using extern-prelude does not allow use of std/core

[nikomatsakis]

Example (play):

#![feature(extern_prelude)]

mod foo {
    pub fn test() {
        let x = std::cmp::min(2, 3);
        assert_eq!(x, 2);
    }
}

fn main() {
    foo::test();
}

I would expect this to work. (I presume it doesn't because cargo doesn't pass --extern std.)

cc @petrochenkov @Manishearth

[petrochenkov]

std is not passed via --extern, so it can't work through extern prelude.

It needs a separate (library!) addition - pub use crate as std; in libstd prelude (which unfortunately doesn't work right now due to some bug I haven't had time to investigate yet).

[nikomatsakis]

@petrochenkov

can't work through extern prelude

Can't is a strong word. =) Linking in std is "well known" to rustc so we certainly could make it work, I would imagine. However, I don't really care how we implement it, I just want the example to work. =)

[Manishearth]

We should just insert std in extern-prelude if the crate is not no-std? (and something similar for core)

[mbrubeck]

Similarly core should work if the crate is no_std (unless it is also no_core).

CC #44660

[petrochenkov]

I think we can do this in the compiler temporarily and then move to the library prelude once bugs are fixed.