Error code E0374

CoerceUnsized was implemented on a struct which does not contain a field with an unsized type.

Example of erroneous code:

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

struct Foo<T: ?Sized> {
    a: i32,
}

// error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
    where T: CoerceUnsized<U> {}
}

An unsized type is any type where the compiler does not know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

CoerceUnsized is used to coerce one struct containing an unsized type into another struct containing a different unsized type. If the struct doesn't have any fields of unsized types then you don't need explicit coercion to get the types you want. To fix this you can either not try to implement CoerceUnsized or you can add a field that is unsized to the struct.

Example:

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

// We don't need to impl `CoerceUnsized` here.
struct Foo {
    a: i32,
}

// We add the unsized type field to the struct.
struct Bar<T: ?Sized> {
    a: i32,
    b: T,
}

// The struct has an unsized field so we can implement
// `CoerceUnsized` for it.
impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
    where T: CoerceUnsized<U> {}
}

Note that CoerceUnsized is mainly used by smart pointers like Box, Rc and Arc to be able to mark that they can coerce unsized types that they are pointing at.