Compiler should offer suggestions from other implemented traits when methods aren't found

[chris-morgan]

Explanation of the problem

Here is some sample code:

struct Foo;

impl Foo {
    fn foo(&mut self, buf: &[u8]) {
        self.write(buf);
    }
}

impl std::rt::io::Writer for Foo {
    fn write(&mut self, buf: &[u8]) {
    }

    fn flush(&mut self) {
    }
}

fn main() {
}

Expected (by me) behaviour: code should compile happily.

Actual behaviour: well, here is the compiler's opinion of this code:

foo.rs:5:8: 5:24 error: type `&mut Foo` does not implement any method in scope named `write`
foo.rs:5         self.write(buf);
                 ^~~~~~~~~~~~~~~~
error: aborting due to previous error

First thought: huh? But I implemented that method just there, see?

If I add use std::rt::io::Writer; to the top of the file, it will work. This is, as I now realise, because only traits used in the current scope are searched for a matching method; this is a necessary evil, lest such things as conflicts arise, yet it need not be this evil, where it gives no real indication of the problem (sure, the "in scope" bit hints obliquely at the problem, but not at all clearly).

Now, what should be done

In such a case, where a method is not found in the current scope, the compiler should scan all impl blocks that it knows about for that type and see if it can find any that matches the description, and then suggest them (hopefully it).

For example, the error message might then read

error: type &mut Foo does not implement any method in scope named write; you probably want to use std::rt::io::Writer;

[thestinger]

Triage bump, this still hasn't really been investigated as a way of improving the error messages.

[michaelsproul]

I had a crack at implementing Chris' suggestions as my first journey into the compiler. I failed, but I'm fairly sure the current arguments to check_method_call don't contain enough information to allow suggestions to be made. Someone with better knowledge of the compiler might be able to suggest where we could find the inactive/unimported implementations?

[nrc]

cc me

[nikomatsakis]

yes I've been wanting to do this, I would be happy to mentor.