A proposal for how to enable the use constants with pattern matching

proposal.md

At the moment one of the limitations of pattern matching is the inability to use variables in the scope of the match.

pcwalton suggests using const:

const origin: Point = Point { x: 0, y: 0 };

let foo = Point { x: 12, y: 42 }

match p {
    const origin          => io::println(~"the point lies at the origin"),
    Point { origin.x, y } => io::println(fmt!("the point lies on the x axis, at y=%?", y)),
    Point { x, origin.y } => io::println(fmt!("the point lies on the y axis, at x=%?", x)),
    const foo             => io::println(~"the point is at the point \"foo\""),
    Point { x, y }        => io::println(fmt!("The point is at (%?, %?)", x, y))
}

nmatsakis suggests using the == operator:

const origin: Point = Point { x: 0, y: 0 };

let foo = Point { x: 12, y: 42 }

match p {
    == origin             => io::println(~"the point lies at the origin"),
    Point { origin.x, y } => io::println(fmt!("the point lies on the x axis, at y=%?", y)),
    Point { x, origin.y } => io::println(fmt!("the point lies on the y axis, at x=%?", x)),
    == foo                => io::println(~"the point is at the point \"foo\""),
    Point { x, y }        => io::println(fmt!("The point is at (%?, %?)", x, y))
}

How about something that looks like capture clauses:

match p |origin, foo| {
    origin                => io::println(~"the point lies at the origin"),
    Point { origin.x, y } => io::println(fmt!("the point lies on the x axis, at y=%?", y)),
    Point { x, origin.y } => io::println(fmt!("the point lies on the y axis, at x=%?", x)),
    foo                   => io::println(~"the point is at the point \"foo\""),
    Point { x, y }        => io::println(fmt!("The point is at (%?, %?)", x, y))
}