Type-directed method resolution does not work through pointers

[Dretch]

I think that the following code should work, but it does not compile:

use io::WriterUtil;

struct NotAWriter {x: int}

pub impl NotAWriter {
    fn write_be_u32(y: u32) {}
}

fn main() {
    let x = &(NotAWriter {x: 1});
    x.write_be_u32(0);
}

The error from rustc is:

test-method-scope.rs:15:4: 15:22 error: failed to find an implementation of trait @core::io::Writer for <V2>
test-method-scope.rs:15     x.write_be_u32(0);
                            ^~~~~~~~~~~~~~~~~~

I don't think there should be an error here. NotAWriter is not a Writer, it just shares a method name with WriterUtil (which is defined for all Writers).

If x is not a pointer then rustc works fine, i.e.:

let x = NotAWriter {x: 1};
x.write_be_u32(0);

[nikomatsakis]

The problem here is that, currently, method resolution does not attempt to do full trait resolution. That is, write_be_u32 is defined on the WriterUtil trait which has an impl for all W where W:Writer. So method resolution unifies W with NotAWriter but leaves the actual resolution of whether NotAWriter implements Writer for a later step. This is sometimes good (because all types may not yet be known) but sometimes bad.

[nikomatsakis]

Not critical for 0.6; removing milestone

[nikomatsakis]

cc #5527

[catamorphism]

Reproduced with 0252c30 . Nominating for milestone 5, production-ready.

[emberian]

Reproduces with

use std::io::WriterUtil;

struct NotAWriter {x: int}

impl NotAWriter {
    fn write_be_u32(y: u32) {}
}

fn main() {
    let x = &(NotAWriter {x: 1});
    x.write_be_u32(0);
}

I think this is #5898. @nikomatsakis?

[graydon]

accepted for well-defined milestone

[pcwalton]

I don't believe this is backwards incompatible. Nominating.

[pnkfelix]

Not a 1.0 issue; now P-high. Possibly linked / solvable via trait reform (#5527)

[steveklabnik]

Importing old_io::Writer rather than WriterUtil, since that's where this method lives today, we get:

hello.rs:11:15: 11:30 error: type `&NotAWriter` does not implement any method in scope named `write_be_u32`
hello.rs:11             x.write_be_u32(0);
                          ^~~~~~~~~~~~~~~

Seems good!