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+`CoerceUnsized` was implemented on a struct which contains more than one field
+with an unsized type.
+
+Erroneous code example:
+
+```compile_fail,E0375
+#![feature(coerce_unsized)]
+use std::ops::CoerceUnsized;
+
+struct Foo<T: ?Sized, U: ?Sized> {
+    a: i32,
+    b: T,
+    c: U,
+}
+
+// error: Struct `Foo` has more than one unsized field.
+impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
+```
+
+A struct with more than one field containing an unsized type cannot implement
+`CoerceUnsized`. This only occurs when you are trying to coerce one of the
+types in your struct to another type in the struct. In this case we try to
+impl `CoerceUnsized` from `T` to `U` which are both types that the struct
+takes. 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.
+
+`CoerceUnsized` only allows for coercion from a structure with a single
+unsized type field to another struct with a single unsized type field.
+In fact Rust only allows for a struct to have one unsized type in a struct
+and that unsized type must be the last field in the struct. So having two
+unsized types in a single struct is not allowed by the compiler. To fix this
+use only one field containing an unsized type in the struct and then use
+multiple structs to manage each unsized type field you need.
+
+Example:
+
+```
+#![feature(coerce_unsized)]
+use std::ops::CoerceUnsized;
+
+struct Foo<T: ?Sized> {
+    a: i32,
+    b: T,
+}
+
+impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
+    where T: CoerceUnsized<U> {}
+
+fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
+    Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
+}
+```
+
+[1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait