The code:

def foo(x: 1):
    pass

Currently gives:

$ buildifier foo.bzl
foo.bzl:1:11: syntax error near :

That code is accepted by python3. I think it would be useful to support in buildifer in particular because types are an extension that some Starlark implementations support, much the same way that nested-def is a Starlark extension that is supported by buildifier.

If the above is accepted, I'd be happy to do a PR to enable it.