~Foo should auto-borrow to &Foo when casting to a trait object

[sfackler]

It'd be nice if this would work:

trait Foo {}
impl<'self> Foo for &'self str {}
let a = ~"";
let b = &a as &Foo;

You currently have to manually borrow

trait Foo {}
impl<'self> Foo for &'self str {}
let a = ~"";
let b: &str = a;
let b = &b as &Foo;

[nikomatsakis]

I feel like this is a dup but can't find the other issue. I agree; this shouldn't be too hard. cc me.

[jdm]

Dup of #10039.

[sfackler]

I don't think this is a dup of #10039. Auto-borrowing of a ~MyStruct to a &MyStruct seems distinct from auto casting &MyStruct to &SomeTrait.

[nikomatsakis]

Agreed, not a dup of that particular issue.

[alexcrichton]

Due to RFC 37, I don't think that this is as high priority any more, and it should go through the RFC process if so (it's a language change).