Out pointers

Note

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Out pointers are a fundamental building block of in-place initialisation: at the core level it is hard to imagine any way of building the feature on a concrete machine without bringing in out pointers.

The viewpoint of the out pointer approach to in-place initialisation is therefore that Rust should not attempt to hide the existence of out pointers behind special traits, attributes, or compiler magic but should instead make them available as a first class citizen of the language.

The basic idea is a pair of move-only reference-like types Uninit and InPlace:

struct Uninit<'a, T>(&'a mut MaybeUninit<T>, Invariant<'a>);
struct InPlace<'a, T>(Box<T, InPlaceAlloc<'a>>, Invariant<'a>);

fn init_x<'a>(x: Uninit<'a, X>) -> InPlace<'a, X> { ... }

fn main() {
    // Note: we'd actually need generativity here to produce a truly unique lifetime for x.
    let mut x = MaybeUninit::<X>::uninit();
    let x = init_x(Uninit::from(&mut x));
}

Approaches

• Custom types (as above)
• &uninit

Resources

In-place initialization via outptrs, Jul 8 2025

• Introduced InPlace<T> as a Box with custom allocator.
• Very influential.

Thoughts on “out”-pointer, Nov 12 2025

Backlinks

• In-place initialization