Inconsistency between destructuring assignment and pattern matching with tuples

[bstrie]

This code fails to compile:

fn main() {
    let bar = ~(1,2);
    foo(bar);
}

fn foo<T,U>(x: &(T,U)) {
    let (ref a, ref b) = *x;  // error: moving out of dereference of immutable & pointer
}

...but this (seemingly) logically equivalent does compile:

fn main() {
    let bar = ~(1,2);
    foo(bar);
}

fn foo<T,U>(x: &(T,U)) {
    let (a, b) = match *x {
        (ref a, ref b) => (a, b)
    };
}

[nikomatsakis]

Dup of #4384