Borrow checker should exploit information provided by match alternatives

[zwarich]

The following code gives a borrow checker error because the borrow of a in the result of a.foo() overlaps with the mutable borrow of a.bar():

struct A<S>;

impl<S> A<S> {
    fn foo<'a>(&'a self) -> Option<&'a uint> { None }
    fn bar(&mut self) { }
}

fn baz<S>(a: &mut A<S>) -> bool {
    match a.foo() {
        Some(i) => true,
        None => { a.bar(); false }
    }
}

The borrow checker should be able to recognize that the None alternative of matching an Option<&T> can't contain a borrowed pointer.

[huonw]

Thanks for filing, I believe this is covered by #6393 and/or #10390.