Rust: no_std application

#![no_std] is a crate-level attribute that indicates that the crate will link to the core-crate instead of the std-crate. The libcore crate in turn is a platform-agnostic subset of the std crate which makes no assumptions about the system the program will run on. As such, it provides APIs for language primitives like floats, strings and slices, as well as APIs that expose processor features like atomic operations and SIMD instructions.

Project

> cargo new shared --lib --vcs none
> cargo new program --bin --vcs none

> touch Cargo.toml .cargo/config.toml
> touch shared/src/_start.s
# Cargo.toml

[workspace]
members = ["shared", "launcher"]
resolver = "2"

[profile.dev]
panic = "abort"
debug = true
incremental = true
lto = false
opt-level = 0

[profile.release]
panic = "abort"
debug = false
incremental = false
lto = true
opt-level = 3
codegen-units = 1
strip = "symbols"
# .config/config.toml

[target.'cfg(target_family="unix")']
rustflags=[
    "-Cdefault-linker-libraries=off",
    "-Clink-arg=-nostdlib"
]

[target.'cfg(target_family="windows")']
rustflags=[
    "-Clink-arg=/NODEFAULTLIB",
    "-Clink-arg=/SUBSYSTEM:WINDOWS",
    "-Clink-arg=/ENTRY:_start"
]
; shared/src/_start.s

.global _start

.text
    _start:
        xor     rbp, rbp
        call    start
        mov     edi, eax
        mov     eax, 0xe7
        syscall
        hlt

Shared

# shared/lib.rs

#![no_std]

use core::arch::global_asm;
use core::panic::PanicInfo;

#[cfg(all(target_arch="x86_64", target_os="linux"))]
global_asm!(include_str!("_start.s"));

#[panic_handler]
fn panic(_panic_info: &PanicInfo) -> ! {
    loop {}
}

#[no_mangle]
pub extern "C" fn start() -> i32 {
    0
}

pub fn foo() -> i32 {
    256
}

Program

# program/Cargo.toml

[dependencies]
shared = { path = "../shared" }
# program/main.rs

#![no_std]
#![no_main]

use shared;

#[no_mangle]
extern fn main() -> i32 {
    shared::foo()
}

Build

> cargo build

Compiling shared v0.1.0 (/media/dev/project/shared)
Compiling launcher v0.1.0 (/media/dev/project/program)

Finished dev [unoptimized + debuginfo] target(s) in 0.11s