ICE: 'Non-constant path in constant expr' when matching on enum constants

[brendanzab]

This could be related to #6533 and #6449

pub enum MouseButton {
    MOUSE_BUTTON_1,
    MOUSE_BUTTON_2,
    MOUSE_BUTTON_3,
    MOUSE_BUTTON_4,
    MOUSE_BUTTON_5,
    MOUSE_BUTTON_6,
    MOUSE_BUTTON_7,
    MOUSE_BUTTON_8,
}

pub static MOUSE_BUTTON_LEFT    : MouseButton = MOUSE_BUTTON_1;
pub static MOUSE_BUTTON_RIGHT   : MouseButton = MOUSE_BUTTON_2;
pub static MOUSE_BUTTON_MIDDLE  : MouseButton = MOUSE_BUTTON_3;

fn mouse_button_to_str(button: MouseButton) -> ~str {
    match button {
        MOUSE_BUTTON_LEFT     => ~"Left",
        MOUSE_BUTTON_RIGHT    => ~"Right",
        MOUSE_BUTTON_MIDDLE   => ~"Middle",
        MOUSE_BUTTON_4        => ~"4",
        MOUSE_BUTTON_5        => ~"5",
        MOUSE_BUTTON_6        => ~"6",
        MOUSE_BUTTON_7        => ~"7",
        MOUSE_BUTTON_8        => ~"8",
    }
}

fn main() {}

Error:

$ RUST_LOG=rustc=1,::rt::backtrace rustc enum-match-ice.rs
rust: task failed at 'Non-constant path in constant expr', /------/rust/src/librustc/middle/const_eval.rs:245
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further details and report the results to github.com/mozilla/rust/issues
rust: task failed at 'explicit failure', /------/rust/src/librustc/rustc.rc:355
rust: domain main @0x7ffc9a00ba10 root task failed

[graydon]

accepted for production-ready milestone

[metajack]

This no longer ICEs. Instead you get a reasonable error message:

$ rustc glfw.rs
glfw.rs:13:48: 13:62 error: Non-constant path in constant expr
glfw.rs:13 pub static MOUSE_BUTTON_RIGHT   : MouseButton = MOUSE_BUTTON_2;
                                                           ^~~~~~~~~~~~~~