Tagging pointers in Rust

I've been learning some Rust after hearing about their 1.0 release. I'm very impressed by the compiler, the documentation, and the language itself.

The first thing I wrote in Rust was a routine to tag pointers. (If you haven't heard of pointer tagging, it's just a way of storing information within a pointer. Pointers tend to be at least 32-bit aligned, leaving the two least-significant bits of the pointer free for tags.) It works by using Rust's unsafe-cast mechanism (std::mem::transmute) to do a little bit flipping:

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fn tag_pointer<T>(p: &T, bit: u8) -> &T { unsafe { let pod: usize = mem::transmute(p); let pod = pod | (1 << bit); mem::transmute(pod) } } fn untag_pointer<T>(p: &T, bit: u8) -> &T { unsafe { let pod: usize = mem::transmute(p); let pod = pod & (!(1 << bit) as usize); mem::transmute(pod) } } fn check_pointer_tag<T>(p: &T, bit: u8) -> u8 { unsafe { let pod: usize = mem::transmute(p); ((pod >> bit) & 1) as u8 } }

To check that this works, we can do:

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let p = &0; // Take the address of an integer. let pn = tag_pointer(p, 0); assert_eq!(check_pointer_tag(p, 0), 0); assert_eq!(check_pointer_tag(pn, 0), 1); assert_eq!(p, untag_pointer(pn, 0));

N.b: it isn't safe to dereference a tagged pointer :-). Here is a safe, alternate version which implements Deref and DerefMut.

This code isn't particularly tricky or interesting, but maybe it'll be useful to someone.