Stop ICE when referring to foreign static values in statics and consts

[ghost]

(I am new to Git and Github so I'm really sorry if I messed up somewhere)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @huonw (or someone else) soon.

[pcwalton]

I'd just remove this comment.

[pcwalton]

r=me with comment removed. Thanks!

[alexcrichton]

Could you also be sure to add a test case for this as well?

[ghost]

Updated. Is the test case ok?

[alexcrichton]

Thanks! Could you also squash the commits together as well?

[ghost]

Ok, I think that worked.

[thestinger]

Isn't it already impossible to refer to the value of a static in a constant expression?

[ghost]

It is. There is still an error when you do that, it's just more useful than an ICE.

E.g. compiling this code:

extern {
    static x: i32;
}

const X: i32 = x;

fn main() { }

before:

$ rustc main.rs
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'expected item, found foreign item x::x (id=5)', .../rust/src/libsyntax/ast_map/mod.rs:327

after:

$ rustc main.rs
main.rs:5:16: 5:17 error: cannot refer to other statics by value, use the address-of operator or a constant instead
main.rs:5 const X: i32 = x;
                         ^
main.rs:5:16: 5:17 error: constants cannot refer to other statics, insert an intermediate constant instead
main.rs:5 const X: i32 = x;
                         ^