Tracking issue for rust-lang/rfcs#66: better temporary lifetimes.

Some unresolved questions to be settled when implementing:

1. This implies that the lifetimes of temporaries is not known until after typeck. I think this is ok but it is a phase change which can sometimes be tricky (currently temporary lifetimes are known before typeck).
2. We have to specify the precise rules over when a temporary is extended. There are various subtle cases to be considered:

• Clearly we must consider whether the parameter type is a reference with a lifetime that also appears in the return type. Does the variance in the return type matter? (I think: no, too subtle and not worth it.)
• When do we decide what the type of the parameter is? Do we consider the declared type, the type after inference, or a hybrid?

Some examples where this matters:

fn identity<T>(x: T) -> T { x }

// Are these the same or different?

foo(&3);
foo::<&int>(&3);

My take: Probably we should just consider the fully inferred type.

Unresolved questions

• Resolve the concern about semantic changes described in comment.