raw_vec.rs.html -- source
Description: Source to the Rust file `liballoc/raw_vec.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use core::cmp;
    use core::mem;
    use core::ops::Drop;
    use core::ptr::{self, Unique};
    use core::slice;
    use heap::{Alloc, Layout, Heap};
    use super::boxed::Box;

    /// 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::empty() on zero-sized types
    /// * Produces Unique::empty() on zero-length allocations
    /// * 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
    /// * Aborts on OOM
    /// * Avoids freeing Unique::empty()
    /// * Contains a ptr::Unique and thus endows the user with all related benefits
    ///
    /// 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 a RawVec always forces its capacity to be usize::MAX for zero-sized types.
    /// This enables you to use capacity growing logic catch the overflows in your length
    /// that might occur with zero-sized types.
    ///
    /// However this means that you need to be careful when roundtripping this type
    /// with a `Box<[T]>`: `cap()` won't yield the len. However `with_capacity`,
    /// `shrink_to_fit`, and `from_box` will actually set RawVec's private capacity
    /// field. This allows zero-sized types to not be special-cased by consumers of
    /// this type.
    #[allow(missing_debug_implementations)]
    pub struct RawVec<T, A: Alloc = Heap> {
        ptr: Unique<T>,
        cap: usize,
        a: A,
    }

    impl<T, A: Alloc> RawVec<T, A> {
        /// Like `new` but parameterized over the choice of allocator for
        /// the returned RawVec.
        pub fn new_in(a: A) -> Self {
            // !0 is usize::MAX. This branch should be stripped at compile time.
            let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };

            // Unique::empty() doubles as "unallocated" and "zero-sized allocation"
            RawVec {
                ptr: Unique::empty(),
                cap,
                a,
            }
        }

        /// Like `with_capacity` but parameterized over the choice of
        /// allocator for the returned RawVec.
        #[inline]
        pub fn with_capacity_in(cap: usize, a: A) -> Self {
            RawVec::allocate_in(cap, false, a)
        }

        /// Like `with_capacity_zeroed` but parameterized over the choice
        /// of allocator for the returned RawVec.
        #[inline]
        pub fn with_capacity_zeroed_in(cap: usize, a: A) -> Self {
            RawVec::allocate_in(cap, true, a)
        }

        fn allocate_in(cap: usize, zeroed: bool, mut a: A) -> Self {
            unsafe {
                let elem_size = mem::size_of::<T>();

                let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
                alloc_guard(alloc_size);

                // handles ZSTs and `cap = 0` alike
                let ptr = if alloc_size == 0 {
                    mem::align_of::<T>() as *mut u8
                } else {
                    let align = mem::align_of::<T>();
                    let result = if zeroed {
                        a.alloc_zeroed(Layout::from_size_align(alloc_size, align).unwrap())
                    } else {
                        a.alloc(Layout::from_size_align(alloc_size, align).unwrap())
                    };
                    match result {
                        Ok(ptr) => ptr,
                        Err(err) => a.oom(err),
                    }
                };

                RawVec {
                    ptr: Unique::new_unchecked(ptr as *mut _),
                    cap,
                    a,
                }
            }
        }
    }

    impl<T> RawVec<T, Heap> {
        /// Creates the biggest possible RawVec (on the system heap)
        /// without allocating. If T has positive size, then this makes a
        /// RawVec with capacity 0. If T has 0 size, then it makes a
        /// RawVec with capacity `usize::MAX`. Useful for implementing
        /// delayed allocation.
        pub fn new() -> Self {
            Self::new_in(Heap)
        }

        /// Creates a RawVec (on the system heap) with exactly the
        /// capacity and alignment requirements for a `[T; cap]`. This is
        /// equivalent to calling RawVec::new when `cap` is 0 or T is
        /// zero-sized. Note that if `T` is zero-sized this means you will
        /// *not* get a RawVec with the requested capacity!
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        #[inline]
        pub fn with_capacity(cap: usize) -> Self {
            RawVec::allocate_in(cap, false, Heap)
        }

        /// Like `with_capacity` but guarantees the buffer is zeroed.
        #[inline]
        pub fn with_capacity_zeroed(cap: usize) -> Self {
            RawVec::allocate_in(cap, true, Heap)
        }
    }

    impl<T, A: Alloc> RawVec<T, A> {
        /// Reconstitutes a RawVec from a pointer, capacity, and allocator.
        ///
        /// # Undefined Behavior
        ///
        /// The ptr must be allocated (via the given allocator `a`), and with the given capacity. The
        /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
        /// If the ptr and capacity come from a RawVec created via `a`, then this is guaranteed.
        pub unsafe fn from_raw_parts_in(ptr: *mut T, cap: usize, a: A) -> Self {
            RawVec {
                ptr: Unique::new_unchecked(ptr),
                cap,
                a,
            }
        }
    }

    impl<T> RawVec<T, Heap> {
        /// Reconstitutes a RawVec from a pointer, capacity.
        ///
        /// # Undefined Behavior
        ///
        /// The ptr must be allocated (on the system heap), and with the given capacity. The
        /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
        /// If the ptr and capacity come from a RawVec, then this is guaranteed.
        pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
            RawVec {
                ptr: Unique::new_unchecked(ptr),
                cap,
                a: Heap,
            }
        }

        /// Converts a `Box<[T]>` into a `RawVec<T>`.
        pub fn from_box(mut slice: Box<[T]>) -> Self {
            unsafe {
                let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
                mem::forget(slice);
                result
            }
        }
    }

    impl<T, A: Alloc> RawVec<T, A> {
        /// Gets a raw pointer to the start of the allocation. Note that this is
        /// Unique::empty() if `cap = 0` or T is zero-sized. In the former case, you must
        /// be careful.
        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 cap(&self) -> usize {
            if mem::size_of::<T>() == 0 {
                !0
            } else {
                self.cap
            }
        }

        /// Returns a shared reference to the allocator backing this RawVec.
        pub fn alloc(&self) -> &A {
            &self.a
        }

        /// Returns a mutable reference to the allocator backing this RawVec.
        pub fn alloc_mut(&mut self) -> &mut A {
            &mut self.a
        }

        fn current_layout(&self) -> Option<Layout> {
            if 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 align = mem::align_of::<T>();
                    let size = mem::size_of::<T>() * self.cap;
                    Some(Layout::from_size_align_unchecked(size, align))
                }
            }
        }

        /// Doubles the size of the type's backing allocation. This is common enough
        /// to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// This function is ideal for when pushing elements one-at-a-time because
        /// you don't need to incur the costs of the more general computations
        /// reserve needs to do to guard against overflow. You do however need to
        /// manually check if your `len == cap`.
        ///
        /// # Panics
        ///
        /// * Panics if T is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(alloc)]
        /// # extern crate alloc;
        /// # use std::ptr;
        /// # use alloc::raw_vec::RawVec;
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T> MyVec<T> {
        ///     pub fn push(&mut self, elem: T) {
        ///         if self.len == self.buf.cap() { self.buf.double(); }
        ///         // double would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         unsafe {
        ///             ptr::write(self.buf.ptr().offset(self.len as isize), elem);
        ///         }
        ///         self.len += 1;
        ///     }
        /// }
        /// # fn main() {
        /// #   let mut vec = MyVec { buf: RawVec::new(), len: 0 };
        /// #   vec.push(1);
        /// # }
        /// ```
        #[inline(never)]
        #[cold]
        pub fn double(&mut self) {
            unsafe {
                let elem_size = mem::size_of::<T>();

                // since we set the capacity to usize::MAX when elem_size is
                // 0, getting to here necessarily means the RawVec is overfull.
                assert!(elem_size != 0, "capacity overflow");

                let (new_cap, uniq) = match self.current_layout() {
                    Some(cur) => {
                        // Since we guarantee that we never allocate more than
                        // isize::MAX bytes, `elem_size * self.cap <= isize::MAX` as
                        // a precondition, so this can't overflow. Additionally the
                        // alignment will never be too large as to "not be
                        // satisfiable", so `Layout::from_size_align` will always
                        // return `Some`.
                        //
                        // tl;dr; we bypass runtime checks due to dynamic assertions
                        // in this module, allowing us to use
                        // `from_size_align_unchecked`.
                        let new_cap = 2 * self.cap;
                        let new_size = new_cap * elem_size;
                        let new_layout = Layout::from_size_align_unchecked(new_size, cur.align());
                        alloc_guard(new_size);
                        let ptr_res = self.a.realloc(self.ptr.as_ptr() as *mut u8,
                                                     cur,
                                                     new_layout);
                        match ptr_res {
                            Ok(ptr) => (new_cap, Unique::new_unchecked(ptr as *mut T)),
                            Err(e) => self.a.oom(e),
                        }
                    }
                    None => {
                        // skip to 4 because tiny Vec's are dumb; but not if that
                        // would cause overflow
                        let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
                        match self.a.alloc_array::<T>(new_cap) {
                            Ok(ptr) => (new_cap, ptr),
                            Err(e) => self.a.oom(e),
                        }
                    }
                };
                self.ptr = uniq;
                self.cap = new_cap;
            }
        }

        /// Attempts to double the size of the type's backing allocation in place. This is common
        /// enough to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// Returns true if the reallocation attempt has succeeded, or false otherwise.
        ///
        /// # Panics
        ///
        /// * Panics if T is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        #[inline(never)]
        #[cold]
        pub fn double_in_place(&mut self) -> bool {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let old_layout = match self.current_layout() {
                    Some(layout) => layout,
                    None => return false, // nothing to double
                };

                // since we set the capacity to usize::MAX when elem_size is
                // 0, getting to here necessarily means the RawVec is overfull.
                assert!(elem_size != 0, "capacity overflow");

                // Since we guarantee that we never allocate more than isize::MAX
                // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
                // this can't overflow.
                //
                // Similarly like with `double` above we can go straight to
                // `Layout::from_size_align_unchecked` as we know this won't
                // overflow and the alignment is sufficiently small.
                let new_cap = 2 * self.cap;
                let new_size = new_cap * elem_size;
                alloc_guard(new_size);
                let ptr = self.ptr() as *mut _;
                let new_layout = Layout::from_size_align_unchecked(new_size, old_layout.align());
                match self.a.grow_in_place(ptr, old_layout, new_layout) {
                    Ok(_) => {
                        // We can't directly divide `size`.
                        self.cap = new_cap;
                        true
                    }
                    Err(_) => {
                        false
                    }
                }
            }
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` 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 `used_cap` exceeds `self.cap()`, 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 requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
            unsafe {
                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity.
                // Wrapping in case they gave a bad `used_cap`.
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
                    return;
                }

                // Nothing we can really do about these checks :(
                let new_cap = used_cap.checked_add(needed_extra_cap).expect("capacity overflow");
                let new_layout = match Layout::array::<T>(new_cap) {
                    Some(layout) => layout,
                    None => panic!("capacity overflow"),
                };
                alloc_guard(new_layout.size());
                let res = match self.current_layout() {
                    Some(layout) => {
                        let old_ptr = self.ptr.as_ptr() as *mut u8;
                        self.a.realloc(old_ptr, layout, new_layout)
                    }
                    None => self.a.alloc(new_layout),
                };
                let uniq = match res {
                    Ok(ptr) => Unique::new_unchecked(ptr as *mut T),
                    Err(e) => self.a.oom(e),
                };
                self.ptr = uniq;
                self.cap = new_cap;
            }
        }

        /// Calculates the buffer's new size given that it'll hold `used_cap +
        /// needed_extra_cap` elements. This logic is used in amortized reserve methods.
        /// Returns `(new_capacity, new_alloc_size)`.
        fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize) -> usize {
            // Nothing we can really do about these checks :(
            let required_cap = used_cap.checked_add(needed_extra_cap)
                .expect("capacity overflow");
            // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
            let double_cap = self.cap * 2;
            // `double_cap` guarantees exponential growth.
            cmp::max(double_cap, required_cap)
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` elements. If it doesn't already have
        /// enough capacity, will reallocate enough space plus comfortable slack
        /// space to get amortized `O(1)` behavior. Will limit this behavior
        /// if it would needlessly cause itself to panic.
        ///
        /// If `used_cap` exceeds `self.cap()`, 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.
        ///
        /// This is ideal for implementing a bulk-push operation like `extend`.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(alloc)]
        /// # extern crate alloc;
        /// # use std::ptr;
        /// # use alloc::raw_vec::RawVec;
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T: Clone> MyVec<T> {
        ///     pub fn push_all(&mut self, elems: &[T]) {
        ///         self.buf.reserve(self.len, elems.len());
        ///         // reserve would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         for x in elems {
        ///             unsafe {
        ///                 ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
        ///             }
        ///             self.len += 1;
        ///         }
        ///     }
        /// }
        /// # fn main() {
        /// #   let mut vector = MyVec { buf: RawVec::new(), len: 0 };
        /// #   vector.push_all(&[1, 3, 5, 7, 9]);
        /// # }
        /// ```
        pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
            unsafe {
                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity.
                // Wrapping in case they give a bad `used_cap`
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
                    return;
                }

                let new_cap = self.amortized_new_size(used_cap, needed_extra_cap);

                let new_layout = match Layout::array::<T>(new_cap) {
                    Some(layout) => layout,
                    None => panic!("capacity overflow"),
                };
                // FIXME: may crash and burn on over-reserve
                alloc_guard(new_layout.size());
                let res = match self.current_layout() {
                    Some(layout) => {
                        let old_ptr = self.ptr.as_ptr() as *mut u8;
                        self.a.realloc(old_ptr, layout, new_layout)
                    }
                    None => self.a.alloc(new_layout),
                };
                let uniq = match res {
                    Ok(ptr) => Unique::new_unchecked(ptr as *mut T),
                    Err(e) => self.a.oom(e),
                };
                self.ptr = uniq;
                self.cap = new_cap;
            }
        }

        /// Attempts to ensure that the buffer contains at least enough space to hold
        /// `used_cap + needed_extra_cap` elements. If it doesn't already have
        /// enough capacity, will reallocate in place enough space plus comfortable slack
        /// space to get amortized `O(1)` behavior. Will limit this behaviour
        /// if it would needlessly cause itself to panic.
        ///
        /// If `used_cap` exceeds `self.cap()`, 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.
        ///
        /// Returns true if the reallocation attempt has succeeded, or false otherwise.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
            unsafe {
                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity. If the current `cap` is 0, we can't
                // reallocate in place.
                // Wrapping in case they give a bad `used_cap`
                let old_layout = match self.current_layout() {
                    Some(layout) => layout,
                    None => return false,
                };
                if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
                    return false;
                }

                let new_cap = self.amortized_new_size(used_cap, needed_extra_cap);

                // Here, `cap < used_cap + needed_extra_cap <= new_cap`
                // (regardless of whether `self.cap - used_cap` wrapped).
                // Therefore we can safely call grow_in_place.

                let ptr = self.ptr() as *mut _;
                let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
                // FIXME: may crash and burn on over-reserve
                alloc_guard(new_layout.size());
                match self.a.grow_in_place(ptr, old_layout, new_layout) {
                    Ok(_) => {
                        self.cap = new_cap;
                        true
                    }
                    Err(_) => {
                        false
                    }
                }
            }
        }

        /// Shrinks the allocation down to the specified amount. 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, amount: usize) {
            let elem_size = mem::size_of::<T>();

            // Set the `cap` because they might be about to promote to a `Box<[T]>`
            if elem_size == 0 {
                self.cap = amount;
                return;
            }

            // This check is my waterloo; it's the only thing Vec wouldn't have to do.
            assert!(self.cap >= amount, "Tried to shrink to a larger capacity");

            if amount == 0 {
                // We want to create a new zero-length vector within the
                // same allocator.  We use ptr::write to avoid an
                // erroneous attempt to drop the contents, and we use
                // ptr::read to sidestep condition against destructuring
                // types that implement Drop.

                unsafe {
                    let a = ptr::read(&self.a as *const A);
                    self.dealloc_buffer();
                    ptr::write(self, RawVec::new_in(a));
                }
            } else if self.cap != amount {
                unsafe {
                    // We know here that our `amount` is greater than zero. This
                    // implies, via the assert above, that capacity is also greater
                    // than zero, which means that we've got a current layout that
                    // "fits"
                    //
                    // We also know that `self.cap` is greater than `amount`, and
                    // consequently we don't need runtime checks for creating either
                    // layout
                    let old_size = elem_size * self.cap;
                    let new_size = elem_size * amount;
                    let align = mem::align_of::<T>();
                    let old_layout = Layout::from_size_align_unchecked(old_size, align);
                    let new_layout = Layout::from_size_align_unchecked(new_size, align);
                    match self.a.realloc(self.ptr.as_ptr() as *mut u8,
                                         old_layout,
                                         new_layout) {
                        Ok(p) => self.ptr = Unique::new_unchecked(p as *mut T),
                        Err(err) => self.a.oom(err),
                    }
                }
                self.cap = amount;
            }
        }
    }

    impl<T> RawVec<T, Heap> {
        /// Converts the entire buffer into `Box<[T]>`.
        ///
        /// While it is not *strictly* Undefined Behavior to call
        /// this procedure while some of the RawVec is uninitialized,
        /// it certainly makes it trivial to trigger it.
        ///
        /// Note that this will correctly reconstitute any `cap` changes
        /// that may have been performed. (see description of type for details)
        pub unsafe fn into_box(self) -> Box<[T]> {
            // NOTE: not calling `cap()` here, actually using the real `cap` field!
            let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
            let output: Box<[T]> = Box::from_raw(slice);
            mem::forget(self);
            output
        }
    }

    impl<T, A: Alloc> RawVec<T, A> {
        /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
        pub unsafe fn dealloc_buffer(&mut self) {
            let elem_size = mem::size_of::<T>();
            if elem_size != 0 {
                if let Some(layout) = self.current_layout() {
                    let ptr = self.ptr() as *mut u8;
                    self.a.dealloc(ptr, layout);
                }
            }
        }
    }

    unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
        /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
        fn drop(&mut self) {
            unsafe { self.dealloc_buffer(); }
        }
    }

    // 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) {
        if mem::size_of::<usize>() < 8 {
            assert!(alloc_size <= ::core::isize::MAX as usize,
                    "capacity overflow");
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn allocator_param() {
            use allocator::{Alloc, AllocErr};

            // Writing a test of integration between third-party
            // allocators and RawVec is a little tricky because the RawVec
            // API does not expose fallible allocation methods, so we
            // cannot check what happens when allocator is exhausted
            // (beyond detecting a panic).
            //
            // Instead, this just checks that the RawVec methods do at
            // least go through the Allocator API when it reserves
            // storage.

            // A dumb allocator that consumes a fixed amount of fuel
            // before allocation attempts start failing.
            struct BoundedAlloc { fuel: usize }
            unsafe impl Alloc for BoundedAlloc {
                unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr> {
                    let size = layout.size();
                    if size > self.fuel {
                        return Err(AllocErr::Unsupported { details: "fuel exhausted" });
                    }
                    match Heap.alloc(layout) {
                        ok @ Ok(_) => { self.fuel -= size; ok }
                        err @ Err(_) => err,
                    }
                }
                unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout) {
                    Heap.dealloc(ptr, layout)
                }
            }

            let a = BoundedAlloc { fuel: 500 };
            let mut v: RawVec<u8, _> = RawVec::with_capacity_in(50, a);
            assert_eq!(v.a.fuel, 450);
            v.reserve(50, 150); // (causes a realloc, thus using 50 + 150 = 200 units of fuel)
            assert_eq!(v.a.fuel, 250);
        }

        #[test]
        fn reserve_does_not_overallocate() {
            {
                let mut v: RawVec<u32> = RawVec::new();
                // First `reserve` allocates like `reserve_exact`
                v.reserve(0, 9);
                assert_eq!(9, v.cap());
            }

            {
                let mut v: RawVec<u32> = RawVec::new();
                v.reserve(0, 7);
                assert_eq!(7, v.cap());
                // 97 if more than double of 7, so `reserve` should work
                // like `reserve_exact`.
                v.reserve(7, 90);
                assert_eq!(97, v.cap());
            }

            {
                let mut v: RawVec<u32> = RawVec::new();
                v.reserve(0, 12);
                assert_eq!(12, v.cap());
                v.reserve(12, 3);
                // 3 is less than half of 12, so `reserve` must grow
                // exponentially. At the time of writing this test grow
                // factor is 2, so new capacity is 24, however, grow factor
                // of 1.5 is OK too. Hence `>= 18` in assert.
                assert!(v.cap() >= 12 + 12 / 2);
            }
        }

    }

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