rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

/// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok());

/// ```

///

/// [`Box`]es can also be used to store trait objects by coercing their type:

///

/// ```

/// trait FooTrait {}

///

/// struct FooStruct;

/// impl FooTrait for FooStruct {}

///

/// let _ = KBox::new(FooStruct, GFP_KERNEL)? as KBox<dyn FooTrait>;

/// # Ok::<(), Error>(())

/// ```

///

/// # Invariants

///

/// `self.0` is always properly aligned and either points to memory allocated with `A` or, for

/// zero-sized types, is a dangling, well aligned pointer.

#[repr(transparent)]

pub struct Box<T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);

#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]

pub struct Box<#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, pointee)] T: ?Sized, A: Allocator>(

    NonNull<T>,

    PhantomData<A>,

);

// This is to allow coercion from `Box<T, A>` to `Box<U, A>` if `T` can be converted to the

// dynamically-sized type (DST) `U`.

#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]

impl<T, U, A> core::ops::CoerceUnsized<Box<U, A>> for Box<T, A>

where

    T: ?Sized + core::marker::Unsize<U>,

    U: ?Sized,

    A: Allocator,

{

}

// This is to allow `Box<U, A>` to be dispatched on when `Box<T, A>` can be coerced into `Box<U,

// A>`.

#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]

impl<T, U, A> core::ops::DispatchFromDyn<Box<U, A>> for Box<T, A>

where

    T: ?Sized + core::marker::Unsize<U>,

    U: ?Sized,

    A: Allocator,

{

}

/// Type alias for [`Box`] with a [`Kmalloc`] allocator.

///