Minimal `cc-rs` `#![no_std]` example

build.rs

fn main() {
    println!("cargo:rerun-if-changed=build.rs");
    println!("cargo:rerun-if-changed=foo.c");
    cc::Build::new().file("foo.c").compile("foo");
}

Cargo.toml

[package]
name = "example"
version = "0.1.0"
edition = "2021"
publish = false

[build-dependencies]
cc = "1.2.17"

[lib]
path = "lib.rs"

foo.c

int foo(void) {
    return 42;
}

lib.rs

#![no_std]
use core::ffi::c_int;

unsafe extern "C" {
    safe fn foo() -> c_int;
}

pub fn bar() {
    assert_eq!(foo(), 42);
}