Atomic::<u32>::is_lock_free() always returns false for ESP32

[usbalbin]

Hi!

I am experimenting with atomic-rs on an embedded system with an ESP32 mcu using the esp-idf-hal crate with project generated from esp-idf-template.

However I am getting some unexpected results from Atomic::is_lock_free which seems to return false no matter what I throw at it.

Having a peek at how the cfgs are generated i understand it as the build script looking for core::sync::atomic::AtomicU32::compare_exchange being available(for has_atomic_u32). That does work fine in my manuall test below and AtomicU32 is otherwise available and usable.

use esp_idf_sys as _; // If using the `binstart` feature of `esp-idf-sys`, always keep this module imported

fn main() {
    esp_idf_sys::link_patches(); // Added by esp-rs/esp-idf-template

    std::sync::atomic::AtomicU32::compare_exchange; // Works
    core::sync::atomic::AtomicU32::compare_exchange;// Works

    const _: () = assert!(core::mem::align_of::<u32>() == 4); // Works
    const _: () = assert!(core::mem::size_of::<u32>() == 4); // Works

    // Copy pasted from atomic-rs and commented out cfgs
    pub const fn atomic_is_lock_free<T>() -> bool {
        let size = core::mem::size_of::<T>();
        let align = core::mem::align_of::<T>();
    
        (/*cfg!(has_atomic_u8) & */ (size == 1) & (align >= 1))
            | (/*cfg!(has_atomic_u16) &*/ (size == 2) & (align >= 2))
            | (/*cfg!(has_atomic_u32) &*/ (size == 4) & (align >= 4))
            | (/*cfg!(has_atomic_u64) &*/ (size == 8) & (align >= 8))
            | (/*cfg!(has_atomic_u128) &*/ (size == 16) & (align >= 16))
    }

    const _: () = assert!(atomic_is_lock_free::<u32>()); // Works fine

    const _: () = assert!(atomic::Atomic::<u8>::is_lock_free()); // Does not work
    const _: () = assert!(atomic::Atomic::<u16>::is_lock_free());// Does not work
    const _: () = assert!(atomic::Atomic::<u32>::is_lock_free());// Does not work
    const _: () = assert!(atomic::Atomic::<u64>::is_lock_free());// Does not work
    
    println!("Hello, world!");
}

I do have

[unstable]
build-std = ["core"]

in my project. Could this be cuviper/autocfg#34 ? Is there any solution/work around :) My intention is to use this with my own enum type

[Amanieu]

cfg(target_has_atomic = "N") is now stable (since Rust 1.60 I believe), so we could switch to using that instead of autocfg-based detection. A PR would be welcome!

[usbalbin]

Great, thanks! I will have a look. No guarantees though :)