Doesn't need type annotations. Annotate as much or as little as you'd like. The types are inferred by the language (and, again, are guaranteed correct)
How is this achieved? E.g. you npm install module ‘foo’ which has no types and takes no part in a rescript compilation process. Does it force you to type-narrow all the values returned from foo.funcname()?
E.g., if a type returned from a function is nullable then you will be forced to handle that explicitly via pattern matching I imagine, rather than being able to dereference it without checking.
Take Rust, calls to C code are inherently unsafe, and must be marked as so. You're leaving Rust's source code and type system and calling into external functions that you may not even have access to source code for and that the compiler just can't feasibly vouch for.