By-value self methods on trait objects have bad error messages

[alexcrichton]

If I have ownership of a trait object, I would expect to be able to call a by-value self method defined on the trait. For example:

trait A { fn foo(self) {} }    

fn foo(a: ~A) {                
    a.foo();                   
}    
struct B;                      
impl A for B {}                

fn main() {                    
    foo(~B as ~A);             
}

currently yields the error of:

foo.rs:4:4: 4:12 error: type `~A:Send` does not implement any method in scope named `foo`
foo.rs:4     a.foo();
             ^~~~~~~~
error: aborting due to previous error

Nominating for the backcompat-lang 1.0 milestone.

[thestinger]

This would be difficult because the size of the type stored in the trait is unknown at compile-time. A by-value self parameter is currently implement poorly via indirection, but that shouldn't be relied upon. I don't think this should be attempted before self is passed properly to avoid blocking fixing that issue.

[catamorphism]

The bug is to improve the error message.

[catamorphism]

Not 1.0

[nikomatsakis]

I don't think it is generally possible to codegen this. We should just give a better error msg.

[alexcrichton]

Updating title to no longer reflect that this is a bug in that it should work, but rather it is a bug in that it should have a better error message.

[alexcrichton]

I just realized that you can indeed write fn foo(~self) {} on the trait, so perhaps the error message could include something along the lines of that. It's the only real way to have ownership of a trait object anyway.

[alexcrichton]

Closing in favor of #10672