Compiling #![no_std] crates with --test gives unhelpful error message

[veddan]

test-error.rs:

#![no_std]

Compiling:

$> rustc --test test-error.rs 
test-error.rs:1:1: 1:4 error: unresolved import `std::slice::AsSlice`. Maybe a missing `extern crate std`?
test-error.rs:1 #![no_std]
                ^~~
error: aborting due to previous error

This isn't very helpful. Changing the file to

#![no_std]
extern crate core;
mod std { mod slice { pub use core::slice::AsSlice; } }

instead gives the error message

test-error.rs:1:1: 1:1 error: failed to resolve. Could not find `os` in `std`.
test-error.rs:1 #![no_std]
                ^
test-error.rs:1:1: 1:1 error: unresolved name `std::os::args`.
test-error.rs:1 #![no_std]
                ^
error: aborting due to 2 previous errors

The error always refers to the first line of the file, so if you have feature gates before #![no_std], the error will refer to them.

[steveklabnik]

This has been resolved: an empty crate with just ![no_std] compiles fine with --test.