Rust has a thing called the Guaranteed Niche Optimisation, which says if you make a Sum type, and the Sum type has exactly one variant which is just itself, plus exactly one variant which has a niche (a bit pattern which isn't used by any valid representation of that type) then it promises that your type is the same size as the type with the niche in it.
That is, if you made your own Maybe type which works like Option, it's also guaranteed to get this optimisation, and the optimisation works for any type which the compiler knows has a "niche", not just obvious things like references or small enumerations, the NonZero type, but also e.g. OwnedFd, a type which is a Unix file descriptor - Unix file descriptors cannot be -1, and so logically the bit pattern for -1 serves as a niche for this type.
I really like this feature, and I want to use it more. There's good news and bad news. The good news is that although the Guaranteed Niche Optimisation is the only such guarantee, in practice the Rust compiler will do much more with a niche.
The bad news is that we're not allowed to make new types with their own niches (other than enumerations which automatically get an appropriately sized niche) in stable Rust today. In fact the ability to mark a niche is not only permanently unstable (thus usable in practice only from the Rust stdlib) but it's a compiler internal feature, they're pretty much telling you not to touch this, it can't and won't get stabilized in this form)
But, we do have a good number of useful niches in the standard library, all references, the NonNull pointers (if you use pointers for something), the NonZero types, the booleans, small C-style enumerations, OwnedFd, that's quite a lot of possibilities.
The main thing I want, and the reason I tried to make more movement happen (but I have done very little for about a year) is BalancedIx a suite of types like NonZero, but missing the most negative values of the signed integers. You very rarely need -128 on an 8-bit signed integer, and it's kind of a nuisance, so BalancedI8 would be the same size, it loses -128 and in exchange Option<BalancedI8> is the same size and now abs does what you expected, two for the price!
That is, if you made your own Maybe type which works like Option, it's also guaranteed to get this optimisation, and the optimisation works for any type which the compiler knows has a "niche", not just obvious things like references or small enumerations, the NonZero type, but also e.g. OwnedFd, a type which is a Unix file descriptor - Unix file descriptors cannot be -1, and so logically the bit pattern for -1 serves as a niche for this type.
I really like this feature, and I want to use it more. There's good news and bad news. The good news is that although the Guaranteed Niche Optimisation is the only such guarantee, in practice the Rust compiler will do much more with a niche.
The bad news is that we're not allowed to make new types with their own niches (other than enumerations which automatically get an appropriately sized niche) in stable Rust today. In fact the ability to mark a niche is not only permanently unstable (thus usable in practice only from the Rust stdlib) but it's a compiler internal feature, they're pretty much telling you not to touch this, it can't and won't get stabilized in this form)
But, we do have a good number of useful niches in the standard library, all references, the NonNull pointers (if you use pointers for something), the NonZero types, the booleans, small C-style enumerations, OwnedFd, that's quite a lot of possibilities.
The main thing I want, and the reason I tried to make more movement happen (but I have done very little for about a year) is BalancedIx a suite of types like NonZero, but missing the most negative values of the signed integers. You very rarely need -128 on an 8-bit signed integer, and it's kind of a nuisance, so BalancedI8 would be the same size, it loses -128 and in exchange Option<BalancedI8> is the same size and now abs does what you expected, two for the price!