnsized<U> for Foo<T> {}
}

In this example, the error occurs because the CoerceUnsized trait is implemented for a type `U` that is not a struct.

Corrected Code Example

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

struct Foo<T> {
    a: T,
}

// The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
}

In this example, the error is resolved by implementing the CoerceUnsized trait for a struct `Foo<U>`.

Working with CoerceUnsized