variable in macro output incorrectly reported as unused

[metajack]

In the code here:

macro_rules! handle_heading_element(
    ($tag:expr, $string:expr, $ctor:ident, $type_id:expr, $level:expr) => (
        if eq_slice($tag, $string) {
            let _element = ~HTMLHeadingElement {
                parent: Element::new($type_id, ($tag).to_str()),
                level: $level
            };
            unsafe {
                return Node::as_abstract_node(_element);
            }
        }
    )
)

(taken from jack's servo repo.)

The compiler will complain that element is unused unless I prefix it with _, even though it is clearly used shortly after.

[pnkfelix]

Nominating for milestone 5 (production-ready), as this is analogous to issue #4381 which was accepted for that milestone 3 weeks ago.

[graydon]

accepted for production-ready milestone

[catamorphism]

Bug triage. Milestone still looks fine.

[pnkfelix]

This need not block 1.0, since macro_rules are feature gated. Assigning P-low.

[pnkfelix]

(also suspected not-a-bug-anymore as fixed, but need to check...)

[huonw]

Seems to be fixed, e.g.

#![feature(macro_rules)]

struct Element;

macro_rules! foo {
    ($tag: expr, $string: expr) => {
        if $tag == $string {
            let element = box Element;
            unsafe {
                return std::mem::transmute::<_, uint>(element);
            }
        }
    }
}

fn bar() -> uint {
    foo!("a", "b");
    0
}

fn main() {
    bar();
}