// SPDX-License-Identifier: GPL-2.0 //! Extensions to [`Box`] for fallible allocations. use super::{AllocError, Flags};use alloc::boxed::Box;use core::{mem::MaybeUninit, ptr, result::Result}; /// Extensions to [`Box`].pub trait BoxExt<T>: Sized {    /// Allocates a new box.    ///    /// The allocation may fail, in which case an error is returned.    fn new(x: T, flags: Flags) -> Result<Self, AllocError>;     /// Allocates a new uninitialised box.    ///    /// The allocation may fail, in which case an error is returned.    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;     /// Drops the contents, but keeps the allocation.    ///    /// # Examples    ///    /// ```    /// use kernel::alloc::{flags, box_ext::BoxExt};    /// let value = Box::new([0; 32], flags::GFP_KERNEL)?;    /// assert_eq!(*value, [0; 32]);    /// let mut value = Box::drop_contents(value);    /// // Now we can re-use `value`:    /// value.write([1; 32]);    /// // SAFETY: We just wrote to it.    /// let value = unsafe { value.assume_init() };    /// assert_eq!(*value, [1; 32]);    /// # Ok::<(), Error>(())    /// ```    fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;} impl<T> BoxExt<T> for Box<T> {    fn new(x: T, flags: Flags) -> Result<Self, AllocError> {        let mut b = <Self as BoxExt<_>>::new_uninit(flags)?;        b.write(x);        // SAFETY: We just wrote to it.        Ok(unsafe { b.assume_init() })    }     #[cfg(any(test, testlib))]    fn new_uninit(_flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {        Ok(Box::new_uninit())    }     #[cfg(not(any(test, testlib)))]    fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {        let ptr = if core::mem::size_of::<MaybeUninit<T>>() == 0 {            core::ptr::NonNull::<_>::dangling().as_ptr()        } else {            let layout = core::alloc::Layout::new::<MaybeUninit<T>>();             // SAFETY: Memory is being allocated (first arg is null). The only other source of            // safety issues is sleeping on atomic context, which is addressed by klint. Lastly,            // the type is not a SZT (checked above).            let ptr =                unsafe { super::allocator::krealloc_aligned(core::ptr::null_mut(), layout, flags) };            if ptr.is_null() {                return Err(AllocError);            }             ptr.cast::<MaybeUninit<T>>()        };         // SAFETY: For non-zero-sized types, we allocate above using the global allocator. For        // zero-sized types, we use `NonNull::dangling`.        Ok(unsafe { Box::from_raw(ptr) })    }     fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {        let ptr = Box::into_raw(this);        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.        unsafe { ptr::drop_in_place(ptr) };         // CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.        let ptr = ptr.cast::<MaybeUninit<T>>();         // SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for        // reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.        unsafe { Box::from_raw(ptr) }    }}