Replace Mutex with cordyceps::TransferStack for Epochs and make no_std compatible

[ilslv]

Hi! I've recently saw a talk and was wondering what will it take to remove Mutex lock on ReadHandle initialisation and drop.

The simplest approach I could come boils down to the following idea: what if ReadHandles wouldn't bother themselves with removal of their Epochs from shared Epochs and would just drop the Arc. Then WriteHandle can check for ReadHandle liveliness with Arc::strong_count since every Arc has a strong count of 1 or 2. After that it became obvious that something like cordyceps::TransferStack can be used to semantically replace Epochs from "shared storage of Epochs that is accessed both by ReadHandles and WriteHandle" to "a kinda channel to let WriteHandle know about new ReadHandles".

I also made library no_std compatible as mentioned in #101.

I think more tests should be added before merging, but before investing more time I would love to here about approach in general and haven't I missed something.

[ilslv]

cordyceps has a following note

This is a hobby project. I’m working on it in my spare time, for my own personal use. I’m very happy to share it with the broader Rust community, and contributions and bug reports are always welcome. However, please remember that I’m working on this library for fun, and if it stops being fun…well, you get the idea.

Anyway, feel free to use and enjoy this crate, and to contribute back as much as you want to!

But implementation looks very solid and embassy-executor uses it

[ilslv]

I would say this is my main concern with the implementation. Would this be a dealbreaker?

[codecov]

Codecov Report

❌ Patch coverage is 90.54054% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.6%. Comparing base (1b9b8f4) to head (42a5df7).

Files with missing lines	Patch %	Lines
src/aliasing.rs	60.0%	2 Missing ⚠️
src/read/factory.rs	0.0%	2 Missing ⚠️
src/write.rs	94.2%	2 Missing ⚠️
src/read.rs	80.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/epochs.rs	100.0% <100.0%> (ø)
src/lib.rs	84.2% <100.0%> (+5.6%)	⬆️
src/read/guard.rs	43.4% <ø> (ø)
src/read.rs	56.5% <80.0%> (-1.9%)	⬇️
src/aliasing.rs	27.2% <60.0%> (ø)
src/read/factory.rs	0.0% <0.0%> (ø)
src/write.rs	73.3% <94.2%> (+3.4%)	⬆️

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[ilslv]

cordyceps declares MSRV at 1.61.0, but actually it's 1.66.0. I'll report it to upstream a little later