Embedded: Mutex + critical sections + hardware interrupts. The Mutex isn't std::Mutex, but a hardware-specific one. (E.g. ARM) works in a similar way. The default syntax is a mess of RefCell, Cell, Option, with the requisite < and > characters. (The Option is so you can insert a value in on init, then it's never None) etc. I work around this with macros.

PC applications (GUI, 3D etc): std::thread and MPSC receivers/transmitters which update a state struct when they're ready. Then poll in the GUI's event loop or w/e. I believe this workflow has worked for me in lieu of Arc because I have been structuring programs with a single main thread, then sub threads which perform non-blocking tasks, then return data to the main thread.

Also: If it's a primitive type, Atomics are nice.