Trait object coercion to borrowed trait does not work with methods on `&self`

[Kimundi]

This might not be a bug, but while writing extension traits for a trait object I discovered that calling a &self method of a trait implemented on a &Trait does not work if it needs to borrow from a ~Trait or @Trait first. This complicates things, because you need to explicit borrow to a &Trait prior to making the call.

As comparison, these three combinations work:

trait Bar{} 
impl Bar for uint {} 

// self by value
trait Foo { fn baz(self); }  
impl<'self> Foo for &'self Bar { fn baz(self) { println("baz"); } }; 

// borrowed trait object
let x = &5u as &Bar; 

x.baz();
> baz

trait Bar{}
impl Bar for uint {}

// self by value
trait Foo {fn baz(self);}
impl<'self> Foo for &'self Bar { fn baz(self) { println("baz"); } };

// owned trait object
let x = ~5u as ~Bar; 

x.baz();
> baz

trait Bar{}
impl Bar for uint {}

// self by reference
trait Foo {fn baz(&self);} 
impl<'self> Foo for &'self Bar { fn baz(&self) { println("baz"); } };

// borrowed trait object
let x = &5u as &Bar; 

x.baz();
> baz

While this one fails to compile:

trait Bar{}
impl Bar for uint {} 

// self by reference
trait Foo {fn baz(&self);} 
impl<'self> Foo for &'self Bar { fn baz(&self) { println("baz"); } };

// owned trait object
let x = ~5u as ~Bar; 

x.baz();
> error: type `~main::Bar:Send` does not implement any method in scope named `baz`

And this one is the workaround necessary to make it work:

trait Bar{}
impl Bar for uint {} 

// self by reference
trait Foo {fn baz(&self);} 
impl<'self> Foo for &'self Bar { fn baz(&self) { println("baz"); } };

// owned trait object
let x = ~5u as ~Bar; 

{let tmp: &Bar = x; tmp}.baz();
> baz

[nikomatsakis]

I think this will improve with DST, in that it would be possible to implement:

impl<'self> Foo for Bar { ... }

and then the required type of self would not be & & Bar but rather &Bar. I believe this would address the issue. Incidentally, the reason that this works for slices is that we added a special hack to address this scenario -- we could do the same for traits but I'd rather do DST.

[nikomatsakis]

cc #6308

[reem]

Triage: No change. Third example still fails to compile in the same way, this is symptomatic of larger issues around trait objects until DST lands.

[steveklabnik]

Triage: I am bad at DST, and so couldn't update the code sample. What's the state of this now?

[steveklabnik]

Triage: with no response in a year, and no real comments since 2013, and with tons of changes, I'm just going to close this one.