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+`CoerceUnsized` was implemented on a struct which does not contain a field with
+an unsized type.
+
+Example of erroneous code:
+
+```compile_fail,E0374
+#![feature(coerce_unsized)]
+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][1] 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.
+
+[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+
+`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:
+
+```
+#![feature(coerce_unsized)]
+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.