ramp 0.7.0

// This file is modified from
// https://raw.githubusercontent.com/rust-lang/rust/c212fc4aa7719613e5254e9051ea03a93558fef4/library/alloc/src/raw_vec.rs

use core::alloc::LayoutError;
use core::intrinsics;
use core::mem;
use core::ops::Drop;
use core::ptr::{NonNull, Unique};

use alloc::alloc::handle_alloc_error;
use alloc::alloc::{Allocator, Global, Layout};
use alloc::collections::TryReserveError;
use alloc::collections::TryReserveErrorKind::*;

/// A low-level utility for more ergonomically allocating, reallocating, and deallocating
/// a buffer of memory on the heap without having to worry about all the corner cases
/// involved. This type is excellent for building your own data structures like Vec and VecDeque.
/// In particular:
///
/// * Produces `Unique::dangling()` on zero-sized types.
/// * Produces `Unique::dangling()` on zero-length allocations.
/// * Avoids freeing `Unique::dangling()`.
/// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
/// * Guards against 32-bit systems allocating more than isize::MAX bytes.
/// * Guards against overflowing your length.
/// * Calls `handle_alloc_error` for fallible allocations.
/// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
/// * Uses the excess returned from the allocator to use the largest available capacity.
///
/// This type does not in anyway inspect the memory that it manages. When dropped it *will*
/// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
/// to handle the actual things *stored* inside of a `RawVec`.
///
/// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
/// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
/// `Box<[T]>`, since `capacity()` won't yield the length.
#[allow(missing_debug_implementations)]
pub(crate) struct RawVec<T, A: Allocator = Global> {
    ptr: Unique<T>,
    cap: usize,
    alloc: A,
}

impl<T, A: Allocator> RawVec<T, A> {
    /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
    ///
    /// # Safety
    ///
    /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
    /// `capacity`.
    /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
    /// systems). ZST vectors may have a capacity up to `usize::MAX`.
    /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
    /// guaranteed.
    #[inline]
    pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
        Self {
            ptr: Unique::new_unchecked(ptr),
            cap: capacity,
            alloc,
        }
    }

    /// Gets a raw pointer to the start of the allocation. Note that this is
    /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
    /// be careful.
    #[inline]
    pub fn ptr(&self) -> *mut T {
        self.ptr.as_ptr()
    }

    /// Gets the capacity of the allocation.
    ///
    /// This will always be `usize::MAX` if `T` is zero-sized.
    #[inline(always)]
    pub fn capacity(&self) -> usize {
        if mem::size_of::<T>() == 0 {
            usize::MAX
        } else {
            self.cap
        }
    }

    fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
        if mem::size_of::<T>() == 0 || self.cap == 0 {
            None
        } else {
            // We have an allocated chunk of memory, so we can bypass runtime
            // checks to get our current layout.
            unsafe {
                let layout = Layout::array::<T>(self.cap).unwrap_unchecked();
                Some((self.ptr.cast().into(), layout))
            }
        }
    }

    /// Ensures that the buffer contains at least enough space to hold `len +
    /// additional` elements. If it doesn't already, will reallocate the
    /// minimum possible amount of memory necessary. Generally this will be
    /// exactly the amount of memory necessary, but in principle the allocator
    /// is free to give back more than we asked for.
    ///
    /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
    /// the requested space. This is not really unsafe, but the unsafe code
    /// *you* write that relies on the behavior of this function may break.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    pub fn reserve_exact(&mut self, len: usize, additional: usize) {
        handle_reserve(self.try_reserve_exact(len, additional));
    }

    /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
    pub fn try_reserve_exact(
        &mut self,
        len: usize,
        additional: usize,
    ) -> Result<(), TryReserveError> {
        if self.needs_to_grow(len, additional) {
            self.grow_exact(len, additional)
        } else {
            Ok(())
        }
    }

    /// Shrinks the buffer down to the specified capacity. If the given amount
    /// is 0, actually completely deallocates.
    ///
    /// # Panics
    ///
    /// Panics if the given amount is *larger* than the current capacity.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    pub fn shrink_to_fit(&mut self, cap: usize) {
        handle_reserve(self.shrink(cap));
    }
}

impl<T, A: Allocator> RawVec<T, A> {
    /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
    /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
    fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
        additional > self.capacity().wrapping_sub(len)
    }

    fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
        // Allocators currently return a `NonNull<[u8]>` whose length matches
        // the size requested. If that ever changes, the capacity here should
        // change to `ptr.len() / mem::size_of::<T>()`.
        self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
        self.cap = cap;
    }

    // The constraints on this method are much the same as those on
    // `grow_amortized`, but this method is usually instantiated less often so
    // it's less critical.
    fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
        if mem::size_of::<T>() == 0 {
            // Since we return a capacity of `usize::MAX` when the type size is
            // 0, getting to here necessarily means the `RawVec` is overfull.
            return Err(CapacityOverflow.into());
        }

        let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
        let new_layout = Layout::array::<T>(cap);

        // `finish_grow` is non-generic over `T`.
        let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
        self.set_ptr_and_cap(ptr, cap);
        Ok(())
    }

    fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
        assert!(
            cap <= self.capacity(),
            "Tried to shrink to a larger capacity"
        );

        let (ptr, layout) = if let Some(mem) = self.current_memory() {
            mem
        } else {
            return Ok(());
        };

        let ptr = unsafe {
            // `Layout::array` cannot overflow here because it would have
            // overflowed earlier when capacity was larger.
            let new_layout = Layout::array::<T>(cap).unwrap_unchecked();
            self.alloc
                .shrink(ptr, layout, new_layout)
                .map_err(|_| AllocError {
                    layout: new_layout,
                    non_exhaustive: (),
                })?
        };
        self.set_ptr_and_cap(ptr, cap);
        Ok(())
    }
}

// This function is outside `RawVec` to minimize compile times. See the comment
// above `RawVec::grow_amortized` for details. (The `A` parameter isn't
// significant, because the number of different `A` types seen in practice is
// much smaller than the number of `T` types.)
#[inline(never)]
fn finish_grow<A>(
    new_layout: Result<Layout, LayoutError>,
    current_memory: Option<(NonNull<u8>, Layout)>,
    alloc: &mut A,
) -> Result<NonNull<[u8]>, TryReserveError>
where
    A: Allocator,
{
    // Check for the error here to minimize the size of `RawVec::grow_*`.
    let new_layout = new_layout.map_err(|_| CapacityOverflow)?;

    alloc_guard(new_layout.size())?;

    let memory = if let Some((ptr, old_layout)) = current_memory {
        debug_assert_eq!(old_layout.align(), new_layout.align());
        unsafe {
            // The allocator checks for alignment equality
            intrinsics::assume(old_layout.align() == new_layout.align());
            alloc.grow(ptr, old_layout, new_layout)
        }
    } else {
        alloc.allocate(new_layout)
    };

    memory.map_err(|_| {
        AllocError {
            layout: new_layout,
            non_exhaustive: (),
        }
        .into()
    })
}

impl<T, A: Allocator> Drop for RawVec<T, A> {
    /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
    fn drop(&mut self) {
        if let Some((ptr, layout)) = self.current_memory() {
            unsafe { self.alloc.deallocate(ptr, layout) }
        }
    }
}

// Central function for reserve error handling.
#[inline]
fn handle_reserve(result: Result<(), TryReserveError>) {
    match result.map_err(|e| e.kind()) {
        Err(CapacityOverflow) => capacity_overflow(),
        Err(AllocError { layout, .. }) => handle_alloc_error(layout),
        Ok(()) => { /* yay */ }
    }
}

// We need to guarantee the following:
// * We don't ever allocate `> isize::MAX` byte-size objects.
// * We don't overflow `usize::MAX` and actually allocate too little.
//
// On 64-bit we just need to check for overflow since trying to allocate
// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
// an extra guard for this in case we're running on a platform which can use
// all 4GB in user-space, e.g., PAE or x32.

#[inline]
fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
    if usize::BITS < 64 && alloc_size > isize::MAX as usize {
        Err(CapacityOverflow.into())
    } else {
        Ok(())
    }
}

// One central function responsible for reporting capacity overflows. This'll
// ensure that the code generation related to these panics is minimal as there's
// only one location which panics rather than a bunch throughout the module.
fn capacity_overflow() -> ! {
    panic!("capacity overflow");
}