To expand on the sibling answer: sort of! Rust will do niche optimisation, but for references and NonNull pointers this is limited to "the value 0 is invalid and can thus be used as a niche". But Rust does not (currently) take advantage of alignment niches in pointers. Nor does it use high bit on architectures where you know your whole theoretical address space isn't actually in use.

Is doing that manually worth it? Usually not, but for some core types (classical example is strings) or in language runtimes it can be.

Would it be awesome if this could be done automatically? Absolutely, but I understand it is a large change, and the plan is to later build upon the pattern types that are currently work in progress (and would allow you to specify custom ranged integer typed).

I mean, kinda, sorta? Rust's guaranteed niche optimisation means Option<&T> [which might be Some(&T) or just None] is promised to be the same size in memory as &T the reference to a T

So that's one tiny use of this sort of idea which is guaranteed unnecessary in Rust, and indeed although it isn't guaranteed the optimiser will typically spot less obvious opportunities so that Option<Option<bool>> which might be None, or Some(None) or Some(Some(true)) or Some(Some(false)) is the same size (one byte) as bool.

But hiding stuff in a pointer is applicable in places your Rust compiler won't try to take advantage unless you do something like this. A novel tiny String-like type I saw recently does this, https://crates.io/crates/cold-string ColdString is 8 bytes, if your text is 8 or fewer bytes of UTF-8 then you're done, that'll fit, but, if you have more text ColdString allocates on the heap to store not only your text but also its length and so it needs to actually "be" in some sense a raw pointer to that structure, but if the string is shorter that pointer is nonsense, we've hidden our text in the pointer itself.

Implementation requires knowing how pointers work, and how UTF-8 encoding works. I actually really like one of the other Rust tiny strings, CompactString but if you have a lot of very small strings (e.g. UK postcodes fit great) then ColdString might be as much as three times smaller than your existing Rust or C++ approach and it's really hard to beat that for such use cases.

Edited: To remove suggestion ColdString has a distinct storage capacity, this isn't intended as a conventional string buffer, it can't grow after creation

Nope. Rust stabilized strict provenance over a year ago. Some details about aliasing aren't tied down, but so long as you can obey the strict provenance rules you're golden today in Rust to hide flags in pointers etc.

https://blog.rust-lang.org/2025/01/09/Rust-1.84.0/#strict-pr...