// Copyright 2023 The Fuchsia Authors

//

// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0

// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT

// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.

// This file may not be copied, modified, or distributed except according to

// those terms.

use core::{

    cmp::Ordering,

    fmt::{self, Debug, Display, Formatter},

    hash::Hash,

    mem::{self, ManuallyDrop},

    ops::{Deref, DerefMut},

    ptr,

};

use super::*;

/// A type with no alignment requirement.

///

/// An `Unalign` wraps a `T`, removing any alignment requirement. `Unalign<T>`

/// has the same size and bit validity as `T`, but not necessarily the same

/// alignment [or ABI]. This is useful if a type with an alignment requirement

/// needs to be read from a chunk of memory which provides no alignment

/// guarantees.

///

/// Since `Unalign` has no alignment requirement, the inner `T` may not be

/// properly aligned in memory. There are five ways to access the inner `T`:

/// - by value, using [`get`] or [`into_inner`]

/// - by reference inside of a callback, using [`update`]

/// - fallibly by reference, using [`try_deref`] or [`try_deref_mut`]; these can

///   fail if the `Unalign` does not satisfy `T`'s alignment requirement at

///   runtime

/// - unsafely by reference, using [`deref_unchecked`] or

///   [`deref_mut_unchecked`]; it is the caller's responsibility to ensure that

///   the `Unalign` satisfies `T`'s alignment requirement

/// - (where `T: Unaligned`) infallibly by reference, using [`Deref::deref`] or

///   [`DerefMut::deref_mut`]

///

/// [or ABI]: https://github.com/google/zerocopy/issues/164

/// [`get`]: Unalign::get

/// [`into_inner`]: Unalign::into_inner

/// [`update`]: Unalign::update

/// [`try_deref`]: Unalign::try_deref

/// [`try_deref_mut`]: Unalign::try_deref_mut

/// [`deref_unchecked`]: Unalign::deref_unchecked

/// [`deref_mut_unchecked`]: Unalign::deref_mut_unchecked

// NOTE: This type is sound to use with types that need to be dropped. The

// reason is that the compiler-generated drop code automatically moves all

// values to aligned memory slots before dropping them in-place. This is not

// well-documented, but it's hinted at in places like [1] and [2]. However, this

// also means that `T` must be `Sized`; unless something changes, we can never

// support unsized `T`. [3]

//

// [1] https://github.com/rust-lang/rust/issues/54148#issuecomment-420529646

// [2] https://github.com/google/zerocopy/pull/126#discussion_r1018512323

// [3] https://github.com/google/zerocopy/issues/209

#[allow(missing_debug_implementations)]

#[derive(Default, Copy)]

#[cfg_attr(

    any(feature = "derive", test),

    derive(KnownLayout, FromZeroes, FromBytes, AsBytes, Unaligned)

)]

#[repr(C, packed)]

pub struct Unalign<T>(T);

#[cfg(not(any(feature = "derive", test)))]

impl_known_layout!(T => Unalign<T>);

safety_comment! {

    /// SAFETY:

    /// - `Unalign<T>` is `repr(packed)`, so it is unaligned regardless of the

    ///   alignment of `T`, and so we don't require that `T: Unaligned`

    /// - `Unalign<T>` has the same bit validity as `T`, and so it is

    ///   `FromZeroes`, `FromBytes`, or `AsBytes` exactly when `T` is as well.

    impl_or_verify!(T => Unaligned for Unalign<T>);

    impl_or_verify!(T: FromZeroes => FromZeroes for Unalign<T>);

    impl_or_verify!(T: FromBytes => FromBytes for Unalign<T>);

    impl_or_verify!(T: AsBytes => AsBytes for Unalign<T>);

}

// Note that `Unalign: Clone` only if `T: Copy`. Since the inner `T` may not be

// aligned, there's no way to safely call `T::clone`, and so a `T: Clone` bound

// is not sufficient to implement `Clone` for `Unalign`.

impl<T: Copy> Clone for Unalign<T> {

    #[inline(always)]

    fn clone(&self) -> Unalign<T> {

        *self

    }

}

impl<T> Unalign<T> {

    /// Constructs a new `Unalign`.

    #[inline(always)]

    pub const fn new(val: T) -> Unalign<T> {

        Unalign(val)

    }

    /// Consumes `self`, returning the inner `T`.

    #[inline(always)]

    pub const fn into_inner(self) -> T {

        // Use this instead of `mem::transmute` since the latter can't tell

        // that `Unalign<T>` and `T` have the same size.

        #[repr(C)]

        union Transmute<T> {

            u: ManuallyDrop<Unalign<T>>,

            t: ManuallyDrop<T>,

        }

        // SAFETY: Since `Unalign` is `#[repr(C, packed)]`, it has the same

        // layout as `T`. `ManuallyDrop<U>` is guaranteed to have the same

        // layout as `U`, and so `ManuallyDrop<Unalign<T>>` has the same layout

        // as `ManuallyDrop<T>`. Since `Transmute<T>` is `#[repr(C)]`, its `t`

        // and `u` fields both start at the same offset (namely, 0) within the

        // union.

        //

        // We do this instead of just destructuring in order to prevent

        // `Unalign`'s `Drop::drop` from being run, since dropping is not

        // supported in `const fn`s.

        //

        // TODO(https://github.com/rust-lang/rust/issues/73255): Destructure

        // instead of using unsafe.

        unsafe { ManuallyDrop::into_inner(Transmute { u: ManuallyDrop::new(self) }.t) }

    }

    /// Attempts to return a reference to the wrapped `T`, failing if `self` is

    /// not properly aligned.

    ///

    /// If `self` does not satisfy `mem::align_of::<T>()`, then it is unsound to

    /// return a reference to the wrapped `T`, and `try_deref` returns `None`.

    ///

    /// If `T: Unaligned`, then `Unalign<T>` implements [`Deref`], and callers

    /// may prefer [`Deref::deref`], which is infallible.

    #[inline(always)]

    pub fn try_deref(&self) -> Option<&T> {

        if !util::aligned_to::<_, T>(self) {

            return None;

        }

        // SAFETY: `deref_unchecked`'s safety requirement is that `self` is

        // aligned to `align_of::<T>()`, which we just checked.

        unsafe { Some(self.deref_unchecked()) }

    }

    /// Attempts to return a mutable reference to the wrapped `T`, failing if

    /// `self` is not properly aligned.

    ///

    /// If `self` does not satisfy `mem::align_of::<T>()`, then it is unsound to

    /// return a reference to the wrapped `T`, and `try_deref_mut` returns

    /// `None`.

    ///

    /// If `T: Unaligned`, then `Unalign<T>` implements [`DerefMut`], and

    /// callers may prefer [`DerefMut::deref_mut`], which is infallible.

    #[inline(always)]

    pub fn try_deref_mut(&mut self) -> Option<&mut T> {

        if !util::aligned_to::<_, T>(&*self) {

            return None;

        }

        // SAFETY: `deref_mut_unchecked`'s safety requirement is that `self` is

        // aligned to `align_of::<T>()`, which we just checked.

        unsafe { Some(self.deref_mut_unchecked()) }

    }

    /// Returns a reference to the wrapped `T` without checking alignment.

    ///

    /// If `T: Unaligned`, then `Unalign<T>` implements[ `Deref`], and callers

    /// may prefer [`Deref::deref`], which is safe.

    ///

    /// # Safety

    ///

    /// If `self` does not satisfy `mem::align_of::<T>()`, then

    /// `self.deref_unchecked()` may cause undefined behavior.

    #[inline(always)]

    pub const unsafe fn deref_unchecked(&self) -> &T {

        // SAFETY: `Unalign<T>` is `repr(transparent)`, so there is a valid `T`

        // at the same memory location as `self`. It has no alignment guarantee,

        // but the caller has promised that `self` is properly aligned, so we

        // know that it is sound to create a reference to `T` at this memory

        // location.

        //

        // We use `mem::transmute` instead of `&*self.get_ptr()` because

        // dereferencing pointers is not stable in `const` on our current MSRV

        // (1.56 as of this writing).

        unsafe { mem::transmute(self) }

    }

    /// Returns a mutable reference to the wrapped `T` without checking

    /// alignment.

    ///

    /// If `T: Unaligned`, then `Unalign<T>` implements[ `DerefMut`], and

    /// callers may prefer [`DerefMut::deref_mut`], which is safe.

    ///

    /// # Safety

    ///

    /// If `self` does not satisfy `mem::align_of::<T>()`, then

    /// `self.deref_mut_unchecked()` may cause undefined behavior.

    #[inline(always)]

    pub unsafe fn deref_mut_unchecked(&mut self) -> &mut T {

        // SAFETY: `self.get_mut_ptr()` returns a raw pointer to a valid `T` at

        // the same memory location as `self`. It has no alignment guarantee,

        // but the caller has promised that `self` is properly aligned, so we

        // know that the pointer itself is aligned, and thus that it is sound to

        // create a reference to a `T` at this memory location.

        unsafe { &mut *self.get_mut_ptr() }

    }

    /// Gets an unaligned raw pointer to the inner `T`.

    ///

    /// # Safety

    ///

    /// The returned raw pointer is not necessarily aligned to

    /// `align_of::<T>()`. Most functions which operate on raw pointers require

    /// those pointers to be aligned, so calling those functions with the result

    /// of `get_ptr` will be undefined behavior if alignment is not guaranteed

    /// using some out-of-band mechanism. In general, the only functions which

    /// are safe to call with this pointer are those which are explicitly

    /// documented as being sound to use with an unaligned pointer, such as

    /// [`read_unaligned`].

    ///

    /// [`read_unaligned`]: core::ptr::read_unaligned

    #[inline(always)]

    pub const fn get_ptr(&self) -> *const T {

        ptr::addr_of!(self.0)

    }

    /// Gets an unaligned mutable raw pointer to the inner `T`.

    ///

    /// # Safety

    ///

    /// The returned raw pointer is not necessarily aligned to

    /// `align_of::<T>()`. Most functions which operate on raw pointers require

    /// those pointers to be aligned, so calling those functions with the result

    /// of `get_ptr` will be undefined behavior if alignment is not guaranteed

    /// using some out-of-band mechanism. In general, the only functions which

    /// are safe to call with this pointer are those which are explicitly

    /// documented as being sound to use with an unaligned pointer, such as

    /// [`read_unaligned`].

    ///

    /// [`read_unaligned`]: core::ptr::read_unaligned

    // TODO(https://github.com/rust-lang/rust/issues/57349): Make this `const`.

    #[inline(always)]

    pub fn get_mut_ptr(&mut self) -> *mut T {

        ptr::addr_of_mut!(self.0)

    }

    /// Sets the inner `T`, dropping the previous value.

    // TODO(https://github.com/rust-lang/rust/issues/57349): Make this `const`.

    #[inline(always)]

    pub fn set(&mut self, t: T) {

        *self = Unalign::new(t);

    }

    /// Updates the inner `T` by calling a function on it.

    ///

    /// If [`T: Unaligned`], then `Unalign<T>` implements [`DerefMut`], and that

    /// impl should be preferred over this method when performing updates, as it

    /// will usually be faster and more ergonomic.

    ///

    /// For large types, this method may be expensive, as it requires copying

    /// `2 * size_of::<T>()` bytes. \[1\]

    ///

    /// \[1\] Since the inner `T` may not be aligned, it would not be sound to

    /// invoke `f` on it directly. Instead, `update` moves it into a

    /// properly-aligned location in the local stack frame, calls `f` on it, and

    /// then moves it back to its original location in `self`.

    ///

    /// [`T: Unaligned`]: Unaligned

    #[inline]

    pub fn update<O, F: FnOnce(&mut T) -> O>(&mut self, f: F) -> O {

        // On drop, this moves `copy` out of itself and uses `ptr::write` to

        // overwrite `slf`.

        struct WriteBackOnDrop<T> {

            copy: ManuallyDrop<T>,

            slf: *mut Unalign<T>,

        }

        impl<T> Drop for WriteBackOnDrop<T> {

            fn drop(&mut self) {

                // SAFETY: We never use `copy` again as required by

                // `ManuallyDrop::take`.

                let copy = unsafe { ManuallyDrop::take(&mut self.copy) };

                // SAFETY: `slf` is the raw pointer value of `self`. We know it

                // is valid for writes and properly aligned because `self` is a

                // mutable reference, which guarantees both of these properties.

                unsafe { ptr::write(self.slf, Unalign::new(copy)) };

            }

        }

        // SAFETY: We know that `self` is valid for reads, properly aligned, and

        // points to an initialized `Unalign<T>` because it is a mutable

        // reference, which guarantees all of these properties.

        //

        // Since `T: !Copy`, it would be unsound in the general case to allow

        // both the original `Unalign<T>` and the copy to be used by safe code.

        // We guarantee that the copy is used to overwrite the original in the

        // `Drop::drop` impl of `WriteBackOnDrop`. So long as this `drop` is

        // called before any other safe code executes, soundness is upheld.

        // While this method can terminate in two ways (by returning normally or

        // by unwinding due to a panic in `f`), in both cases, `write_back` is

        // dropped - and its `drop` called - before any other safe code can

        // execute.

        let copy = unsafe { ptr::read(self) }.into_inner();

        let mut write_back = WriteBackOnDrop { copy: ManuallyDrop::new(copy), slf: self };

        let ret = f(&mut write_back.copy);

        drop(write_back);

        ret

    }

}

impl<T: Copy> Unalign<T> {

    /// Gets a copy of the inner `T`.

    // TODO(https://github.com/rust-lang/rust/issues/57349): Make this `const`.

    #[inline(always)]

    pub fn get(&self) -> T {

        let Unalign(val) = *self;

        val

    }

}

impl<T: Unaligned> Deref for Unalign<T> {

    type Target = T;

    #[inline(always)]

    fn deref(&self) -> &T {

        // SAFETY: `deref_unchecked`'s safety requirement is that `self` is

        // aligned to `align_of::<T>()`. `T: Unaligned` guarantees that

        // `align_of::<T>() == 1`, and all pointers are one-aligned because all

        // addresses are divisible by 1.

        unsafe { self.deref_unchecked() }

    }

}

impl<T: Unaligned> DerefMut for Unalign<T> {

    #[inline(always)]

    fn deref_mut(&mut self) -> &mut T {

        // SAFETY: `deref_mut_unchecked`'s safety requirement is that `self` is

        // aligned to `align_of::<T>()`. `T: Unaligned` guarantees that

        // `align_of::<T>() == 1`, and all pointers are one-aligned because all

        // addresses are divisible by 1.

        unsafe { self.deref_mut_unchecked() }

    }

}

impl<T: Unaligned + PartialOrd> PartialOrd<Unalign<T>> for Unalign<T> {

    #[inline(always)]

    fn partial_cmp(&self, other: &Unalign<T>) -> Option<Ordering> {

        PartialOrd::partial_cmp(self.deref(), other.deref())

    }

}

impl<T: Unaligned + Ord> Ord for Unalign<T> {

    #[inline(always)]

    fn cmp(&self, other: &Unalign<T>) -> Ordering {

        Ord::cmp(self.deref(), other.deref())

    }

}

impl<T: Unaligned + PartialEq> PartialEq<Unalign<T>> for Unalign<T> {

    #[inline(always)]

    fn eq(&self, other: &Unalign<T>) -> bool {

        PartialEq::eq(self.deref(), other.deref())

    }

}

impl<T: Unaligned + Eq> Eq for Unalign<T> {}

impl<T: Unaligned + Hash> Hash for Unalign<T> {

    #[inline(always)]

    fn hash<H>(&self, state: &mut H)

    where

        H: Hasher,

    {

        self.deref().hash(state);

    }

}

impl<T: Unaligned + Debug> Debug for Unalign<T> {

    #[inline(always)]

    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {

        Debug::fmt(self.deref(), f)

    }

}

impl<T: Unaligned + Display> Display for Unalign<T> {

    #[inline(always)]

    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {

        Display::fmt(self.deref(), f)

    }

}

#[cfg(test)]

mod tests {

    use core::panic::AssertUnwindSafe;

    use super::*;

    use crate::util::testutil::*;

    /// A `T` which is guaranteed not to satisfy `align_of::<A>()`.

    ///

    /// It must be the case that `align_of::<T>() < align_of::<A>()` in order

    /// fot this type to work properly.

    #[repr(C)]

    struct ForceUnalign<T, A> {

        // The outer struct is aligned to `A`, and, thanks to `repr(C)`, `t` is

        // placed at the minimum offset that guarantees its alignment. If

        // `align_of::<T>() < align_of::<A>()`, then that offset will be

        // guaranteed *not* to satisfy `align_of::<A>()`.

        _u: u8,

        t: T,

        _a: [A; 0],

    }

    impl<T, A> ForceUnalign<T, A> {

        const fn new(t: T) -> ForceUnalign<T, A> {

            ForceUnalign { _u: 0, t, _a: [] }

        }

    }

    #[test]

    fn test_unalign() {

        // Test methods that don't depend on alignment.

        let mut u = Unalign::new(AU64(123));

        assert_eq!(u.get(), AU64(123));

        assert_eq!(u.into_inner(), AU64(123));

        assert_eq!(u.get_ptr(), <*const _>::cast::<AU64>(&u));

        assert_eq!(u.get_mut_ptr(), <*mut _>::cast::<AU64>(&mut u));

        u.set(AU64(321));

        assert_eq!(u.get(), AU64(321));

        // Test methods that depend on alignment (when alignment is satisfied).

        let mut u: Align<_, AU64> = Align::new(Unalign::new(AU64(123)));

        assert_eq!(u.t.try_deref(), Some(&AU64(123)));

        assert_eq!(u.t.try_deref_mut(), Some(&mut AU64(123)));

        // SAFETY: The `Align<_, AU64>` guarantees proper alignment.

        assert_eq!(unsafe { u.t.deref_unchecked() }, &AU64(123));

        // SAFETY: The `Align<_, AU64>` guarantees proper alignment.

        assert_eq!(unsafe { u.t.deref_mut_unchecked() }, &mut AU64(123));

        *u.t.try_deref_mut().unwrap() = AU64(321);

        assert_eq!(u.t.get(), AU64(321));

        // Test methods that depend on alignment (when alignment is not

        // satisfied).

        let mut u: ForceUnalign<_, AU64> = ForceUnalign::new(Unalign::new(AU64(123)));

        assert_eq!(u.t.try_deref(), None);

        assert_eq!(u.t.try_deref_mut(), None);

        // Test methods that depend on `T: Unaligned`.

        let mut u = Unalign::new(123u8);

        assert_eq!(u.try_deref(), Some(&123));

        assert_eq!(u.try_deref_mut(), Some(&mut 123));

        assert_eq!(u.deref(), &123);

        assert_eq!(u.deref_mut(), &mut 123);

        *u = 21;

        assert_eq!(u.get(), 21);

        // Test that some `Unalign` functions and methods are `const`.

        const _UNALIGN: Unalign<u64> = Unalign::new(0);

        const _UNALIGN_PTR: *const u64 = _UNALIGN.get_ptr();

        const _U64: u64 = _UNALIGN.into_inner();

        // Make sure all code is considered "used".

        //

        // TODO(https://github.com/rust-lang/rust/issues/104084): Remove this

        // attribute.

        #[allow(dead_code)]

        const _: () = {

            let x: Align<_, AU64> = Align::new(Unalign::new(AU64(123)));

            // Make sure that `deref_unchecked` is `const`.

            //

            // SAFETY: The `Align<_, AU64>` guarantees proper alignment.

            let au64 = unsafe { x.t.deref_unchecked() };

            match au64 {

                AU64(123) => {}

                _ => unreachable!(),

            }

        };

    }

    #[test]

    fn test_unalign_update() {

        let mut u = Unalign::new(AU64(123));

        u.update(|a| a.0 += 1);

        assert_eq!(u.get(), AU64(124));

        // Test that, even if the callback panics, the original is still

        // correctly overwritten. Use a `Box` so that Miri is more likely to

        // catch any unsoundness (which would likely result in two `Box`es for

        // the same heap object, which is the sort of thing that Miri would

        // probably catch).

        let mut u = Unalign::new(Box::new(AU64(123)));

        let res = std::panic::catch_unwind(AssertUnwindSafe(|| {

            u.update(|a| {

                a.0 += 1;

                panic!();

            })

        }));

        assert!(res.is_err());

        assert_eq!(u.into_inner(), Box::new(AU64(124)));

    }

}