A type T with #[must_use] does not imply that Pin<T> is #[must_use]. This means that there is no hint to .await a Pin<Box<dyn Future>>, which is one of the current workarounds for the lack of aysnc fn in traits. This is used by async-trait in its expansion, and in some crates that have traits that "ought to" have async fns.

See also #39524, which would propagate must_use "everywhere" but was rejected, and #62228, which special-cased must_use propagation through Box only.

cc @varkor, who indicated they would provide mentoring advice.