This feature wouldn't make a lot of sense in most cases in Rust. Rust has full type interference inside functions, so if we're inside the function body we can allow the type to be inferred, partially or entirely. For example Vec<_> says this is a Vec of something but we're not specifying what it's a Vec of.

In the function signature, Rust deliberately doesn't have inference, you must write down the types and decltype would not be acceptable for that purpose.