# vec.rs - source
Source of the Rust file `C:\Users\zenn\.cargo\registry\src\index.crates.io-6f17d22bba15001f\heapless-0.7.16\src\vec.rs`.

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    1600

    use core::{
        cmp::Ordering, convert::TryFrom, fmt, hash, iter::FromIterator, mem::MaybeUninit, ops, ptr,
        slice,
    };
    use hash32;

    /// A fixed capacity [`Vec`](https://doc.rust-lang.org/std/vec/struct.Vec.html)
    ///
    /// # Examples
    ///
    /// ```
    /// use heapless::Vec;
    ///
    ///
    /// // A vector with a fixed capacity of 8 elements allocated on the stack
    /// let mut vec = Vec::<_, 8>::new();
    /// vec.push(1);
    /// vec.push(2);
    ///
    /// assert_eq!(vec.len(), 2);
    /// assert_eq!(vec[0], 1);
    ///
    /// assert_eq!(vec.pop(), Some(2));
    /// assert_eq!(vec.len(), 1);
    ///
    /// vec[0] = 7;
    /// assert_eq!(vec[0], 7);
    ///
    /// vec.extend([1, 2, 3].iter().cloned());
    ///
    /// for x in &vec {
    ///     println!("{}", x);
    /// }
    /// assert_eq!(*vec, [7, 1, 2, 3]);
    /// ```
    pub struct Vec<T, const N: usize> {
        // NOTE order is important for optimizations. the `len` first layout lets the compiler optimize
        // `new` to: reserve stack space and zero the first word. With the fields in the reverse order
        // the compiler optimizes `new` to `memclr`-ing the *entire* stack space, including the `buffer`
        // field which should be left uninitialized. Optimizations were last checked with Rust 1.60
        len: usize,

        buffer: [MaybeUninit<T>; N],
    }

    impl<T, const N: usize> Vec<T, N> {
        const ELEM: MaybeUninit<T> = MaybeUninit::uninit();
        const INIT: [MaybeUninit<T>; N] = [Self::ELEM; N]; // important for optimization of `new`

        /// Constructs a new, empty vector with a fixed capacity of `N`
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// // allocate the vector on the stack
        /// let mut x: Vec<u8, 16> = Vec::new();
        ///
        /// // allocate the vector in a static variable
        /// static mut X: Vec<u8, 16> = Vec::new();
        /// ```
        /// `Vec` `const` constructor; wrap the returned value in [`Vec`](../struct.Vec.html)
        pub const fn new() -> Self {
            // Const assert N >= 0
            crate::sealed::greater_than_eq_0::<N>();

            Self {
                len: 0,
                buffer: Self::INIT,
            }
        }

        /// Constructs a new vector with a fixed capacity of `N` and fills it
        /// with the provided slice.
        ///
        /// This is equivalent to the following code:
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut v: Vec<u8, 16> = Vec::new();
        /// v.extend_from_slice(&[1, 2, 3]).unwrap();
        /// ```
        #[inline]
        pub fn from_slice(other: &[T]) -> Result<Self, ()>
        where
            T: Clone,
        {
            let mut v = Vec::new();
            v.extend_from_slice(other)?;
            Ok(v)
        }

        /// Clones a vec into a new vec
        pub(crate) fn clone(&self) -> Self
        where
            T: Clone,
        {
            let mut new = Self::new();
            // avoid `extend_from_slice` as that introduces a runtime check / panicking branch
            for elem in self {
                unsafe {
                    new.push_unchecked(elem.clone());
                }
            }
            new
        }

        /// Returns a raw pointer to the vector’s buffer.
        pub fn as_ptr(&self) -> *const T {
            self.buffer.as_ptr() as *const T
        }

        /// Returns a raw pointer to the vector’s buffer, which may be mutated through.
        pub fn as_mut_ptr(&mut self) -> *mut T {
            self.buffer.as_mut_ptr() as *mut T
        }

        /// Extracts a slice containing the entire vector.
        ///
        /// Equivalent to `&s[..]`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        /// let buffer: Vec<u8, 5> = Vec::from_slice(&[1, 2, 3, 5, 8]).unwrap();
        /// assert_eq!(buffer.as_slice(), &[1, 2, 3, 5, 8]);
        /// ```
        pub fn as_slice(&self) -> &[T] {
            // NOTE(unsafe) avoid bound checks in the slicing operation
            // &buffer[..self.len]
            unsafe { slice::from_raw_parts(self.buffer.as_ptr() as *const T, self.len) }
        }

        /// Returns the contents of the vector as an array of length `M` if the length
        /// of the vector is exactly `M`, otherwise returns `Err(self)`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        /// let buffer: Vec<u8, 42> = Vec::from_slice(&[1, 2, 3, 5, 8]).unwrap();
        /// let array: [u8; 5] = buffer.into_array().unwrap();
        /// assert_eq!(array, [1, 2, 3, 5, 8]);
        /// ```
        pub fn into_array<const M: usize>(self) -> Result<[T; M], Self> {
            if self.len() == M {
                // This is how the unstable `MaybeUninit::array_assume_init` method does it
                let array = unsafe { (&self.buffer as *const _ as *const [T; M]).read() };

                // We don't want `self`'s destructor to be called because that would drop all the
                // items in the array
                core::mem::forget(self);

                Ok(array)
            } else {
                Err(self)
            }
        }

        /// Extracts a mutable slice containing the entire vector.
        ///
        /// Equivalent to `&s[..]`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        /// let mut buffer: Vec<u8, 5> = Vec::from_slice(&[1, 2, 3, 5, 8]).unwrap();
        /// buffer[0] = 9;
        /// assert_eq!(buffer.as_slice(), &[9, 2, 3, 5, 8]);
        /// ```
        pub(crate) fn as_mut_slice(&mut self) -> &mut [T] {
            // NOTE(unsafe) avoid bound checks in the slicing operation
            // &mut buffer[..self.len]
            unsafe { slice::from_raw_parts_mut(self.buffer.as_mut_ptr() as *mut T, self.len) }
        }

        /// Returns the maximum number of elements the vector can hold.
        pub const fn capacity(&self) -> usize {
            N
        }

        /// Clears the vector, removing all values.
        pub fn clear(&mut self) {
            self.truncate(0);
        }

        /// Extends the vec from an iterator.
        ///
        /// # Panic
        ///
        /// Panics if the vec cannot hold all elements of the iterator.
        pub fn extend<I>(&mut self, iter: I)
        where
            I: IntoIterator<Item = T>,
        {
            for elem in iter {
                self.push(elem).ok().unwrap()
            }
        }

        /// Clones and appends all elements in a slice to the `Vec`.
        ///
        /// Iterates over the slice `other`, clones each element, and then appends
        /// it to this `Vec`. The `other` vector is traversed in-order.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut vec = Vec::<u8, 8>::new();
        /// vec.push(1).unwrap();
        /// vec.extend_from_slice(&[2, 3, 4]).unwrap();
        /// assert_eq!(*vec, [1, 2, 3, 4]);
        /// ```
        pub fn extend_from_slice(&mut self, other: &[T]) -> Result<(), ()>
        where
            T: Clone,
        {
            if self.len + other.len() > self.capacity() {
                // won't fit in the `Vec`; don't modify anything and return an error
                Err(())
            } else {
                for elem in other {
                    unsafe {
                        self.push_unchecked(elem.clone());
                    }
                }
                Ok(())
            }
        }

        /// Removes the last element from a vector and returns it, or `None` if it's empty
        pub fn pop(&mut self) -> Option<T> {
            if self.len != 0 {
                Some(unsafe { self.pop_unchecked() })
            } else {
                None
            }
        }

        /// Appends an `item` to the back of the collection
        ///
        /// Returns back the `item` if the vector is full
        pub fn push(&mut self, item: T) -> Result<(), T> {
            if self.len < self.capacity() {
                unsafe { self.push_unchecked(item) }
                Ok(())
            } else {
                Err(item)
            }
        }

        /// Removes the last element from a vector and returns it
        ///
        /// # Safety
        ///
        /// This assumes the vec to have at least one element.
        pub unsafe fn pop_unchecked(&mut self) -> T {
            debug_assert!(!self.is_empty());

            self.len -= 1;
            (self.buffer.get_unchecked_mut(self.len).as_ptr() as *const T).read()
        }

        /// Appends an `item` to the back of the collection
        ///
        /// # Safety
        ///
        /// This assumes the vec is not full.
        pub unsafe fn push_unchecked(&mut self, item: T) {
            // NOTE(ptr::write) the memory slot that we are about to write to is uninitialized. We
            // use `ptr::write` to avoid running `T`'s destructor on the uninitialized memory
            debug_assert!(!self.is_full());

            *self.buffer.get_unchecked_mut(self.len) = MaybeUninit::new(item);

            self.len += 1;
        }

        /// Shortens the vector, keeping the first `len` elements and dropping the rest.
        pub fn truncate(&mut self, len: usize) {
            // This is safe because:
            //
            // * the slice passed to `drop_in_place` is valid; the `len > self.len`
            //   case avoids creating an invalid slice, and
            // * the `len` of the vector is shrunk before calling `drop_in_place`,
            //   such that no value will be dropped twice in case `drop_in_place`
            //   were to panic once (if it panics twice, the program aborts).
            unsafe {
                // Note: It's intentional that this is `>` and not `>=`.
                //       Changing it to `>=` has negative performance
                //       implications in some cases. See rust-lang/rust#78884 for more.
                if len > self.len {
                    return;
                }
                let remaining_len = self.len - len;
                let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
                self.len = len;
                ptr::drop_in_place(s);
            }
        }

        /// Resizes the Vec in-place so that len is equal to new_len.
        ///
        /// If new_len is greater than len, the Vec is extended by the
        /// difference, with each additional slot filled with value. If
        /// new_len is less than len, the Vec is simply truncated.
        ///
        /// See also [`resize_default`](struct.Vec.html#method.resize_default).
        pub fn resize(&mut self, new_len: usize, value: T) -> Result<(), ()>
        where
            T: Clone,
        {
            if new_len > self.capacity() {
                return Err(());
            }

            if new_len > self.len {
                while self.len < new_len {
                    self.push(value.clone()).ok();
                }
            } else {
                self.truncate(new_len);
            }

            Ok(())
        }

        /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
        ///
        /// If `new_len` is greater than `len`, the `Vec` is extended by the
        /// difference, with each additional slot filled with `Default::default()`.
        /// If `new_len` is less than `len`, the `Vec` is simply truncated.
        ///
        /// See also [`resize`](struct.Vec.html#method.resize).
        pub fn resize_default(&mut self, new_len: usize) -> Result<(), ()>
        where
            T: Clone + Default,
        {
            self.resize(new_len, T::default())
        }

        /// Forces the length of the vector to `new_len`.
        ///
        /// This is a low-level operation that maintains none of the normal
        /// invariants of the type. Normally changing the length of a vector
        /// is done using one of the safe operations instead, such as
        /// [`truncate`], [`resize`], [`extend`], or [`clear`].
        ///
        /// [`truncate`]: #method.truncate
        /// [`resize`]: #method.resize
        /// [`extend`]: https://doc.rust-lang.org/stable/core/iter/trait.Extend.html#tymethod.extend
        /// [`clear`]: #method.clear
        ///
        /// # Safety
        ///
        /// - `new_len` must be less than or equal to [`capacity()`].
        /// - The elements at `old_len..new_len` must be initialized.
        ///
        /// [`capacity()`]: #method.capacity
        ///
        /// # Examples
        ///
        /// This method can be useful for situations in which the vector
        /// is serving as a buffer for other code, particularly over FFI:
        ///
        /// ```no_run
        /// # #![allow(dead_code)]
        /// use heapless::Vec;
        ///
        /// # // This is just a minimal skeleton for the doc example;
        /// # // don't use this as a starting point for a real library.
        /// # pub struct StreamWrapper { strm: *mut core::ffi::c_void }
        /// # const Z_OK: i32 = 0;
        /// # extern "C" {
        /// #     fn deflateGetDictionary(
        /// #         strm: *mut core::ffi::c_void,
        /// #         dictionary: *mut u8,
        /// #         dictLength: *mut usize,
        /// #     ) -> i32;
        /// # }
        /// # impl StreamWrapper {
        /// pub fn get_dictionary(&self) -> Option<Vec<u8, 32768>> {
        ///     // Per the FFI method's docs, "32768 bytes is always enough".
        ///     let mut dict = Vec::new();
        ///     let mut dict_length = 0;
        ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
        ///     // 1. `dict_length` elements were initialized.
        ///     // 2. `dict_length` <= the capacity (32_768)
        ///     // which makes `set_len` safe to call.
        ///     unsafe {
        ///         // Make the FFI call...
        ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
        ///         if r == Z_OK {
        ///             // ...and update the length to what was initialized.
        ///             dict.set_len(dict_length);
        ///             Some(dict)
        ///         } else {
        ///             None
        ///         }
        ///     }
        /// }
        /// # }
        /// ```
        ///
        /// While the following example is sound, there is a memory leak since
        /// the inner vectors were not freed prior to the `set_len` call:
        ///
        /// ```
        /// use core::iter::FromIterator;
        /// use heapless::Vec;
        ///
        /// let mut vec = Vec::<Vec<u8, 3>, 3>::from_iter(
        ///     [
        ///         Vec::from_iter([1, 0, 0].iter().cloned()),
        ///         Vec::from_iter([0, 1, 0].iter().cloned()),
        ///         Vec::from_iter([0, 0, 1].iter().cloned()),
        ///     ]
        ///     .iter()
        ///     .cloned()
        /// );
        /// // SAFETY:
        /// // 1. `old_len..0` is empty so no elements need to be initialized.
        /// // 2. `0 <= capacity` always holds whatever `capacity` is.
        /// unsafe {
        ///     vec.set_len(0);
        /// }
        /// ```
        ///
        /// Normally, here, one would use [`clear`] instead to correctly drop
        /// the contents and thus not leak memory.
        pub unsafe fn set_len(&mut self, new_len: usize) {
            debug_assert!(new_len <= self.capacity());

            self.len = new_len
        }

        /// Removes an element from the vector and returns it.
        ///
        /// The removed element is replaced by the last element of the vector.
        ///
        /// This does not preserve ordering, but is O(1).
        ///
        /// # Panics
        ///
        /// Panics if `index` is out of bounds.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///// use heapless::consts::*;
        ///
        /// let mut v: Vec<_, 8> = Vec::new();
        /// v.push("foo").unwrap();
        /// v.push("bar").unwrap();
        /// v.push("baz").unwrap();
        /// v.push("qux").unwrap();
        ///
        /// assert_eq!(v.swap_remove(1), "bar");
        /// assert_eq!(&*v, ["foo", "qux", "baz"]);
        ///
        /// assert_eq!(v.swap_remove(0), "foo");
        /// assert_eq!(&*v, ["baz", "qux"]);
        /// ```
        pub fn swap_remove(&mut self, index: usize) -> T {
            assert!(index < self.len);
            unsafe { self.swap_remove_unchecked(index) }
        }

        /// Removes an element from the vector and returns it.
        ///
        /// The removed element is replaced by the last element of the vector.
        ///
        /// This does not preserve ordering, but is O(1).
        ///
        /// # Safety
        ///
        ///  Assumes `index` within bounds.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut v: Vec<_, 8> = Vec::new();
        /// v.push("foo").unwrap();
        /// v.push("bar").unwrap();
        /// v.push("baz").unwrap();
        /// v.push("qux").unwrap();
        ///
        /// assert_eq!(unsafe { v.swap_remove_unchecked(1) }, "bar");
        /// assert_eq!(&*v, ["foo", "qux", "baz"]);
        ///
        /// assert_eq!(unsafe { v.swap_remove_unchecked(0) }, "foo");
        /// assert_eq!(&*v, ["baz", "qux"]);
        /// ```
        pub unsafe fn swap_remove_unchecked(&mut self, index: usize) -> T {
            let length = self.len();
            debug_assert!(index < length);
            let value = ptr::read(self.as_ptr().add(index));
            let base_ptr = self.as_mut_ptr();
            ptr::copy(base_ptr.add(length - 1), base_ptr.add(index), 1);
            self.len -= 1;
            value
        }

        /// Returns true if the vec is full
        #[inline]
        pub fn is_full(&self) -> bool {
            self.len == self.capacity()
        }

        /// Returns true if the vec is empty
        #[inline]
        pub fn is_empty(&self) -> bool {
            self.len == 0
        }

        /// Returns `true` if `needle` is a prefix of the Vec.
        ///
        /// Always returns `true` if `needle` is an empty slice.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let v: Vec<_, 8> = Vec::from_slice(b"abc").unwrap();
        /// assert_eq!(v.starts_with(b""), true);
        /// assert_eq!(v.starts_with(b"ab"), true);
        /// assert_eq!(v.starts_with(b"bc"), false);
        /// ```
        #[inline]
        pub fn starts_with(&self, needle: &[T]) -> bool
        where
            T: PartialEq,
        {
            let n = needle.len();
            self.len >= n && needle == &self[..n]
        }

        /// Returns `true` if `needle` is a suffix of the Vec.
        ///
        /// Always returns `true` if `needle` is an empty slice.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let v: Vec<_, 8> = Vec::from_slice(b"abc").unwrap();
        /// assert_eq!(v.ends_with(b""), true);
        /// assert_eq!(v.ends_with(b"ab"), false);
        /// assert_eq!(v.ends_with(b"bc"), true);
        /// ```
        #[inline]
        pub fn ends_with(&self, needle: &[T]) -> bool
        where
            T: PartialEq,
        {
            let (v, n) = (self.len(), needle.len());
            v >= n && needle == &self[v - n..]
        }

        /// Inserts an element at position `index` within the vector, shifting all
        /// elements after it to the right.
        ///
        /// Returns back the `element` if the vector is full.
        ///
        /// # Panics
        ///
        /// Panics if `index > len`.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut vec: Vec<_, 8> = Vec::from_slice(&[1, 2, 3]).unwrap();
        /// vec.insert(1, 4);
        /// assert_eq!(vec, [1, 4, 2, 3]);
        /// vec.insert(4, 5);
        /// assert_eq!(vec, [1, 4, 2, 3, 5]);
        /// ```
        pub fn insert(&mut self, index: usize, element: T) -> Result<(), T> {
            let len = self.len();
            if index > len {
                panic!(
                    "insertion index (is {}) should be <= len (is {})",
                    index, len
                );
            }

            // check there's space for the new element
            if self.is_full() {
                return Err(element);
            }

            unsafe {
                // infallible
                // The spot to put the new value
                {
                    let p = self.as_mut_ptr().add(index);
                    // Shift everything over to make space. (Duplicating the
                    // `index`th element into two consecutive places.)
                    ptr::copy(p, p.offset(1), len - index);
                    // Write it in, overwriting the first copy of the `index`th
                    // element.
                    ptr::write(p, element);
                }
                self.set_len(len + 1);
            }

            Ok(())
        }

        /// Removes and returns the element at position `index` within the vector,
        /// shifting all elements after it to the left.
        ///
        /// Note: Because this shifts over the remaining elements, it has a
        /// worst-case performance of *O*(*n*). If you don't need the order of
        /// elements to be preserved, use [`swap_remove`] instead. If you'd like to
        /// remove elements from the beginning of the `Vec`, consider using
        /// [`Deque::pop_front`] instead.
        ///
        /// [`swap_remove`]: Vec::swap_remove
        /// [`Deque::pop_front`]: crate::Deque::pop_front
        ///
        /// # Panics
        ///
        /// Panics if `index` is out of bounds.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut v: Vec<_, 8> = Vec::from_slice(&[1, 2, 3]).unwrap();
        /// assert_eq!(v.remove(1), 2);
        /// assert_eq!(v, [1, 3]);
        /// ```
        pub fn remove(&mut self, index: usize) -> T {
            let len = self.len();
            if index >= len {
                panic!("removal index (is {}) should be < len (is {})", index, len);
            }
            unsafe {
                // infallible
                let ret;
                {
                    // the place we are taking from.
                    let ptr = self.as_mut_ptr().add(index);
                    // copy it out, unsafely having a copy of the value on
                    // the stack and in the vector at the same time.
                    ret = ptr::read(ptr);

                    // Shift everything down to fill in that spot.
                    ptr::copy(ptr.offset(1), ptr, len - index - 1);
                }
                self.set_len(len - 1);
                ret
            }
        }

        /// Retains only the elements specified by the predicate.
        ///
        /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
        /// This method operates in place, visiting each element exactly once in the
        /// original order, and preserves the order of the retained elements.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut vec: Vec<_, 8> = Vec::from_slice(&[1, 2, 3, 4]).unwrap();
        /// vec.retain(|&x| x % 2 == 0);
        /// assert_eq!(vec, [2, 4]);
        /// ```
        ///
        /// Because the elements are visited exactly once in the original order,
        /// external state may be used to decide which elements to keep.
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut vec: Vec<_, 8> = Vec::from_slice(&[1, 2, 3, 4, 5]).unwrap();
        /// let keep = [false, true, true, false, true];
        /// let mut iter = keep.iter();
        /// vec.retain(|_| *iter.next().unwrap());
        /// assert_eq!(vec, [2, 3, 5]);
        /// ```
        pub fn retain<F>(&mut self, mut f: F)
        where
            F: FnMut(&T) -> bool,
        {
            self.retain_mut(|elem| f(elem));
        }

        /// Retains only the elements specified by the predicate, passing a mutable reference to it.
        ///
        /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
        /// This method operates in place, visiting each element exactly once in the
        /// original order, and preserves the order of the retained elements.
        ///
        /// # Examples
        ///
        /// ```
        /// use heapless::Vec;
        ///
        /// let mut vec: Vec<_, 8> = Vec::from_slice(&[1, 2, 3, 4]).unwrap();
        /// vec.retain_mut(|x| if *x <= 3 {
        ///     *x += 1;
        ///     true
        /// } else {
        ///     false
        /// });
        /// assert_eq!(vec, [2, 3, 4]);
        /// ```
        pub fn retain_mut<F>(&mut self, mut f: F)
        where
            F: FnMut(&mut T) -> bool,
        {
            let original_len = self.len();
            // Avoid double drop if the drop guard is not executed,
            // since we may make some holes during the process.
            unsafe { self.set_len(0) };

            // Vec: [Kept, Kept, Hole, Hole, Hole, Hole, Unchecked, Unchecked]
            //      |<-              processed len   ->| ^- next to check
            //                  |<-  deleted cnt     ->|
            //      |<-              original_len                          ->|
            // Kept: Elements which predicate returns true on.
            // Hole: Moved or dropped element slot.
            // Unchecked: Unchecked valid elements.
            //
            // This drop guard will be invoked when predicate or `drop` of element panicked.
            // It shifts unchecked elements to cover holes and `set_len` to the correct length.
            // In cases when predicate and `drop` never panick, it will be optimized out.
            struct BackshiftOnDrop<'a, T, const N: usize> {
                v: &'a mut Vec<T, N>,
                processed_len: usize,
                deleted_cnt: usize,
                original_len: usize,
            }

            impl<T, const N: usize> Drop for BackshiftOnDrop<'_, T, N> {
                fn drop(&mut self) {
                    if self.deleted_cnt > 0 {
                        // SAFETY: Trailing unchecked items must be valid since we never touch them.
                        unsafe {
                            ptr::copy(
                                self.v.as_ptr().add(self.processed_len),
                                self.v
                                    .as_mut_ptr()
                                    .add(self.processed_len - self.deleted_cnt),
                                self.original_len - self.processed_len,
                            );
                        }
                    }
                    // SAFETY: After filling holes, all items are in contiguous memory.
                    unsafe {
                        self.v.set_len(self.original_len - self.deleted_cnt);
                    }
                }
            }

            let mut g = BackshiftOnDrop {
                v: self,
                processed_len: 0,
                deleted_cnt: 0,
                original_len,
            };

            fn process_loop<F, T, const N: usize, const DELETED: bool>(
                original_len: usize,
                f: &mut F,
                g: &mut BackshiftOnDrop<'_, T, N>,
            ) where
                F: FnMut(&mut T) -> bool,
            {
                while g.processed_len != original_len {
                    let p = g.v.as_mut_ptr();
                    // SAFETY: Unchecked element must be valid.
                    let cur = unsafe { &mut *p.add(g.processed_len) };
                    if !f(cur) {
                        // Advance early to avoid double drop if `drop_in_place` panicked.
                        g.processed_len += 1;
                        g.deleted_cnt += 1;
                        // SAFETY: We never touch this element again after dropped.
                        unsafe { ptr::drop_in_place(cur) };
                        // We already advanced the counter.
                        if DELETED {
                            continue;
                        } else {
                            break;
                        }
                    }
                    if DELETED {
                        // SAFETY: `deleted_cnt` > 0, so the hole slot must not overlap with current element.
                        // We use copy for move, and never touch this element again.
                        unsafe {
                            let hole_slot = p.add(g.processed_len - g.deleted_cnt);
                            ptr::copy_nonoverlapping(cur, hole_slot, 1);
                        }
                    }
                    g.processed_len += 1;
                }
            }

            // Stage 1: Nothing was deleted.
            process_loop::<F, T, N, false>(original_len, &mut f, &mut g);

            // Stage 2: Some elements were deleted.
            process_loop::<F, T, N, true>(original_len, &mut f, &mut g);

            // All item are processed. This can be optimized to `set_len` by LLVM.
            drop(g);
        }
    }

    // Trait implementations

    impl<T, const N: usize> Default for Vec<T, N> {
        fn default() -> Self {
            Self::new()
        }
    }

    impl<T, const N: usize> fmt::Debug for Vec<T, N>
    where
        T: fmt::Debug,
    {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            <[T] as fmt::Debug>::fmt(self, f)
        }
    }

    impl<const N: usize> fmt::Write for Vec<u8, N> {
        fn write_str(&mut self, s: &str) -> fmt::Result {
            match self.extend_from_slice(s.as_bytes()) {
                Ok(()) => Ok(()),
                Err(_) => Err(fmt::Error),
            }
        }
    }

    impl<T, const N: usize> Drop for Vec<T, N> {
        fn drop(&mut self) {
            // We drop each element used in the vector by turning into a &mut[T]
            unsafe {
                ptr::drop_in_place(self.as_mut_slice());
            }
        }
    }

    impl<'a, T: Clone, const N: usize> TryFrom<&'a [T]> for Vec<T, N> {
        type Error = ();

        fn try_from(slice: &'a [T]) -> Result<Self, Self::Error> {
            Vec::from_slice(slice)
        }
    }

    impl<T, const N: usize> Extend<T> for Vec<T, N> {
        fn extend<I>(&mut self, iter: I)
        where
            I: IntoIterator<Item = T>,
        {
            self.extend(iter)
        }
    }

    impl<'a, T, const N: usize> Extend<&'a T> for Vec<T, N>
    where
        T: 'a + Copy,
    {
        fn extend<I>(&mut self, iter: I)
        where
            I: IntoIterator<Item = &'a T>,
        {
            self.extend(iter.into_iter().cloned())
        }
    }

    impl<T, const N: usize> hash::Hash for Vec<T, N>
    where
        T: core::hash::Hash,
    {
        fn hash<H: hash::Hasher>(&self, state: &mut H) {
            <[T] as hash::Hash>::hash(self, state)
        }
    }

    impl<T, const N: usize> hash32::Hash for Vec<T, N>
    where
        T: hash32::Hash,
    {
        fn hash<H: hash32::Hasher>(&self, state: &mut H) {
            <[T] as hash32::Hash>::hash(self, state)
        }
    }

    impl<'a, T, const N: usize> IntoIterator for &'a Vec<T, N> {
        type Item = &'a T;
        type IntoIter = slice::Iter<'a, T>;

        fn into_iter(self) -> Self::IntoIter {
            self.iter()
        }
    }

    impl<'a, T, const N: usize> IntoIterator for &'a mut Vec<T, N> {
        type Item = &'a mut T;
        type IntoIter = slice::IterMut<'a, T>;

        fn into_iter(self) -> Self::IntoIter {
            self.iter_mut()
        }
    }

    impl<T, const N: usize> FromIterator<T> for Vec<T, N> {
        fn from_iter<I>(iter: I) -> Self
        where
            I: IntoIterator<Item = T>,
        {
            let mut vec = Vec::new();
            for i in iter {
                vec.push(i).ok().expect("Vec::from_iter overflow");
            }
            vec
        }
    }

    /// An iterator that moves out of an [`Vec`][`Vec`].
    ///
    /// This struct is created by calling the `into_iter` method on [`Vec`][`Vec`].
    ///
    /// [`Vec`]: (https://doc.rust-lang.org/std/vec/struct.Vec.html)
    ///
    pub struct IntoIter<T, const N: usize> {
        vec: Vec<T, N>,
        next: usize,
    }

    impl<T, const N: usize> Iterator for IntoIter<T, N> {
        type Item = T;
        fn next(&mut self) -> Option<Self::Item> {
            if self.next < self.vec.len() {
                let item = unsafe {
                    (self.vec.buffer.get_unchecked_mut(self.next).as_ptr() as *const T).read()
                };
                self.next += 1;
                Some(item)
            } else {
                None
            }
        }
    }

    impl<T, const N: usize> Clone for IntoIter<T, N>
    where
        T: Clone,
    {
        fn clone(&self) -> Self {
            let mut vec = Vec::new();

            if self.next < self.vec.len() {
                let s = unsafe {
                    slice::from_raw_parts(
                        (self.vec.buffer.as_ptr() as *const T).add(self.next),
                        self.vec.len() - self.next,
                    )
                };
                vec.extend_from_slice(s).ok();
            }

            Self { vec, next: 0 }
        }
    }

    impl<T, const N: usize> Drop for IntoIter<T, N> {
        fn drop(&mut self) {
            unsafe {
                // Drop all the elements that have not been moved out of vec
                ptr::drop_in_place(&mut self.vec.as_mut_slice()[self.next..]);
                // Prevent dropping of other elements
                self.vec.len = 0;
            }
        }
    }

    impl<T, const N: usize> IntoIterator for Vec<T, N> {
        type Item = T;
        type IntoIter = IntoIter<T, N>;

        fn into_iter(self) -> Self::IntoIter {
            IntoIter { vec: self, next: 0 }
        }
    }

    impl<A, B, const N1: usize, const N2: usize> PartialEq<Vec<B, N2>> for Vec<A, N1>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<B, N2>) -> bool {
            <[A]>::eq(self, &**other)
        }
    }

    // Vec<A, N> == [B]
    impl<A, B, const N: usize> PartialEq<[B]> for Vec<A, N>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &[B]) -> bool {
            <[A]>::eq(self, &other[..])
        }
    }

    // [B] == Vec<A, N>
    impl<A, B, const N: usize> PartialEq<Vec<A, N>> for [B]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<A, N>) -> bool {
            <[A]>::eq(other, &self[..])
        }
    }

    // Vec<A, N> == &[B]
    impl<A, B, const N: usize> PartialEq<&[B]> for Vec<A, N>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &&[B]) -> bool {
            <[A]>::eq(self, &other[..])
        }
    }

    // &[B] == Vec<A, N>
    impl<A, B, const N: usize> PartialEq<Vec<A, N>> for &[B]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<A, N>) -> bool {
            <[A]>::eq(other, &self[..])
        }
    }

    // Vec<A, N> == &mut [B]
    impl<A, B, const N: usize> PartialEq<&mut [B]> for Vec<A, N>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &&mut [B]) -> bool {
            <[A]>::eq(self, &other[..])
        }
    }

    // &mut [B] == Vec<A, N>
    impl<A, B, const N: usize> PartialEq<Vec<A, N>> for &mut [B]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<A, N>) -> bool {
            <[A]>::eq(other, &self[..])
        }
    }

    // Vec<A, N> == [B; M]
    // Equality does not require equal capacity
    impl<A, B, const N: usize, const M: usize> PartialEq<[B; M]> for Vec<A, N>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &[B; M]) -> bool {
            <[A]>::eq(self, &other[..])
        }
    }

    // [B; M] == Vec<A, N>
    // Equality does not require equal capacity
    impl<A, B, const N: usize, const M: usize> PartialEq<Vec<A, N>> for [B; M]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<A, N>) -> bool {
            <[A]>::eq(other, &self[..])
        }
    }

    // Vec<A, N> == &[B; M]
    // Equality does not require equal capacity
    impl<A, B, const N: usize, const M: usize> PartialEq<&[B; M]> for Vec<A, N>
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &&[B; M]) -> bool {
            <[A]>::eq(self, &other[..])
        }
    }

    // &[B; M] == Vec<A, N>
    // Equality does not require equal capacity
    impl<A, B, const N: usize, const M: usize> PartialEq<Vec<A, N>> for &[B; M]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &Vec<A, N>) -> bool {
            <[A]>::eq(other, &self[..])
        }
    }

    // Implements Eq if underlying data is Eq
    impl<T, const N: usize> Eq for Vec<T, N> where T: Eq {}

    impl<T, const N1: usize, const N2: usize> PartialOrd<Vec<T, N2>> for Vec<T, N1>
    where
        T: PartialOrd,
    {
        fn partial_cmp(&self, other: &Vec<T, N2>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
    }

    impl<T, const N: usize> Ord for Vec<T, N>
    where
        T: Ord,
    {
        #[inline]
        fn cmp(&self, other: &Self) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }

    impl<T, const N: usize> ops::Deref for Vec<T, N> {
        type Target = [T];

        fn deref(&self) -> &[T] {
            self.as_slice()
        }
    }

    impl<T, const N: usize> ops::DerefMut for Vec<T, N> {
        fn deref_mut(&mut self) -> &mut [T] {
            self.as_mut_slice()
        }
    }

    impl<T, const N: usize> AsRef<Vec<T, N>> for Vec<T, N> {
        #[inline]
        fn as_ref(&self) -> &Self {
            self
        }
    }

    impl<T, const N: usize> AsMut<Vec<T, N>> for Vec<T, N> {
        #[inline]
        fn as_mut(&mut self) -> &mut Self {
            self
        }
    }

    impl<T, const N: usize> AsRef<[T]> for Vec<T, N> {
        #[inline]
        fn as_ref(&self) -> &[T] {
            self
        }
    }

    impl<T, const N: usize> AsMut<[T]> for Vec<T, N> {
        #[inline]
        fn as_mut(&mut self) -> &mut [T] {
            self
        }
    }

    impl<T, const N: usize> Clone for Vec<T, N>
    where
        T: Clone,
    {
        fn clone(&self) -> Self {
            self.clone()
        }
    }

    #[cfg(test)]
    mod tests {
        use crate::Vec;
        use core::fmt::Write;

        #[test]
        fn static_new() {
            static mut _V: Vec<i32, 4> = Vec::new();
        }

        #[test]
        fn stack_new() {
            let mut _v: Vec<i32, 4> = Vec::new();
        }

        #[test]
        fn is_full_empty() {
            let mut v: Vec<i32, 4> = Vec::new();

            assert!(v.is_empty());
            assert!(!v.is_full());

            v.push(1).unwrap();
            assert!(!v.is_empty());
            assert!(!v.is_full());

            v.push(1).unwrap();
            assert!(!v.is_empty());
            assert!(!v.is_full());

            v.push(1).unwrap();
            assert!(!v.is_empty());
            assert!(!v.is_full());

            v.push(1).unwrap();
            assert!(!v.is_empty());
            assert!(v.is_full());
        }

        #[test]
        fn drop() {
            droppable!();

            {
                let mut v: Vec<Droppable, 2> = Vec::new();
                v.push(Droppable::new()).ok().unwrap();
                v.push(Droppable::new()).ok().unwrap();
                v.pop().unwrap();
            }

            assert_eq!(Droppable::count(), 0);

            {
                let mut v: Vec<Droppable, 2> = Vec::new();
                v.push(Droppable::new()).ok().unwrap();
                v.push(Droppable::new()).ok().unwrap();
            }

            assert_eq!(Droppable::count(), 0);
        }

        #[test]
        fn eq() {
            let mut xs: Vec<i32, 4> = Vec::new();
            let mut ys: Vec<i32, 8> = Vec::new();

            assert_eq!(xs, ys);

            xs.push(1).unwrap();
            ys.push(1).unwrap();

            assert_eq!(xs, ys);
        }

        #[test]
        fn cmp() {
            let mut xs: Vec<i32, 4> = Vec::new();
            let mut ys: Vec<i32, 4> = Vec::new();

            assert_eq!(xs, ys);

            xs.push(1).unwrap();
            ys.push(2).unwrap();

            assert!(xs < ys);
        }

        #[test]
        fn cmp_heterogenous_size() {
            let mut xs: Vec<i32, 4> = Vec::new();
            let mut ys: Vec<i32, 8> = Vec::new();

            assert_eq!(xs, ys);

            xs.push(1).unwrap();
            ys.push(2).unwrap();

            assert!(xs < ys);
        }

        #[test]
        fn cmp_with_arrays_and_slices() {
            let mut xs: Vec<i32, 12> = Vec::new();
            xs.push(1).unwrap();

            let array = [1];

            assert_eq!(xs, array);
            assert_eq!(array, xs);

            assert_eq!(xs, array.as_slice());
            assert_eq!(array.as_slice(), xs);

            assert_eq!(xs, &array);
            assert_eq!(&array, xs);

            let longer_array = [1; 20];

            assert_ne!(xs, longer_array);
            assert_ne!(longer_array, xs);
        }

        #[test]
        fn full() {
            let mut v: Vec<i32, 4> = Vec::new();

            v.push(0).unwrap();
            v.push(1).unwrap();
            v.push(2).unwrap();
            v.push(3).unwrap();

            assert!(v.push(4).is_err());
        }

        #[test]
        fn iter() {
            let mut v: Vec<i32, 4> = Vec::new();

            v.push(0).unwrap();
            v.push(1).unwrap();
            v.push(2).unwrap();
            v.push(3).unwrap();

            let mut items = v.iter();

            assert_eq!(items.next(), Some(&0));
            assert_eq!(items.next(), Some(&1));
            assert_eq!(items.next(), Some(&2));
            assert_eq!(items.next(), Some(&3));
            assert_eq!(items.next(), None);
        }

        #[test]
        fn iter_mut() {
            let mut v: Vec<i32, 4> = Vec::new();

            v.push(0).unwrap();
            v.push(1).unwrap();
            v.push(2).unwrap();
            v.push(3).unwrap();

            let mut items = v.iter_mut();

            assert_eq!(items.next(), Some(&mut 0));
            assert_eq!(items.next(), Some(&mut 1));
            assert_eq!(items.next(), Some(&mut 2));
            assert_eq!(items.next(), Some(&mut 3));
            assert_eq!(items.next(), None);
        }

        #[test]
        fn collect_from_iter() {
            let slice = &[1, 2, 3];
            let vec: Vec<i32, 4> = slice.iter().cloned().collect();
            assert_eq!(&vec, slice);
        }

        #[test]
        #[should_panic]
        fn collect_from_iter_overfull() {
            let slice = &[1, 2, 3];
            let _vec = slice.iter().cloned().collect::<Vec<_, 2>>();
        }

        #[test]
        fn iter_move() {
            let mut v: Vec<i32, 4> = Vec::new();
            v.push(0).unwrap();
            v.push(1).unwrap();
            v.push(2).unwrap();
            v.push(3).unwrap();

            let mut items = v.into_iter();

            assert_eq!(items.next(), Some(0));
            assert_eq!(items.next(), Some(1));
            assert_eq!(items.next(), Some(2));
            assert_eq!(items.next(), Some(3));
            assert_eq!(items.next(), None);
        }

        #[test]
        fn iter_move_drop() {
            droppable!();

            {
                let mut vec: Vec<Droppable, 2> = Vec::new();
                vec.push(Droppable::new()).ok().unwrap();
                vec.push(Droppable::new()).ok().unwrap();
                let mut items = vec.into_iter();
                // Move all
                let _ = items.next();
                let _ = items.next();
            }

            assert_eq!(Droppable::count(), 0);

            {
                let mut vec: Vec<Droppable, 2> = Vec::new();
                vec.push(Droppable::new()).ok().unwrap();
                vec.push(Droppable::new()).ok().unwrap();
                let _items = vec.into_iter();
                // Move none
            }

            assert_eq!(Droppable::count(), 0);

            {
                let mut vec: Vec<Droppable, 2> = Vec::new();
                vec.push(Droppable::new()).ok().unwrap();
                vec.push(Droppable::new()).ok().unwrap();
                let mut items = vec.into_iter();
                let _ = items.next(); // Move partly
            }

            assert_eq!(Droppable::count(), 0);
        }

        #[test]
        fn push_and_pop() {
            let mut v: Vec<i32, 4> = Vec::new();
            assert_eq!(v.len(), 0);

            assert_eq!(v.pop(), None);
            assert_eq!(v.len(), 0);

            v.push(0).unwrap();
            assert_eq!(v.len(), 1);

            assert_eq!(v.pop(), Some(0));
            assert_eq!(v.len(), 0);

            assert_eq!(v.pop(), None);
            assert_eq!(v.len(), 0);
        }

        #[test]
        fn resize_size_limit() {
            let mut v: Vec<u8, 4> = Vec::new();

            v.resize(0, 0).unwrap();
            v.resize(4, 0).unwrap();
            v.resize(5, 0).err().expect("full");
        }

        #[test]
        fn resize_length_cases() {
            let mut v: Vec<u8, 4> = Vec::new();

            assert_eq!(v.len(), 0);

            // Grow by 1
            v.resize(1, 0).unwrap();
            assert_eq!(v.len(), 1);

            // Grow by 2
            v.resize(3, 0).unwrap();
            assert_eq!(v.len(), 3);

            // Resize to current size
            v.resize(3, 0).unwrap();
            assert_eq!(v.len(), 3);

            // Shrink by 1
            v.resize(2, 0).unwrap();
            assert_eq!(v.len(), 2);

            // Shrink by 2
            v.resize(0, 0).unwrap();
            assert_eq!(v.len(), 0);
        }

        #[test]
        fn resize_contents() {
            let mut v: Vec<u8, 4> = Vec::new();

            // New entries take supplied value when growing
            v.resize(1, 17).unwrap();
            assert_eq!(v[0], 17);

            // Old values aren't changed when growing
            v.resize(2, 18).unwrap();
            assert_eq!(v[0], 17);
            assert_eq!(v[1], 18);

            // Old values aren't changed when length unchanged
            v.resize(2, 0).unwrap();
            assert_eq!(v[0], 17);
            assert_eq!(v[1], 18);

            // Old values aren't changed when shrinking
            v.resize(1, 0).unwrap();
            assert_eq!(v[0], 17);
        }

        #[test]
        fn resize_default() {
            let mut v: Vec<u8, 4> = Vec::new();

            // resize_default is implemented using resize, so just check the
            // correct value is being written.
            v.resize_default(1).unwrap();
            assert_eq!(v[0], 0);
        }

        #[test]
        fn write() {
            let mut v: Vec<u8, 4> = Vec::new();
            write!(v, "{:x}", 1234).unwrap();
            assert_eq!(&v[..], b"4d2");
        }

        #[test]
        fn extend_from_slice() {
            let mut v: Vec<u8, 4> = Vec::new();
            assert_eq!(v.len(), 0);
            v.extend_from_slice(&[1, 2]).unwrap();
            assert_eq!(v.len(), 2);
            assert_eq!(v.as_slice(), &[1, 2]);
            v.extend_from_slice(&[3]).unwrap();
            assert_eq!(v.len(), 3);
            assert_eq!(v.as_slice(), &[1, 2, 3]);
            assert!(v.extend_from_slice(&[4, 5]).is_err());
            assert_eq!(v.len(), 3);
            assert_eq!(v.as_slice(), &[1, 2, 3]);
        }

        #[test]
        fn from_slice() {
            // Successful construction
            let v: Vec<u8, 4> = Vec::from_slice(&[1, 2, 3]).unwrap();
            assert_eq!(v.len(), 3);
            assert_eq!(v.as_slice(), &[1, 2, 3]);

            // Slice too large
            assert!(Vec::<u8, 2>::from_slice(&[1, 2, 3]).is_err());
        }

        #[test]
        fn starts_with() {
            let v: Vec<_, 8> = Vec::from_slice(b"ab").unwrap();
            assert!(v.starts_with(&[]));
            assert!(v.starts_with(b""));
            assert!(v.starts_with(b"a"));
            assert!(v.starts_with(b"ab"));
            assert!(!v.starts_with(b"abc"));
            assert!(!v.starts_with(b"ba"));
            assert!(!v.starts_with(b"b"));
        }

        #[test]
        fn ends_with() {
            let v: Vec<_, 8> = Vec::from_slice(b"ab").unwrap();
            assert!(v.ends_with(&[]));
            assert!(v.ends_with(b""));
            assert!(v.ends_with(b"b"));
            assert!(v.ends_with(b"ab"));
            assert!(!v.ends_with(b"abc"));
            assert!(!v.ends_with(b"ba"));
            assert!(!v.ends_with(b"a"));
        }

        #[test]
        fn zero_capacity() {
            let mut v: Vec<u8, 0> = Vec::new();
            // Validate capacity
            assert_eq!(v.capacity(), 0);

            // Make sure there is no capacity
            assert!(v.push(1).is_err());

            // Validate length
            assert_eq!(v.len(), 0);

            // Validate pop
            assert_eq!(v.pop(), None);

            // Validate slice
            assert_eq!(v.as_slice(), &[]);

            // Validate empty
            assert!(v.is_empty());

            // Validate full
            assert!(v.is_full());
        }
    }