Comment by tialaramex
2 days ago
One of the small but important significances of Rust is that it very often gets to have the same machine code as a correct "in-band error signalling" strategy from C but with source code that the maintainers can actually understand.
And really often when your "in-band error signalling" in Rust is more expensive you realise (perhaps to your horror) that your C wouldn't be correct, so you just dodged a bullet. For example Option<OwnedFd> is, like OwnedFd, a 32-bit value, Unix has always reserved -1 to mean invalid, so Rust can treat that as a niche. But the Windows equivalent Option<OwnedHandle> can't do this because, well, cue Raymond Chen: https://devblogs.microsoft.com/oldnewthing/20040302-00/?p=40...
> One of the small but important significances of Rust is that it very often gets to have the same machine code as a correct "in-band error signalling" strategy from C but with source code that the maintainers can actually understand.
But aside from NonZero- and NonNull-based layout optimizations, I don't think Rust is doing anything special to obtain that machine code - it's just relying on LLVM to optimize things.
Well, all of the niche optimisations, so that includes every normal C-style user-defined enumeration because (unlike C or C++) Rust says those aren't just the integers wearing a funny hat.
But sure, for the most part LLVM will do The Right Thing™ here without much help and Rust is just giving better ergonomics to the same backend.