Methods with `&mut Trait` and polymorphic objects can't get encoded

[alexcrichton]

This code

#[crate_type = "lib"];
#[link(name = "foo", vers = "0.1")];

use std::cast;

trait A {}
struct B;
impl A for B {}

fn bar<T>(_: &mut A, _: &T) {}

fn foo<T>(t: &T) {
    let b = B;
    bar(unsafe { cast::transmute(&b as &A) }, t)
}

Dies with

error: internal compiler error: Cannot encode region variables

The transmute is in place to get around #8248, but I think that this may be uncovering a separate issue. Even with the transmute, I think this should compile anyway (although it's still unsafe code).

cc @msullivan

[alexcrichton]

Also cc @nikomatsakis

[alexcrichton]

Some similar code giving the same error, but with less stuff going on:

#[crate_type = "lib"];
#[link(name = "foo", vers = "0.1")];

use std::cast;

trait A {}
struct B;
impl A for B {}

pub fn bar<T>(_: &A) {}

fn foo<T>(b: &A) {
    let b: &A = unsafe { cast::transmute(b) };
    bar::<T>(b)
}

[alexcrichton]

Yay, this works now!

Flagging as needstest