Example doesn't compile after adding MQTT dependency

[rkudryashov]

I'm trying to add support of publishing of MQTT messages to Arduino Uno firmware. There are several crates for this, for example,

https://github.com/00imvj00/mqttrs: mqttrs = { version = "0.4.1", default-features = false }
https://github.com/quartiq/minimq: minimq = "0.5.3"

Both crates depend on https://github.com/japaric/heapless

I add those dependencies to https://github.com/Rahix/avr-hal/blob/main/examples/arduino-uno/Cargo.toml

The error stack:

D:\avr-hal\examples\arduino-uno>cargo run --bin uno-blink
   Compiling heapless v0.7.13
error[E0432]: unresolved import `core::sync::atomic::AtomicUsize`
  --> C:\Users\User\.cargo\registry\src\github.com-1ecc6299db9ec823\heapless-0.7.13\src\pool\singleton\arc.rs:88:26
   |
88 | use core::sync::atomic::{AtomicUsize, Ordering};
   |                          ^^^^^^^^^^^ no `AtomicUsize` in `sync::atomic`

error[E0432]: unresolved import `core::sync::atomic::AtomicPtr`
  --> C:\Users\User\.cargo\registry\src\github.com-1ecc6299db9ec823\heapless-0.7.13\src\pool\llsc.rs:10:26
   |
10 | use core::sync::atomic::{AtomicPtr, Ordering};
   |                          ^^^^^^^^^
   |                          |
   |                          no `AtomicPtr` in `sync::atomic`
   |                          help: a similar name exists in the module: `AtomicI8`

error[E0432]: unresolved import `core::sync::atomic::AtomicUsize`
  --> C:\Users\User\.cargo\registry\src\github.com-1ecc6299db9ec823\heapless-0.7.13\src\spsc.rs:92:26
   |
92 | use core::sync::atomic::{AtomicUsize, Ordering};
   |                          ^^^^^^^^^^^ no `AtomicUsize` in `sync::atomic`

error[E0392]: parameter `T` is never used
   --> C:\Users\User\.cargo\registry\src\github.com-1ecc6299db9ec823\heapless-0.7.13\src\pool\mod.rs:263:17
    |
263 | pub struct Pool<T> {
    |                 ^ unused parameter
    |
    = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
    = help: if you intended `T` to be a const parameter, use `const T: usize` instead

Some errors have detailed explanations: E0392, E0432.
For more information about an error, try `rustc --explain E0392`.
error: could not compile `heapless` due to 4 previous errors

Do you have any thoughts on this? Is there any workable config or workaround?

[Rahix]

The heapless crate has a cas feature which is enabled by default. Maybe disabling that with

[dependencies.heapless]
version = "..."
default-features = false

is enough to get it working. But you might need to do this in the mqtt crate.

If the crates requires atomics from heapless, this won't work of course.

There is a hack but it is very error prone. In your target spec file (e.g. avr-atmega328p.json) you can set

"max-atomic-width": 16

Be warned: There is a high chance that this produces corrupt binaries.

The proper fix looks to be updating the atomic-polyfill dependeny of heapless. From what I can tell, the latest version of that has proper avr support and heapless is just lagging behind...

[rkudryashov]

Thanks! I've tried to bump atomic-polyfill version and disable default features locally. After that just one compilation error remained:

D:\avr-hal\examples\arduino-uno>cargo run --bin uno-blink
   Compiling heapless v0.7.13 (D:\heapless)
error[E0432]: unresolved import `core::sync::atomic::AtomicUsize`
  --> D:\heapless\src\spsc.rs:92:26
   |
92 | use core::sync::atomic::{AtomicUsize, Ordering};
   |                          ^^^^^^^^^^^ no `AtomicUsize` in `sync::atomic`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `heapless` due to previous error

Biut so far I didn't manage to make the example working.

Anyway, it seems the error has nothing to do with avr-hal or mqttrs.

[Rahix]

Agreed, I think this needs to be adressed in heapless.