There are two reasons: fibers with non-native stacks don't mix well with zero-cost FFI with C and other languages with native stacks. For example, Go suffers fr

There are two reasons: fibers with non-native stacks don't mix well with zero-cost FFI with C and other languages with native stacks. For example, Go suffers from this problem. The other problem is that N:M threads require a runtime and not having one is a hard requirement for Rust.