5 releases (3 breaking)

0.4.0 Jul 27, 2023
0.3.0 Aug 10, 2022
0.2.0 Apr 9, 2022
0.1.1 Mar 23, 2022
0.1.0 Mar 21, 2022

#508 in Rust patterns

MIT license

46KB
684 lines

munge makes it easy and safe to destructure MaybeUninits, Cells, UnsafeCells, ManuallyDrops, and more.

Just use the munge! macro to destructure opaque types the same way you'd destructure a value. The munge! macro may be used to perform either borrow destructuring (e.g. let (a, b) = c where c is a reference) or move destructuring (e.g. let (a, b) = c where c is a value) depending on the type.

munge has no features and is always #![no_std].

Examples

munge makes it easy to initialize MaybeUninits:

use {
    ::core::mem::MaybeUninit,
    ::munge::munge,
};

pub struct Example {
    a: u32,
    b: (char, f32),
}

let mut mu = MaybeUninit::<Example>::uninit();

munge!(let Example { a, b: (c, mut f) } = &mut mu);
assert_eq!(a.write(10), &10);
assert_eq!(c.write('x'), &'x');
assert_eq!(f.write(3.14), &3.14);
// Note that `mut` bindings can be reassigned like you'd expect:
f = &mut MaybeUninit::uninit();

// SAFETY: `mu` is completely initialized.
let init = unsafe { mu.assume_init() };
assert_eq!(init.a, 10);
assert_eq!(init.b.0, 'x');
assert_eq!(init.b.1, 3.14);

It can also be used to destructure Cells:

use {
    ::core::cell::Cell,
    ::munge::munge,
};

pub struct Example {
    a: u32,
    b: (char, f32),
}

let value = Example {
    a: 10,
    b: ('x', 3.14),
};
let cell = Cell::<Example>::new(value);

munge!(let Example { a, b: (c, f) } = &cell);
assert_eq!(a.get(), 10);
a.set(42);
assert_eq!(c.get(), 'x');
c.set('!');
assert_eq!(f.get(), 3.14);
f.set(1.41);

let value = cell.into_inner();
assert_eq!(value.a, 42);
assert_eq!(value.b.0, '!');
assert_eq!(value.b.1, 1.41);

You can even extend munge to work with your own types by implementing its Destructure and Restructure traits:

use munge::{Destructure, Restructure, Move, munge};

pub struct Invariant<T>(T);

impl<T> Invariant<T> {
    /// # Safety
    ///
    /// `value` must uphold my custom invariant.
    pub unsafe fn new_unchecked(value: T) -> Self {
        Self(value)
    }

    pub fn unwrap(self) -> T {
        self.0
    }
}

// SAFETY:
// - `Invariant<T>` is destructured by move, so its `Destructuring` type is
//   `Move`.
// - `underlying` returns a pointer to its inner type, so it is guaranteed
//   to be non-null, properly aligned, and valid for reads.
unsafe impl<T> Destructure for Invariant<T> {
    type Underlying = T;
    type Destructuring = Move;

    fn underlying(&mut self) -> *mut Self::Underlying {
        &mut self.0 as *mut Self::Underlying
    }
}

// SAFETY: `restructure` returns an `Invariant<U>` that takes ownership of
// the restructured field because `Invariant<T>` is destructured by move.
unsafe impl<T, U> Restructure<U> for Invariant<T> {
    type Restructured = Invariant<U>;

    unsafe fn restructure(&self, ptr: *mut U) -> Self::Restructured {
        // SAFETY: The caller has guaranteed that `ptr` is a pointer to a
        // subfield of some `T`, so it must be properly aligned, valid for
        // reads, and initialized. We may move the fields because the
        // destructuring type for `Invariant<T>` is `Move`.
        let value = unsafe { ptr.read() };
        Invariant(value)
    }
}

// SAFETY: `(1, 2, 3)` upholds my custom invariant.
let value = unsafe { Invariant::new_unchecked((1, 2, 3)) };
munge!(let (one, two, three) = value);
assert_eq!(one.unwrap(), 1);
assert_eq!(two.unwrap(), 2);
assert_eq!(three.unwrap(), 3);

Dependencies

~0.4–0.8MB
~19K SLoC