Failure to compile with latest `embassy-sync`

[wmmc88]

After embassy-rs/embassy#4320 merged, several bugs in ODP were exposed around improper Sync bounds. One of the major errors is IntrusiveList being used everywhere in OnceLock's when IntrusiveList is !Sync

error[E0277]: `core::cell::Cell<core::option::Option<&'static intrusive_list::IntrusiveNode>>` cannot be shared between threads safely         
   --> D:\.tools\.cargo\git\checkouts\embedded-services-78faf442f1f643c2\a938ac1\embedded-service\src\activity.rs:135:21
    |
135 | static SUBSCRIBERS: OnceLock<IntrusiveList> = OnceLock::new();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^ `core::cell::Cell<core::option::Option<&'static intrusive_list::IntrusiveNode>>` cannot be shared between threads safely
    |
    = help: within `intrusive_list::IntrusiveList`, the trait `Sync` is not implemented for `core::cell::Cell<core::option::Option<&'static intrusive_list::IntrusiveNode>>`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
note: required because it appears within the type `intrusive_list::IntrusiveList`
   --> D:\.tools\.cargo\git\checkouts\embedded-services-78faf442f1f643c2\a938ac1\embedded-service\src\intrusive_list.rs:62:12
    |
62  | pub struct IntrusiveList {
    |            ^^^^^^^^^^^^^
    = note: required for `OnceLock<intrusive_list::IntrusiveList>` to implement `Sync`
    = note: shared static variables must have a type that implements `Sync`

[wmmc88]

I think @JamesHuard had some ideas around ways to remove the Cell in IntrusiveList

[JamesHuard]

InstrusiveList is already in a crit section, so we really just need to rid the implementation of the Cell<> so that we can make it properly Sync

[jerrysxie]

InstrusiveList is already in a crit section, so we really just need to rid the implementation of the Cell<> so that we can make it properly Sync

Is the planned remedy to replace Cell with Mutex in IntrusiveList?

[JamesHuard]

InstrusiveList is already in a crit section, so we really just need to rid the implementation of the Cell<> so that we can make it properly Sync

Is the planned remedy to replace Cell with Mutex in IntrusiveList?

We might be able to replace Cell with raw::CriticalSectionMutex, I don't think we'd be able to use a regular Mutex<> without introducing breaking changes as the node operation APIs would all be made async.

I think the "cleanest" solution would be to leverage SyncUnsafeCell, but that is an unstable feature at the moment. We could replicate this logic internally to intrusive_list, though that would introduce a lot more unsafe {} blocks.

raw::CriticalSectionMutex would also require increasing the amount of unsafe blocks, but that is primarily due to it not being inherently re-entrant. That would be easy to document how the concerns are cleared in place, and probably the easiest approach here

[JamesHuard]

Have a WIP change I'm demoing. There's quite a bit here that breaks aside from just intrusive_list. Will introduce a pattern in a PR we can leverage to use as a drop-in replacement everywhere Cell<> is used unsafely currently

[JamesHuard]

Not fully resolved