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+`CoerceUnsized` was implemented on something that isn't a struct.
+
+Erroneous code example:
+
+```compile_fail,E0376
+#![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][1]
+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.
+
+[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
+
+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.