`extern crate std;` in a `#![no_std]` crate confuses Miri unless it's in the root module

This program:

#![no_std]

fn main() {
    extern crate std;
}

Causes this error in Miri (playground):

error: unsupported operation: can't call foreign function: signal
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/unix/mod.rs:119:17
    |
119 |         assert!(signal(libc::SIGPIPE, libc::SIG_IGN) != libc::SIG_ERR);
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function: signal
    |
    = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
            
    = note: inside `main::std::sys::unix::init::reset_sigpipe` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/unix/mod.rs:119:17
    = note: inside `main::std::sys::unix::init` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/unix/mod.rs:62:5
    = note: inside `main::std::sys_common::rt::init` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/rt.rs:16:9
    = note: inside `main::std::rt::lang_start_internal` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:32:14
    = note: inside `main::std::rt::lang_start::<()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:48:5

But the program runs fine if extern crate std; is in the root module or the crate is not #![no_std].

I encountered this issue when I was trying to access std by using extern crate std; in a #[test] of a #![no_std] crate.

I guess that this is because the stack frame is main::std::... in this case, while this code (cc #1263) only checks for std::...:

miri/src/shims/posix/foreign_items.rs

Line 504 in 453affa

if this.frame().instance.to_string().starts_with("std::sys::unix::") => {

[RalfJung]

I guess that this is because the stack frame is main::std::... in this case, while this code (cc #1263) only checks for std::...:

Yes, I am pretty sure that's it.
How common is it to have extern crate not at the root module...?

[oli-obk]

How common is it to have extern crate not at the root module...?

serde does this a lot

ideally we'd use lang/diagonstic items