[naasking]

Haskell does it automatically as an optimisation: if an algebraic type has fewer than 2-3 cases, then it inline the tag bits directly into the pointer, thus saving an indirection on pattern match.

Some C data structures also make use of low level bit tricks like this to save space and reduce indirections. For instance, the hash-array mapped Trie uses a 32 bit mask to both track which indices of the current node are actually populated, and incidentally, how large the node currently is. It's quite clever.

These are always my foot examples to evaluate any alleged systems programming language. No language less powerful than a theorem prover is currently capable of expressing these idioms safely.

[crzwdjk]

Rust actually does a bit of this internally in the form of the null pointer optimization. If you have an Option<&T> (or Option<*T>) value, it's actually stored as a single pointer-sized value, with None being represented by a null pointer on the assumption that null is not a valid pointer.