empty crates depend on libcore

[gnzlbg]

This crate:

#![crate_type = "rlib"]
#![no_std]

implicitly contains an extern crate core even though it does not use anything from it (not even lang-items).

[jonas-schievink]

Not sure this is a bug - libcore is opt-out just like libstd is, you need #![no_core]

[steveklabnik]

Yes, this is expected behavior; see https://doc.rust-lang.org/stable/reference/crates-and-source-files.html#preludes-and-no_std

[gnzlbg]

Can't do that in stable Rust =/

[steveklabnik]

That's tracked by #29639

[gnzlbg]

The discussion on Discord suggested that this attribute was intended to be perma-unstable.