Error code E0376

CoerceUnsized was implemented on something that isn’t a struct.

Erroneous code example:

#![feature(coerce_unsized)]
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> {}

CoerceUnsized can only be implemented for a struct. Unsized types are already able to be coerced without an implementation of CoerceUnsized whereas a struct containing an unsized type needs to know the unsized type field it’s containing is able to be coerced. An unsized type is any type that the compiler doesn’t know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

The CoerceUnsized trait takes a struct type. Make sure the type you are providing to CoerceUnsized is a struct with only the last field containing an unsized type.

Example:

#![feature(coerce_unsized)]
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> {}

Note that 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.

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