#projection #proc-macro #pin #cell #project

no-std projecture

Easy arbitrary type projections without proc macros

4 releases

0.0.4 Oct 5, 2022
0.0.3 Jul 15, 2022
0.0.2 Jul 4, 2022
0.0.1 Jun 15, 2022

#1983 in Rust patterns

40 downloads per month

MIT license

1.5K SLoC


This is in a proof of concept state and also internally uses a lot of not yet battle-tested unsafe code so use it on your own risk meanwhile if you are good with unsafe rust i would appreciate a soundness review

Allows to do almost arbitrary type projections. In comparison to other crates that do similar things it is more generic, does not require procedural macros and also does not impose additional requirements on target struct, if target struct is located in external crate that crate does not have to explicitly add a support for such projection(pin projection is an exception here).

Although as of now this crate doesn't support enums yet, but it will be added later.

Currently can do following type of projections

  • Destructuring projection (similar to usual let <pattern> but also supports deref pattern, and also works if struct implements Drop which is just not called).
    Note that due to limitations of declaration macros currently unmentioned fields will be leaked.
  • Reference(&, &mut) projection (similar to match ergonomics in let <pattern> but also supports deref pattern)
  • Pin projection
  • Cell projection
  • MaybeUninit projection
  • Atomic(from atomic crate) projection
  • Option projection (which works together with other kinds of projections)
  • RefCell guards projection
  • raw pointers projections (*const T, *mut T, NonNull<T>)

Also adds two types of projectable pointers:

  • generic::GenericPointer - makes it possible to write code that is generic over the reference type.
  • OwningRef - reference that semantically owns data (sometimes referred as &own T in various proposals). On nightly(with nightly feature) it allows you to make object safe traits that accept Self by value.

Where possible, projections can additionally project through a Deref (including dereference by value via DerefOwned).

Here is a general overview of what you can do, see project! macro for more usage details.

#    use std::cell::Cell;
#    use std::marker::PhantomPinned;
#    use std::pin::Pin;
#    use std::rc::Rc;
#    use atomic::Atomic;
#    use projecture::project;
#    use projecture::pin_projectable;
    struct Foo {
        a: Bar,
        b: Rc<Cell<Bar>>,
        c: Pin<Box<Bar>>,
        d: Atomic<Bar>,

    struct Bar(usize,PhantomPinned);
    // needed only for pin projections

    fn test(arg: &Foo) {
        let Foo {
            a: Bar (e, ..),
            b: *Bar{ 0: cell },
            c: *Bar (_, f) ,
            d: Bar(atomic, ..),
        } = arg);
        let _: &usize = e;
        let _: &Cell<usize> = cell;
        let _: Pin<& PhantomPinned> = f;
        let _: &Atomic<usize> = atomic;

        let _: &usize = project!(arg -> a -> 0);
        let _: &Cell<usize> = project!(arg -> b -> 0);
        let _: Pin<& PhantomPinned> = project!(arg -> c -> 1);
        let _: &Atomic<usize> = project!(arg -> d -> 0);

Also allows dependent crates to define their own projections via traits. see atomic module for example of how to do a projection of a transparent field wrapper or Pin for doing projections on a custom reference type

MSRV: 1.54
License: MIT