Rustc can't check constness of foreign globals

[LeoTestard]

The following code cause Rustc to fail:

extern
{
    static foo: u64;
}

static bar: u64 = foo;

fn main()
{
}

With the following message :

task '<unnamed>' failed at 'const not bound to an item', /Users/leo/Downloads/rust/src/librustc/middle/check_const.rs:269
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 to get further details and report the results to github.com/mozilla/rust/issues
task '<unnamed>' failed at 'explicit failure', /Users/leo/Downloads/rust/src/librustc/rustc.rs:391

Shouldn't Rustc be able to compile this code ? Since the foreign global is static, it should be possible to use it in constant expressions, juste like any static declared in Rust code.

[nikomatsakis]

We should fail in a more graceful way, but I suspect we should still
fail. Rust statics can be evaluated at compilation time, but C statics
generally cannot, they are just memory locations basically. I guess
it's plausible that if the C library were available we could inspect
it's read-only segment in some cases, but that sounds very
complicated.

[LeoTestard]

Yeah you're right.
And no, I don't think extract the value from the C library is a viable thing, the extern address could as well not be available until link-time (if it is declared in a linker script, or whatever).
I'm realizing this issue was stupid, foreign globals cannot be reliably evaluated at compile-time.
But still, the compiler should report a proper error message like "Impossible to use foreign global in a constant expression context" rather than failing.

[huonw]

Closing as a dupe of the more modern #14227 (since the compiler behaviour has changed).