The reason for this requirement is lifetime subtyping: https://doc.rust-lang.org/nomicon/subtyping.html https://doc.rust-lang.org/nomicon/subtyping.html Type p

The reason for this requirement is lifetime subtyping: https://doc.rust-lang.org/nomicon/subtyping.html https://doc.rust-lang.org/nomicon/subtyping.html Type parameter variance is inferred from usage (e.g. covariant for normal fields, contravariant for function arguments) and without a usage there's no way to infer it.