Rust unique pointer borrowing

struct A  {
	v	:~[int],
}

impl A	{
	fn foo(&mut self)	{
		//error: illegal borrow unless pure: unique value in aliasable, mutable location
		let pa = &mut self.v[1];
		//note: impure due to assigning to dereference of mutable & pointer
		*pa = 5;
	}
}

fn main()	{
	let mut a = A{v:~[1,2,3]};
	{	//that compiles fine
		let pb = &mut a.v[1];
		*pb = 6;
	}
	a.foo();
}