moveit 0.5.1

A library for safe, in-place construction of Rust (and C++!) objects.
Documentation 
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// Copyright 2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Support for the `alloc` crate, when available.

use core::mem::MaybeUninit;
use core::pin::Pin;

use alloc::boxed::Box;
use alloc::rc::Rc;
use alloc::sync::Arc;

use crate::move_ref::DerefMove;
use crate::move_ref::MoveRef;
use crate::new::EmplaceUnpinned;
use crate::new::TryNew;
use crate::slot::DroppingSlot;

unsafe impl<T> DerefMove for Box<T> {
  type Storage = Box<MaybeUninit<T>>;

  #[inline]
  fn deref_move<'frame>(
    self,
    storage: DroppingSlot<'frame, Self::Storage>,
  ) -> MoveRef<'frame, Self::Target>
  where
    Self: 'frame,
  {
    let cast =
      unsafe { Box::from_raw(Box::into_raw(self).cast::<MaybeUninit<T>>()) };

    let (storage, drop_flag) = storage.put(cast);
    unsafe { MoveRef::new_unchecked(storage.assume_init_mut(), drop_flag) }
  }
}

unsafe impl<T> DerefMove for Pin<Box<T>> {
  type Storage = <Box<T> as DerefMove>::Storage;

  #[inline]
  fn deref_move<'frame>(
    self,
    storage: DroppingSlot<'frame, Self::Storage>,
  ) -> MoveRef<'frame, Self::Target>
  where
    Self: 'frame,
  {
    // Safety: boxes never move their contents
    unsafe { Pin::into_inner_unchecked(self).deref_move(storage) }
  }
}

impl<T> EmplaceUnpinned<T> for Pin<Box<T>> {
  fn try_emplace<N: TryNew<Output = T>>(n: N) -> Result<Self, N::Error> {
    let mut uninit = Box::new(MaybeUninit::<T>::uninit());
    unsafe {
      let pinned = Pin::new_unchecked(&mut *uninit);
      n.try_new(pinned)?;
      Ok(Pin::new_unchecked(Box::from_raw(
        Box::into_raw(uninit).cast::<T>(),
      )))
    }
  }
}

impl<T> EmplaceUnpinned<T> for Pin<Rc<T>> {
  fn try_emplace<N: TryNew<Output = T>>(n: N) -> Result<Self, N::Error> {
    let uninit = Rc::new(MaybeUninit::<T>::uninit());
    unsafe {
      let pinned = Pin::new_unchecked(&mut *(Rc::as_ptr(&uninit) as *mut _));
      n.try_new(pinned)?;
      Ok(Pin::new_unchecked(Rc::from_raw(
        Rc::into_raw(uninit).cast::<T>(),
      )))
    }
  }
}

impl<T> EmplaceUnpinned<T> for Pin<Arc<T>> {
  fn try_emplace<N: TryNew<Output = T>>(n: N) -> Result<Self, N::Error> {
    let uninit = Arc::new(MaybeUninit::<T>::uninit());
    unsafe {
      let pinned = Pin::new_unchecked(&mut *(Arc::as_ptr(&uninit) as *mut _));
      n.try_new(pinned)?;
      Ok(Pin::new_unchecked(Arc::from_raw(
        Arc::into_raw(uninit).cast::<T>(),
      )))
    }
  }
}

#[cfg(test)]
mod tests {
  use crate::move_ref::test::Immovable;
  use crate::moveit;
  use crate::new::mov;
  use crate::Emplace;

  #[test]
  fn test_mov_box() {
    let foo = Box::emplace(Immovable::new());
    moveit!(let _foo = mov(foo));
  }
}