error message for @S as @T cast is unclear

[pnkfelix]

bar.rs:

use core::io::ReaderUtil;
use core::io::Reader;

fn bar(r:@ReaderUtil) -> ~str { r.read_line() }

fn main() {
    let r : @Reader = io::stdin();
    let _m = bar(r as @ReaderUtil);
}

rustc error message:

/tmp/baz.rs:8:17: 8:33 error: can only cast an @-pointer to an @-object, not a trait core::io::Reader
/tmp/baz.rs:8     let _m = bar(r as @ReaderUtil);
                               ^~~~~~~~~~~~~~~~

I read the above, I say "um... r is an @-pointer, not just an unadorned Reader instance..."

(The poor quality of the error message may or may not be a side-effect of the fact that there is an impl Reader for @Reader implementation in io.rs that might be causing the type checker to first treat r as a Reader before passing it to bar; I am not sure. Either way, the message is confusing.)

[catamorphism]

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

[graydon]

sub-bug of #6308

[pnkfelix]

Note that the phrase "cast an @-pointer to an @-object" has two parses in English, which I suspect is part of my confusion:

• (cast (an @-pointer) to an @-object)
• (cast an @-(pointer to an @-object))

[Aatch]

Actually, this more related to the inability to cast between trait objects.

You don't actually have a @-pointer, you have a trait object.

[graydon]

accepted for production-ready milestone

[pnkfelix]

Assigning P-low. (also we may be able to just close this; I need to look at the current state of the various error messages.)

[alexcrichton]

This still exists today:

use std::io;

fn bar(r:~Reader) { }

fn main() {
    let r : ~Reader = ~io::stdin();
    let _m = bar(r as ~Reader);
}

foo.rs:7:18: 7:30 error: can only cast an ~-pointer to a ~-object, not a trait std::io::Reader
foo.rs:7     let _m = bar(r as ~Reader);
                          ^~~~~~~~~~~~
error: aborting due to previous error

[arielb1]

In 0.12 it gives a better error message:

use std::io;

fn bar(r:Box<Reader>) { }

fn main() {
    let r : Box<Reader> = box io::stdin();
    let _m = bar(r as Box<Reader>);
}

gives

<anon>:8:18: 8:34 error: the trait `std::io::Reader` is not implemented for the type `std::io::Reader`
<anon>:8     let _m = bar(r as Box<Reader>);
                          ^~~~~~~~~~~~~~~~
<anon>:8:18: 8:34 note: the trait `std::io::Reader` must be implemented for the cast to the object type `std::io::Reader`
<anon>:8     let _m = bar(r as Box<Reader>);
                          ^~~~~~~~~~~~~~~~
error: aborting due to previous error

through the identity cast should always be allowed, the error-messages are now good.