The thing is, with current tooling, the current design means that I can write a function, type a key combination, and have the type signature printed in a buffer (where I can then copy it into place). And until I do that, there is warning highlighting on the function.

Now in some ways this may seem silly - are you really going to understand code without understanding types? But especially for people new to the language, _or_ when you're dealing with new libraries (if you've ever written a little wrapper around a function from a complicated library, you know what I mean), it's nice to choose whether you want to work from values or from types (where undefined is your friend).

Which isn't to say that think rust's decision is bad, just that having flexibility makes this kind of tooling easier (and I'm assuming here that in all cases, the end result will be all annotated top level functions). And, part of rust's choice was probably to make type checking easier, which is an important thing (especially given how sophisticated the borrow checker is).