https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions

Additionally, a type Foo<T> can implement CoerceUnsized<Foo<U>> when T implements Unsize<U> or CoerceUnsized<Foo<U>>. This allows it to provide a unsized coercion to Foo<U>.

However, the document of CoerceUnsized regarding the corresponding case says:

you can directly implement CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>