This is one of the things that I'm also looking on at Zig like a slow moving car crash about: they claim they are memory safe (or at least "good enough" memory safe if you use the safe optimization level, which is it's own discussion), but they don't have the equivalent to Rust's Send/Sync types. It just so happens that in practice no one was writing enough concurrent Zig code to get bitten by it a lot, I guess...except that now they're working on bringing back first-class async support to the language, which will run futures on other threads and presumably a lot of feet are going to be fired at once that lands.