ICE: cannot encode region variables, for trait object method calls.

[eddyb]

#![crate_type="lib"]
pub trait Foo<'a, T> {
    fn foo(&self);
}
pub fn foo<'a, T>(x: &'a Foo<'a, T>) {
    let x: &'a Foo<T> = x;
    //            ^ the lifetime parameter of Foo is left to be infered.
    x.foo();
    // ^ encoding this method call in metadata triggers an ICE. 
}

This is caused by writeback leaving around ReInfer in the MethodCallee associated with a method call (because MethodOrigin is not type-folded at all).
cc @nikomatsakis