Fearless concurrency, is only fearless for a very fine grained version of it.
In memory resources shared among threads.
Turns out threads also may share resources like out-of-process files, memory mapped regions, shared memory, databases, distributed transactions,.... where the Send and Sync traits is of no help.
Also you happen to forget Haskell has a Software Transactional Memory, and Swift also has similar protocols since version 6, and effects are a thing in OCaml, Scala, Koka, while languages like Dafny, Koka and Idris also provide similar capabilities via proofs.
That's why everybody who cares about OS safety should invest into capability-based operating systems. They take the concept of Send and Sync and implement at the runtime of the entire computer.
In memory resources shared among threads.
Turns out threads also may share resources like out-of-process files, memory mapped regions, shared memory, databases, distributed transactions,.... where the Send and Sync traits is of no help.
Also you happen to forget Haskell has a Software Transactional Memory, and Swift also has similar protocols since version 6, and effects are a thing in OCaml, Scala, Koka, while languages like Dafny, Koka and Idris also provide similar capabilities via proofs.