improve `this function takes * arguments` error message

[0rphon]

when writing a function call, its very helpful to be able to see what types the function takes. rust does this great when you hover over a completed function, but unfortunately, if you dont have all the arguments already in place rust just gives you a vague this function takes * arguments but * arguments were supplied error instead of telling you what types the function actually needs. considering this situation is the most useful time to see a functions input, it would be great if the error showed the function signature/told you what types are needed

[0rphon]

edit: it manages to do this with some functions, but others it refuses to show me the signature. perhaps this is a bug?