Trait object coercion without `as`

[alexcrichton]

When creating a trait object, I shouldn't have to write &mut obj as &mut Trait, I should be able to just write &mut obj and the compiler will coerce it to &mut Trait.

I remember awhile ago that @pcwalton had a patch to do this, but then it fell by the wayside. I was under the impression that we want to get this done, so I'm nominating this for 1.0.

[nikomatsakis]

I really want this. I tried to port the visitor code to use objects and it was horribly tedious to insert the manual coercions. I would be happy to work on it once I get some of my current branches cleared off, but also happy to review anyone else's efforts.

[catamorphism]

High priority but not 1.0

[wycats]

👍 I found this pretty surprising.

Just improving the error message would go a long way here.

[tomdale]

I'd like to give my 👍 to this.

Running into this was confusing as a first-time Rust user, and the code we ended needing to write up became very obtuse. I'm pretty sure I couldn't have figured it out with nagging people in the IRC channel. It makes code that should be straightforward even more annoying if you want to re-use the coerced object in multiple places by pulling it into a variable:

// Where OutputWriter is a struct and OutputStream is a trait it implements
let mut writer = OutputWriter::new(counter.bytecount);
let &mut writer_stream = &mut writer as &mut OutputStream;

do_something_with_writer_stream(writer_stream);

// Calling writer.bytes() moves the original writer out, so
// the compiler complains because there is still a mutable loan outstanding.
writer.bytes();