Rust no_std static lib panic_handler

I want to build a no_std static library with rust. I got the following:

[package]
name = "nostdlb"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["staticlib"]

[profile.dev]
panic = "abort"

[profile.release]
panic = "abort"

lib.rs:

#![no_std]

pub fn add(left: usize, right: usize) -> usize {
    left + right
}

Despite setting the panic behaviour for both dev and release to abort cargo gives the following error:

error: `#[panic_handler]` function required, but not found

error: could not compile `nostdlb` due to previous error

I thought the panic handler is only required when there is no stack unwinding provided by std?

No, you need to write your own one. If you need just aborting, consider using the following one:

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

I also highly recommend to unmangle the function and use C calling convention if you’re going to use this symbol somewhere (not import it the library as a crate, but link it manually):

#[no_mangle]
extern fn add(right: usize, left: usize) -> usize {
   right + left
}