Comment by speedstyle

OP suggests it was always fallible, they switched to a sentinel to mitigate callers dereferencing null, which they wouldn't have with Option/Result. https://git.musl-libc.org/cgit/musl/commit/?id=b3c4cc12

But in your version, you can't preserve backwards compatibility, making an infallible function fallible is a breaking change. For something comparable to libc you might want to (eg Rust std still has mem::uninitialized), but even with security issues, I feel like people would be more inclined to deprecate for a new function (or panic, which does have its own problems).

A good type system also means things are written with better forwards compatibility, and can be safely changed later. The Rust equivalent to `crypt` likely wouldn't exist: a low-level library would just provide hash functions, leaving callers to mitigate DOS etc; a broader library might return some opaque handle with equality/serialization, which you couldn't try to compare to a passwordless user; a high-level library would parse the account store itself. None of these really need to change in incompatible ways, they have all the information to add features and fixes at their own level of abstraction.