Zig's new plan for asynchronous programs

Zig's designers have announced a new approach to asynchronous I/O that avoids the function coloring problem. The design centers on a new generic interface called Io, passed as a parameter to functions that need I/O — similar to how Zig's Allocator interface works. Two built-in implementations are provided: Io.Threaded (uses synchronous syscalls, optionally dispatching to threads) and Io.Evented (uses io_uring, kqueue, or other async backends). Code written against the Io interface works correctly with either backend, making asynchronicity a transparent performance optimization rather than a code-structure concern. Two functions, async() and concurrent(), distinguish between operations that can be parallelized opportunistically versus those that must run simultaneously for correctness. The design avoids new language keywords, letting programmers use existing control-flow primitives like try and defer. Io.Evented is still experimental and incomplete, and a WebAssembly-compatible Io variant is planned. This is one of the last major design questions before Zig's 1.0 release.

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