`.h` files are not something to emulate! External interfaces should be generated by tools where needed.
Edit: Like, seriously. Either the local var is populated by something coming in externally (which is then typable) or, unless your code is too complex / large, it should be easy to see everywhere it's used, and then why would you need that additional typing info?
A classic example of where I might have an inline type annotation in Rust is when I'm doing a non-trivial chain of Future/Result combinators in the middle of a function. It doesn't take much code for your understanding to desync from reality. Annotating "Result<String, IOError>" inline both documents to others what this intermediate value is but also creates better, local errors as the chain is modified.
Complex stuff does generally get factored out into functions, but at the same time, it's nice when you're the one who decides when it makes sense to extract code rather than a limitation of the typing syntax. Those things don't always line up.