External enums are not constant

[thelink2012]

I'm not entirely sure this issue is by design of the language or really a bug, but it feels quite limiting.

When trying to use a enum variant from a external crate on a constant expression rustc fails telling it's not a constant expression although it works fine when it's in the same crate.

Example

Since it involves crates and such it's complicated to post the code only, so here's a project on which the issue can be reproduced easily by just running cargo build:
https://dl.dropboxusercontent.com/u/35476123/Releases/tmp/rust-exenum.tar
main_nope.rs tries to use a external enum in a constant expression and fails to compile.
main_works.rs tries to use a enum from the same crate in a constant expression and works.
You can switch between those two mains in Cargo.toml.

excrate/lib.rs

#[repr(i32)]
pub enum CEnum {
    Value1 = 1,
    Value2 = 2,
}

main_nope.rs

extern crate excrate;
use excrate::CEnum;

#[repr(i32)]
pub enum Enum {
    Value1 = CEnum::Value1 as i32,
    Value2 = CEnum::Value2 as i32,
}

fn main() {
}

main_works.rs

mod excrate {
    #[repr(i32)]
    pub enum CEnum {
        Value1 = 1,
        Value2 = 2,
    }
}
use excrate::CEnum;

#[repr(i32)]
pub enum Enum {
    Value1 = CEnum::Value1 as i32,
    Value2 = CEnum::Value2 as i32,
}

fn main() {
}

Meta

PS C:\Users\LINK\Desktop\rust-test> cargo build --verbose
   Compiling test v0.0.1 (file:///C:/Users/LINK/Desktop/rust-test)
     Running `rustc main_nope.rs --crate-name test --crate-type bin -g --out-dir C:\Users\LINK\Desktop\rust-test\target\
debug --emit=dep-info,link -L dependency=C:\Users\LINK\Desktop\rust-test\target\debug -L dependency=C:\Users\LINK\Deskto
p\rust-test\target\debug\deps --extern excrate=C:\Users\LINK\Desktop\rust-test\target\debug\deps\libexcrate-aed1a7b0d1e3
20a1.rlib`
       Fresh excrate v0.0.1 (file:///C:/Users/LINK/Desktop/rust-test)
main_nope.rs:6:14: 6:27 error: constant evaluation error: non-constant path in constant expr [E0080]
main_nope.rs:6     Value1 = CEnum::Value1 as i32,
                            ^~~~~~~~~~~~~
main_nope.rs:7:14: 7:27 error: constant evaluation error: non-constant path in constant expr [E0080]
main_nope.rs:7     Value2 = CEnum::Value2 as i32,
                            ^~~~~~~~~~~~~
error: aborting due to 2 previous errors
Could not compile `test`.

Caused by:
  Process didn't exit successfully: `rustc main_nope.rs --crate-name test --crate-type bin -g --out-dir C:\Users\LINK\De
sktop\rust-test\target\debug --emit=dep-info,link -L dependency=C:\Users\LINK\Desktop\rust-test\target\debug -L dependen
cy=C:\Users\LINK\Desktop\rust-test\target\debug\deps --extern excrate=C:\Users\LINK\Desktop\rust-test\target\debug\deps\
libexcrate-aed1a7b0d1e320a1.rlib` (exit code: 101)

PS C:\Users\LINK\Desktop\rust-test> rustc --version --verbose
rustc 1.0.0-beta (9854143cb 2015-04-02) (built 2015-04-02)
binary: rustc
commit-hash: 9854143cba679834bc4ef932858cd5303f015a0e
commit-date: 2015-04-02
build-date: 2015-04-02
host: i686-pc-windows-gnu
release: 1.0.0-beta

[ghost]

Related (a dupe, really) to #24106.

[Manishearth]

cc @oli-obk

[Mark-Simulacrum]

No longer ICEs today. Regression test was added for #24106, so closing.