I don't know how C#, but Rust async/await doesn't allocate anything on the heap by itself. So it is not a universal property of all async/await implementations contrary to what you imply in your blog post.
It would also be interesting to try Native AOT compiling both versions…
https://learn.microsoft.com/en-us/dotnet/core/deploying/nati...
Rust also has to allocate (box) when you need recursive async calls: https://rust-lang.github.io/async-book/07_workarounds/04_rec...
What Rust does, it allows to remove one layer of allocations by using Future to store the contents of the state machine (basically, the stack frame of the async function).
This is not at all that different from Go, except that Go preallocates stack without doing any analysis.
Actually... I can fix that! I can use Go's escape analysis machinery to statically check during the compilation if the stack size can be bounded by a lower number than the default 2kb. This way, I can get it to about ~600 bytes per goroutine. It will also help to speed up the code a bit, by eliding the "morestack" checks.
It can be crunched a bit more, by checking if the goroutine uses timers (~100 bytes) and defers (another ~100 bytes). But this will require some tricks.