Error with cast in enum definition.

[Ms2ger]

The following code hits an error:

#![allow(dead_code)]

#[repr(u32)]
enum ValueTag {
    INT32                = 0x1FFF0u32 | (0x01 as u32),
    X,
}

#[repr(u64)]
enum ValueShiftedTag {
    INT32        = ValueTag::INT32 as u64,
    X,
}

fn main() {
    println!("{}", ValueTag::INT32 as u32);
}

/tmp/a.rs:7:42: 7:53 error: constant evaluation error: can't cast this type [E0080]
/tmp/a.rs:7     INT32                = 0x1FFF0u32 | (0x01 as u32),
                                                     ^~~~~~~~~~~

Note that the error doesn't reproduce without the ValueShiftedTag definition.

This blocks the current rust upgrade for Servo.

CC @eddyb.

[mahkoh]

Minimal:

enum X { A = 0 as isize }

enum Y { A = X::A as isize }

fn main() { }

Workaround:

#[repr(u32)] enum X { A = 0u32 | 1u32, B }

#[repr(u64)] enum Y { A = X::A as u64, B }

fn main() { }

[Ms2ger]

Do you also have a workaround for this:

#![allow(dead_code)]

#[repr(u8)]
#[allow(dead_code)]
enum ValueType {
    DOUBLE              = 0x00,
    INT32               = 0x01,
}

#[repr(u32)]
enum ValueTag {
    INT32                = 0x1FFF0u32 | (ValueType::INT32 as u32),
    X,
}

#[repr(u64)]
enum ValueShiftedTag {
    INT32        = ValueTag::INT32 as u64,
    X,
}

fn main() {
    println!("{}", ValueTag::INT32 as u32);
}

?

[SimonSapin]

This doesn’t look like a ICE when I reproduce this.

[mahkoh]

Do you also have a workaround for this:

Nope. Problem seems to come from using as too much.

[SimonSapin]

I’m still not sure what was the root cause, but I worked around this by replacing these enums with a bunch of consts: servo/rust-mozjs#137

Feel free to close.

[ahwatts]

It looks like this happens if it needs to recurse more than once to evaluate a constant, i.e, in order to evaluate PieALaMode, it needs to evaluate ApplePie, which in turn requires it to evaluate Pie and Apple.

#[derive(Debug)]
enum Delicious {
    Pie      =  0x1,
    Apple    =  0x2,
    IceCream = 0x40,
    Vanilla  = 0x80,
    ApplePie = Delicious::Apple as isize | Delicious::Pie as isize,
    PieALaMode = Delicious::ApplePie as isize | Delicious::IceCream as isize | Delicious::Vanilla as isize,
}

fn main() {
    println!("Apple Pie = {:?} ({:?})", Delicious::ApplePie, Delicious::ApplePie as isize);
    println!("Pie à la Mode = {:?} ({:?})", Delicious::PieALaMode, Delicious::PieALaMode as isize);
}

(Sorry, I apparently need to go eat dinner. And dessert.)

[SimonSapin]

Also note that this code used to build with earlier compiler versions.

[frewsxcv]

Update: @Ms2ger's code he pasted currently compiles on nightly. It does not compile on beta or stable.

Playpen link

I'll add a regression test to close this, unless I hear otherwise