Tuple should probably be unsizable by last argument

[VFLashM]

Currently tuples can contain unsized types, but cannot be coerced.
It contrasts with behavior of tuple structs and probably should be changed.

I can see two options here:
a) Forbid tuples with unsized types completely.
b) Forbid tuples with unsized types on any but last position and allow DST coercions.

struct S;
trait T {}
impl T for S {}

struct Wrapper<TT: ?Sized>(i32, TT);

fn main() {
    let sw: Wrapper<S> = Wrapper(0, S);
    let tw: &Wrapper<T> = &sw; // unsizes just fine

    let st: (i32, S) = (0, S);
    let tt: &(i32, T) = &st; // fails to unsize, even though it could
}

// on a side note, why is it a legal type?
// as far as I can tell there is no way to get 
// an instance of this type right now
type Why = (T, i32);