Type-directed method resolution does not work through pointers

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);

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.

Not critical for 0.6; removing milestone

cc #5527

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

@nikomatsakis

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?

accepted for well-defined milestone

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

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

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!

Adds query param for version no (rust-lang#4270)

Merge pull request rust-lang#4270 from rust-lang/rustup-2025-04-12

Merge rust-lang#4270

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Type-directed method resolution does not work through pointers #4270

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Type-directed method resolution does not work through pointers #4270

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nikomatsakis commented on Mar 24, 2013

nikomatsakis commented on Mar 24, 2013

nikomatsakis commented on Mar 24, 2013

catamorphism commented on May 23, 2013

emberian commented on Jul 21, 2013

graydon commented on Aug 1, 2013

pcwalton commented on Jun 26, 2014

pnkfelix commented on Jun 26, 2014

steveklabnik commented on Feb 11, 2015

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