Add API for `Alloc` trait.

Includes `alloc_zeroed` method that `RawVec` has come to depend on.

Exposed private `Layout::from_size_align` ctor to be `pub`, and added
explicit conditions for when it will panic (namely, when `align` is
not power-of-two, or if rounding up `size` to a multiple of `align`
overflows). Normalized all `Layout` construction to go through
`Layout::from_size_align`.

Addressed review feedback regarding `struct Layout` and zero-sized
layouts.

Restrict specification for `dealloc`, adding additional constraint
that the given alignment has to match that used to allocate the block.
(This is a maximally conservative constraint on the alignment. An open
question to resolve (before stabilization) is whether we can return to
a looser constraint such as the one previously specified.)

Split `fn realloc_in_place` into separate `fn grow_in_place` and `fn
shrink_in_place` methods, which have default impls that check against
usable_size for reuse. Make `realloc` default impl try `grow_in_place`
or `shrink_in_place` as appropriate before fallback on
alloc+copy+dealloc.

Drive-by: When reviewing calls to `padding_needed_for`, discovered
what I think was an over-conservative choice for its argument
alignment.  Namely, in `fn extend`, we automatically realign the whole
resulting layout to satisfy both old (self) and new alignments. When
the old alignment exceeds the new, this means we would insert
unnecessary padding. So I changed the code to pass in `next.align`
instead of `new_align` to `padding_needed_for`.

Replaced ref to `realloc_in_place` with `grow_in_place`/`shrink_in_place`.

Revised docs replacing my idiosyncratic style of `fn foo` with just
`foo` when referring to the function or method `foo`.

(Alpha-renamed `Allocator` to `Alloc`.)

Post-rebased, added `Debug` derive for `allocator::Excess` to satisfy
`missing_debug_implementations`.

2 files changed, 990 insertions, 0 deletions

diff --git a/src/liballoc/allocator.rs b/src/liballoc/allocator.rs
new file mode 100644
index 00000000000..89324cf86f6
--- /dev/null
+++ b/src/liballoc/allocator.rs
@@ -0,0 +1,986 @@
+// 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.
+
+#![unstable(feature = "allocator_api",
+            reason = "the precise API and guarantees it provides may be tweaked \
+                      slightly, especially to possibly take into account the \
+                      types being stored to make room for a future \
+                      tracing garbage collector",
+            issue = "27700")]
+
+use core::cmp;
+use core::mem;
+use core::usize;
+use core::ptr::{self, Unique};
+
+/// Represents the combination of a starting address and
+/// a total capacity of the returned block.
+#[derive(Debug)]
+pub struct Excess(pub *mut u8, pub usize);
+
+fn size_align<T>() -> (usize, usize) {
+    (mem::size_of::<T>(), mem::align_of::<T>())
+}
+
+/// Layout of a block of memory.
+///
+/// An instance of `Layout` describes a particular layout of memory.
+/// You build a `Layout` up as an input to give to an allocator.
+///
+/// All layouts have an associated non-negative size and a
+/// power-of-two alignment.
+///
+/// (Note however that layouts are *not* required to have positive
+/// size, even though many allocators require that all memory
+/// requeusts have positive size. A caller to the `Alloc::alloc`
+/// method must either ensure that conditions like this are met, or
+/// use specific allocators with looser requirements.)
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Layout {
+    // size of the requested block of memory, measured in bytes.
+    size: usize,
+
+    // alignment of the requested block of memory, measured in bytes.
+    // we ensure that this is always a power-of-two, because API's
+    // like `posix_memalign` require it and it is a reasonable
+    // constraint to impose on Layout constructors.
+    //
+    // (However, we do not analogously require `align >= sizeof(void*)`,
+    //  even though that is *also* a requirement of `posix_memalign`.)
+    align: usize,
+}
+
+
+// FIXME: audit default implementations for overflow errors,
+// (potentially switching to overflowing_add and
+//  overflowing_mul as necessary).
+
+impl Layout {
+    /// Constructs a `Layout` from a given `size` and `align`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if any of the following conditions are not met:
+    ///
+    /// * `align` must be a power of two,
+    ///
+    /// * `size`, when rounded up to the nearest multiple of `align`,
+    ///    must not overflow (i.e. the rounded value must be less than
+    ///    `usize::MAX`).
+    pub fn from_size_align(size: usize, align: usize) -> Layout {
+        assert!(align.is_power_of_two()); // (this implies align != 0.)
+
+        // Rounded up size is:
+        //   size_rounded_up = (size + align - 1) & !(align - 1);
+        //
+        // We know from above that align != 0. If adding (align - 1)
+        // does not overflow, then rounding up will be fine.
+        //
+        // Conversely, &-masking with !(align - 1) will subtract off
+        // only low-order-bits. Thus if overflow occurs with the sum,
+        // the &-mask cannot subtract enough to undo that overflow.
+        //
+        // Above implies that checking for summation overflow is both
+        // necessary and sufficient.
+        assert!(size <= usize::MAX - (align - 1));
+
+        Layout { size: size, align: align }
+    }
+
+    /// The minimum size in bytes for a memory block of this layout.
+    pub fn size(&self) -> usize { self.size }
+
+    /// The minimum byte alignment for a memory block of this layout.
+    pub fn align(&self) -> usize { self.align }
+
+    /// Constructs a `Layout` suitable for holding a value of type `T`.
+    pub fn new<T>() -> Self {
+        let (size, align) = size_align::<T>();
+        Layout::from_size_align(size, align)
+    }
+
+    /// Produces layout describing a record that could be used to
+    /// allocate backing structure for `T` (which could be a trait
+    /// or other unsized type like a slice).
+    pub fn for_value<T: ?Sized>(t: &T) -> Self {
+        let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
+        Layout::from_size_align(size, align)
+    }
+
+    /// Creates a layout describing the record that can hold a value
+    /// of the same layout as `self`, but that also is aligned to
+    /// alignment `align` (measured in bytes).
+    ///
+    /// If `self` already meets the prescribed alignment, then returns
+    /// `self`.
+    ///
+    /// Note that this method does not add any padding to the overall
+    /// size, regardless of whether the returned layout has a different
+    /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
+    /// will *still* have size 16.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `align` is not a power of two.
+    pub fn align_to(&self, align: usize) -> Self {
+        assert!(align.is_power_of_two());
+        Layout::from_size_align(self.size, cmp::max(self.align, align))
+    }
+
+    /// Returns the amount of padding we must insert after `self`
+    /// to ensure that the following address will satisfy `align`
+    /// (measured in bytes).
+    ///
+    /// E.g. if `self.size` is 9, then `self.padding_needed_for(4)`
+    /// returns 3, because that is the minimum number of bytes of
+    /// padding required to get a 4-aligned address (assuming that the
+    /// corresponding memory block starts at a 4-aligned address).
+    ///
+    /// The return value of this function has no meaning if `align` is
+    /// not a power-of-two.
+    ///
+    /// Note that the utility of the returned value requires `align`
+    /// to be less than or equal to the alignment of the starting
+    /// address for the whole allocated block of memory. One way to
+    /// satisfy this constraint is to ensure `align <= self.align`.
+    pub fn padding_needed_for(&self, align: usize) -> usize {
+        let len = self.size();
+
+        // Rounded up value is:
+        //   len_rounded_up = (len + align - 1) & !(align - 1);
+        // and then we return the padding difference: `len_rounded_up - len`.
+        //
+        // We use modular arithmetic throughout:
+        //
+        // 1. align is guaranteed to be > 0, so align - 1 is always
+        //    valid.
+        //
+        // 2. `len + align - 1` can overflow by at most `align - 1`,
+        //    so the &-mask wth `!(align - 1)` will ensure that in the
+        //    case of overflow, `len_rounded_up` will itself be 0.
+        //    Thus the returned padding, when added to `len`, yields 0,
+        //    which trivially satisfies the alignment `align`.
+        //
+        // (Of course, attempts to allocate blocks of memory whose
+        // size and padding overflow in the above manner should cause
+        // the allocator to yield an error anyway.)
+
+        let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
+        return len_rounded_up.wrapping_sub(len);
+    }
+
+    /// Creates a layout describing the record for `n` instances of
+    /// `self`, with a suitable amount of padding between each to
+    /// ensure that each instance is given its requested size and
+    /// alignment. On success, returns `(k, offs)` where `k` is the
+    /// layout of the array and `offs` is the distance between the start
+    /// of each element in the array.
+    ///
+    /// On arithmetic overflow, returns `None`.
+    pub fn repeat(&self, n: usize) -> Option<(Self, usize)> {
+        let padded_size = match self.size.checked_add(self.padding_needed_for(self.align)) {
+            None => return None,
+            Some(padded_size) => padded_size,
+        };
+        let alloc_size = match padded_size.checked_mul(n) {
+            None => return None,
+            Some(alloc_size) => alloc_size,
+        };
+        Some((Layout::from_size_align(alloc_size, self.align), padded_size))
+    }
+
+    /// Creates a layout describing the record for `self` followed by
+    /// `next`, including any necessary padding to ensure that `next`
+    /// will be properly aligned. Note that the result layout will
+    /// satisfy the alignment properties of both `self` and `next`.
+    ///
+    /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
+    /// record and `offset` is the relative location, in bytes, of the
+    /// start of the `next` embedded witnin the concatenated record
+    /// (assuming that the record itself starts at offset 0).
+    ///
+    /// On arithmetic overflow, returns `None`.
+    pub fn extend(&self, next: Self) -> Option<(Self, usize)> {
+        let new_align = cmp::max(self.align, next.align);
+        let realigned = Layout::from_size_align(self.size, new_align);
+        let pad = realigned.padding_needed_for(next.align);
+        let offset = match self.size.checked_add(pad) {
+            None => return None,
+            Some(offset) => offset,
+        };
+        let new_size = match offset.checked_add(next.size) {
+            None => return None,
+            Some(new_size) => new_size,
+        };
+        Some((Layout::from_size_align(new_size, new_align), offset))
+    }
+
+    /// Creates a layout describing the record for `n` instances of
+    /// `self`, with no padding between each instance.
+    ///
+    /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
+    /// that the repeated instances of `self` will be properly
+    /// aligned, even if a given instance of `self` is properly
+    /// aligned. In other words, if the layout returned by
+    /// `repeat_packed` is used to allocate an array, it is not
+    /// guaranteed that all elements in the array will be properly
+    /// aligned.
+    ///
+    /// On arithmetic overflow, returns `None`.
+    pub fn repeat_packed(&self, n: usize) -> Option<Self> {
+        let size = match self.size().checked_mul(n) {
+            None => return None,
+            Some(scaled) => scaled,
+        };
+        Some(Layout::from_size_align(size, self.align))
+    }
+
+    /// Creates a layout describing the record for `self` followed by
+    /// `next` with no additional padding between the two. Since no
+    /// padding is inserted, the alignment of `next` is irrelevant,
+    /// and is not incoporated *at all* into the resulting layout.
+    ///
+    /// Returns `(k, offset)`, where `k` is layout of the concatenated
+    /// record and `offset` is the relative location, in bytes, of the
+    /// start of the `next` embedded witnin the concatenated record
+    /// (assuming that the record itself starts at offset 0).
+    ///
+    /// (The `offset` is always the same as `self.size()`; we use this
+    ///  signature out of convenience in matching the signature of
+    ///  `extend`.)
+    ///
+    /// On arithmetic overflow, returns `None`.
+    pub fn extend_packed(&self, next: Self) -> Option<(Self, usize)> {
+        let new_size = match self.size().checked_add(next.size()) {
+            None => return None,
+            Some(new_size) => new_size,
+        };
+        Some((Layout::from_size_align(new_size, self.align), self.size()))
+    }
+
+    /// Creates a layout describing the record for a `[T; n]`.
+    ///
+    /// On arithmetic overflow, returns `None`.
+    pub fn array<T>(n: usize) -> Option<Self> {
+        Layout::new::<T>()
+            .repeat(n)
+            .map(|(k, offs)| {
+                debug_assert!(offs == mem::size_of::<T>());
+                k
+            })
+    }
+}
+
+/// The `AllocErr` error specifies whether an allocation failure is
+/// specifically due to resource exhaustion or if it is due to
+/// something wrong when combining the given input arguments with this
+/// allocator.
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum AllocErr {
+    /// Error due to hitting some resource limit or otherwise running
+    /// out of memory. This condition strongly implies that *some*
+    /// series of deallocations would allow a subsequent reissuing of
+    /// the original allocation request to succeed.
+    Exhausted { request: Layout },
+
+    /// Error due to allocator being fundamentally incapable of
+    /// satisfying the original request. This condition implies that
+    /// such an allocation request will never succeed on the given
+    /// allocator, regardless of environment, memory pressure, or
+    /// other contextual conditions.
+    ///
+    /// For example, an allocator that does not support requests for
+    /// large memory blocks might return this error variant.
+    Unsupported { details: &'static str },
+}
+
+impl AllocErr {
+    pub fn invalid_input(details: &'static str) -> Self {
+        AllocErr::Unsupported { details: details }
+    }
+    pub fn is_memory_exhausted(&self) -> bool {
+        if let AllocErr::Exhausted { .. } = *self { true } else { false }
+    }
+    pub fn is_request_unsupported(&self) -> bool {
+        if let AllocErr::Unsupported { .. } = *self { true } else { false }
+    }
+}
+
+/// The `CannotReallocInPlace` error is used when `grow_in_place` or
+/// `shrink_in_place` were unable to reuse the given memory block for
+/// a requested layout.
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub struct CannotReallocInPlace;
+
+/// An implementation of `Alloc` can allocate, reallocate, and
+/// deallocate arbitrary blocks of data described via `Layout`.
+///
+/// Some of the methods require that a memory block be *currently
+/// allocated* via an allocator. This means that:
+///
+/// * the starting address for that memory block was previously
+///   returned by a previous call to an allocation method (`alloc`,
+///   `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
+///   reallocation method (`realloc`, `realloc_excess`, or
+///   `realloc_array`), and
+///
+/// * the memory block has not been subsequently deallocated, where
+///   blocks are deallocated either by being passed to a deallocation
+///   method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
+///   passed to a reallocation method (see above) that returns `Ok`.
+///
+/// A note regarding zero-sized types and zero-sized layouts: many
+/// methods in the `Alloc` trait state that allocation requests
+/// must be non-zero size, or else undefined behavior can result.
+///
+/// * However, some higher-level allocation methods (`alloc_one`,
+///   `alloc_array`) are well-defined on zero-sized types and can
+///   optionally support them: it is left up to the implementor
+///   whether to return `Err`, or to return `Ok` with some pointer.
+///
+/// * If an `Alloc` implementation chooses to return `Ok` in this
+///   case (i.e. the pointer denotes a zero-sized inaccessible block)
+///   then that returned pointer must be considered "currently
+///   allocated". On such an allocator, *all* methods that take
+///   currently-allocated pointers as inputs must accept these
+///   zero-sized pointers, *without* causing undefined behavior.
+///
+/// * In other words, if a zero-sized pointer can flow out of an
+///   allocator, then that allocator must likewise accept that pointer
+///   flowing back into its deallocation and reallocation methods.
+///
+/// Some of the methods require that a layout *fit* a memory block.
+/// What it means for a layout to "fit" a memory block means (or
+/// equivalently, for a memory block to "fit" a layout) is that the
+/// following two conditions must hold:
+///
+/// 1. The block's starting address must be aligned to `layout.align()`.
+///
+/// 2. The block's size must fall in the range `[use_min, use_max]`, where:
+///
+///    * `use_min` is `self.usable_size(layout).0`, and
+///
+///    * `use_max` is the capacity that was (or would have been)
+///      returned when (if) the block was allocated via a call to
+///      `alloc_excess` or `realloc_excess`.
+///
+/// Note that:
+///
+///  * the size of the layout most recently used to allocate the block
+///    is guaranteed to be in the range `[use_min, use_max]`, and
+///
+///  * a lower-bound on `use_max` can be safely approximated by a call to
+///    `usable_size`.
+///
+///  * if a layout `k` fits a memory block (denoted by `ptr`)
+///    currently allocated via an allocator `a`, then it is legal to
+///    use that layout to deallocate it, i.e. `a.dealloc(ptr, k);`.
+pub unsafe trait Alloc {
+
+    // (Note: existing allocators have unspecified but well-defined
+    // behavior in response to a zero size allocation request ;
+    // e.g. in C, `malloc` of 0 will either return a null pointer or a
+    // unique pointer, but will not have arbitrary undefined
+    // behavior. Rust should consider revising the alloc::heap crate
+    // to reflect this reality.)
+
+    /// Returns a pointer meeting the size and alignment guarantees of
+    /// `layout`.
+    ///
+    /// If this method returns an `Ok(addr)`, then the `addr` returned
+    /// will be non-null address pointing to a block of storage
+    /// suitable for holding an instance of `layout`.
+    ///
+    /// The returned block of storage may or may not have its contents
+    /// initialized. (Extension subtraits might restrict this
+    /// behavior, e.g. to ensure initialization to particular sets of
+    /// bit patterns.)
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure that `layout` has non-zero size.
+    ///
+    /// (Extension subtraits might provide more specific bounds on
+    /// behavior, e.g. guarantee a sentinel address or a null pointer
+    /// in response to a zero-size allocation request.)
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `layout` does not meet allocator's size or alignment
+    /// constraints.
+    ///
+    /// Implementations are encouraged to return `Err` on memory
+    /// exhaustion rather than panicking or aborting, but this is not
+    /// a strict requirement. (Specifically: it is *legal* to
+    /// implement this trait atop an underlying native allocation
+    /// library that aborts on memory exhaustion.)
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// allocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr>;
+
+    /// Deallocate the memory referenced by `ptr`.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure all of the following:
+    ///
+    /// * `ptr` must denote a block of memory currently allocated via
+    ///   this allocator,
+    ///
+    /// * `layout` must *fit* that block of memory,
+    ///
+    /// * In addition to fitting the block of memory `layout`, the
+    ///   alignment of the `layout` must match the alignment used
+    ///   to allocate that block of memory.
+    unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout);
+
+    /// Allocator-specific method for signalling an out-of-memory
+    /// condition.
+    ///
+    /// `oom` aborts the thread or process, optionally performing
+    /// cleanup or logging diagnostic information before panicking or
+    /// aborting.
+    ///
+    /// `oom` is meant to be used by clients unable to cope with an
+    /// unsatisfied allocation request (signaled by an error such as
+    /// `AllocErr::Exhausted`), and wish to abandon computation rather
+    /// than attempt to recover locally. Such clients should pass the
+    /// signalling error value back into `oom`, where the allocator
+    /// may incorporate that error value into its diagnostic report
+    /// before aborting.
+    ///
+    /// Implementations of the `oom` method are discouraged from
+    /// infinitely regressing in nested calls to `oom`. In
+    /// practice this means implementors should eschew allocating,
+    /// especially from `self` (directly or indirectly).
+    ///
+    /// Implementions of the allocation and reallocation methods
+    /// (e.g. `alloc`, `alloc_one`, `realloc`) are discouraged from
+    /// panicking (or aborting) in the event of memory exhaustion;
+    /// instead they should return an appropriate error from the
+    /// invoked method, and let the client decide whether to invoke
+    /// this `oom` method in response.
+    fn oom(&mut self, _: AllocErr) -> ! {
+        unsafe { ::core::intrinsics::abort() }
+    }
+
+    // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
+    // usable_size
+
+    /// Returns bounds on the guaranteed usable size of a successful
+    /// allocation created with the specified `layout`.
+    ///
+    /// In particular, if one has a memory block allocated via a given
+    /// allocator `a` and layout `k` where `a.usable_size(k)` returns
+    /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
+    /// layout in the size range [l, u].
+    ///
+    /// (All implementors of `usable_size` must ensure that
+    /// `l <= k.size() <= u`)
+    ///
+    /// Both the lower- and upper-bounds (`l` and `u` respectively)
+    /// are provided, because an allocator based on size classes could
+    /// misbehave if one attempts to deallocate a block without
+    /// providing a correct value for its size (i.e., one within the
+    /// range `[l, u]`).
+    ///
+    /// Clients who wish to make use of excess capacity are encouraged
+    /// to use the `alloc_excess` and `realloc_excess` instead, as
+    /// this method is constrained to report conservative values that
+    /// serve as valid bounds for *all possible* allocation method
+    /// calls.
+    ///
+    /// However, for clients that do not wish to track the capacity
+    /// returned by `alloc_excess` locally, this method is likely to
+    /// produce useful results.
+    fn usable_size(&self, layout: &Layout) -> (usize, usize) {
+        (layout.size(), layout.size())
+    }
+
+    // == METHODS FOR MEMORY REUSE ==
+    // realloc. alloc_excess, realloc_excess
+
+    /// Returns a pointer suitable for holding data described by
+    /// `new_layout`, meeting its size and alignment guarantees. To
+    /// accomplish this, this may extend or shrink the allocation
+    /// referenced by `ptr` to fit `new_layout`.
+    ///
+    /// If this returns `Ok`, then ownership of the memory block
+    /// referenced by `ptr` has been transferred to this
+    /// allocator. The memory may or may not have been freed, and
+    /// should be considered unusable (unless of course it was
+    /// transferred back to the caller again via the return value of
+    /// this method).
+    ///
+    /// If this method returns `Err`, then ownership of the memory
+    /// block has not been transferred to this allocator, and the
+    /// contents of the memory block are unaltered.
+    ///
+    /// For best results, `new_layout` should not impose a different
+    /// alignment constraint than `layout`. (In other words,
+    /// `new_layout.align()` should equal `layout.align()`.) However,
+    /// behavior is well-defined (though underspecified) when this
+    /// constraint is violated; further discussion below.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure all of the following:
+    ///
+    /// * `ptr` must be currently allocated via this allocator,
+    ///
+    /// * `layout` must *fit* the `ptr` (see above). (The `new_layout`
+    ///   argument need not fit it.)
+    ///
+    /// * `new_layout` must have size greater than zero.
+    ///
+    /// * the alignment of `new_layout` is non-zero.
+    ///
+    /// (Extension subtraits might provide more specific bounds on
+    /// behavior, e.g. guarantee a sentinel address or a null pointer
+    /// in response to a zero-size allocation request.)
+    ///
+    /// # Errors
+    ///
+    /// Returns `Err` only if `new_layout` does not match the
+    /// alignment of `layout`, or does not meet the allocator's size
+    /// and alignment constraints of the allocator, or if reallocation
+    /// otherwise fails.
+    ///
+    /// (Note the previous sentence did not say "if and only if" -- in
+    /// particular, an implementation of this method *can* return `Ok`
+    /// if `new_layout.align() != old_layout.align()`; or it can
+    /// return `Err` in that scenario, depending on whether this
+    /// allocator can dynamically adjust the alignment constraint for
+    /// the block.)
+    ///
+    /// Implementations are encouraged to return `Err` on memory
+    /// exhaustion rather than panicking or aborting, but this is not
+    /// a strict requirement. (Specifically: it is *legal* to
+    /// implement this trait atop an underlying native allocation
+    /// library that aborts on memory exhaustion.)
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// reallocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn realloc(&mut self,
+                      ptr: *mut u8,
+                      layout: Layout,
+                      new_layout: Layout) -> Result<*mut u8, AllocErr> {
+        let new_size = new_layout.size();
+        let old_size = layout.size();
+        let aligns_match = layout.align == new_layout.align;
+
+        if new_size >= old_size && aligns_match {
+            if let Ok(()) = self.grow_in_place(ptr, layout.clone(), new_layout.clone()) {
+                return Ok(ptr);
+            }
+        } else if new_size < old_size && aligns_match {
+            if let Ok(()) = self.shrink_in_place(ptr, layout.clone(), new_layout.clone()) {
+                return Ok(ptr);
+            }
+        }
+
+        // otherwise, fall back on alloc + copy + dealloc.
+        let result = self.alloc(new_layout);
+        if let Ok(new_ptr) = result {
+            ptr::copy_nonoverlapping(ptr as *const u8, new_ptr, cmp::min(old_size, new_size));
+            self.dealloc(ptr, layout);
+        }
+        result
+    }
+
+    /// Behaves like `alloc`, but also ensures that the contents
+    /// are set to zero before being returned.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe for the same reasons that `alloc` is.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `layout` does not meet allocator's size or alignment
+    /// constraints, just as in `alloc`.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// allocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<*mut u8, AllocErr> {
+        let size = layout.size();
+        let p = self.alloc(layout);
+        if let Ok(p) = p {
+            ptr::write_bytes(p, 0, size);
+        }
+        p
+    }
+
+    /// Behaves like `alloc`, but also returns the whole size of
+    /// the returned block. For some `layout` inputs, like arrays, this
+    /// may include extra storage usable for additional data.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe for the same reasons that `alloc` is.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `layout` does not meet allocator's size or alignment
+    /// constraints, just as in `alloc`.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// allocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
+        let usable_size = self.usable_size(&layout);
+        self.alloc(layout).map(|p| Excess(p, usable_size.1))
+    }
+
+    /// Behaves like `realloc`, but also returns the whole size of
+    /// the returned block. For some `layout` inputs, like arrays, this
+    /// may include extra storage usable for additional data.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe for the same reasons that `realloc` is.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `layout` does not meet allocator's size or alignment
+    /// constraints, just as in `realloc`.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// reallocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn realloc_excess(&mut self,
+                             ptr: *mut u8,
+                             layout: Layout,
+                             new_layout: Layout) -> Result<Excess, AllocErr> {
+        let usable_size = self.usable_size(&new_layout);
+        self.realloc(ptr, layout, new_layout)
+            .map(|p| Excess(p, usable_size.1))
+    }
+
+    /// Attempts to extend the allocation referenced by `ptr` to fit `new_layout`.
+    ///
+    /// If this returns `Ok`, then the allocator has asserted that the
+    /// memory block referenced by `ptr` now fits `new_layout`, and thus can
+    /// be used to carry data of that layout. (The allocator is allowed to
+    /// expend effort to accomplish this, such as extending the memory block to
+    /// include successor blocks, or virtual memory tricks.)
+    ///
+    /// Regardless of what this method returns, ownership of the
+    /// memory block referenced by `ptr` has not been transferred, and
+    /// the contents of the memory block are unaltered.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure all of the following:
+    ///
+    /// * `ptr` must be currently allocated via this allocator,
+    ///
+    /// * `layout` must *fit* the `ptr` (see above); note the
+    ///   `new_layout` argument need not fit it,
+    ///
+    /// * `new_layout.size()` must not be less than `layout.size()`,
+    ///
+    /// * `new_layout.align()` must equal `layout.align()`.
+    ///
+    /// # Errors
+    ///
+    /// Returns `Err(CannotReallocInPlace)` when the allocator is
+    /// unable to assert that the memory block referenced by `ptr`
+    /// could fit `layout`.
+    ///
+    /// Note that one cannot pass `CannotReallocInPlace` to the `oom`
+    /// method; clients are expected either to be able to recover from
+    /// `grow_in_place` failures without aborting, or to fall back on
+    /// another reallocation method before resorting to an abort.
+    unsafe fn grow_in_place(&mut self,
+                            ptr: *mut u8,
+                            layout: Layout,
+                            new_layout: Layout) -> Result<(), CannotReallocInPlace> {
+        let _ = ptr; // this default implementation doesn't care about the actual address.
+        debug_assert!(new_layout.size >= layout.size);
+        debug_assert!(new_layout.align == layout.align);
+        let (_l, u) = self.usable_size(&layout);
+        // _l <= layout.size()                       [guaranteed by usable_size()]
+        //       layout.size() <= new_layout.size()  [required by this method]
+        if new_layout.size <= u {
+            return Ok(());
+        } else {
+            return Err(CannotReallocInPlace);
+        }
+    }
+
+    /// Attempts to shrink the allocation referenced by `ptr` to fit `new_layout`.
+    ///
+    /// If this returns `Ok`, then the allocator has asserted that the
+    /// memory block referenced by `ptr` now fits `new_layout`, and
+    /// thus can only be used to carry data of that smaller
+    /// layout. (The allocator is allowed to take advantage of this,
+    /// carving off portions of the block for reuse elsewhere.) The
+    /// truncated contents of the block within the smaller layout are
+    /// unaltered, and ownership of block has not been transferred.
+    ///
+    /// If this returns `Err`, then the memory block is considered to
+    /// still represent the original (larger) `layout`. None of the
+    /// block has been carved off for reuse elsewhere, ownership of
+    /// the memory block has not been transferred, and the contents of
+    /// the memory block are unaltered.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure all of the following:
+    ///
+    /// * `ptr` must be currently allocated via this allocator,
+    ///
+    /// * `layout` must *fit* the `ptr` (see above); note the
+    ///   `new_layout` argument need not fit it,
+    ///
+    /// * `new_layout.size()` must not be greater than `layout.size()`
+    ///   (and must be greater than zero),
+    ///
+    /// * `new_layout.align()` must equal `layout.align()`.
+    ///
+    /// # Errors
+    ///
+    /// Returns `Err(CannotReallocInPlace)` when the allocator is
+    /// unable to assert that the memory block referenced by `ptr`
+    /// could fit `layout`.
+    ///
+    /// Note that one cannot pass `CannotReallocInPlace` to the `oom`
+    /// method; clients are expected either to be able to recover from
+    /// `shrink_in_place` failures without aborting, or to fall back
+    /// on another reallocation method before resorting to an abort.
+    unsafe fn shrink_in_place(&mut self,
+                              ptr: *mut u8,
+                              layout: Layout,
+                              new_layout: Layout) -> Result<(), CannotReallocInPlace> {
+        let _ = ptr; // this default implementation doesn't care about the actual address.
+        debug_assert!(new_layout.size <= layout.size);
+        debug_assert!(new_layout.align == layout.align);
+        let (l, _u) = self.usable_size(&layout);
+        //                      layout.size() <= _u  [guaranteed by usable_size()]
+        // new_layout.size() <= layout.size()        [required by this method]
+        if l <= new_layout.size {
+            return Ok(());
+        } else {
+            return Err(CannotReallocInPlace);
+        }
+    }
+
+
+    // == COMMON USAGE PATTERNS ==
+    // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array
+
+    /// Allocates a block suitable for holding an instance of `T`.
+    ///
+    /// Captures a common usage pattern for allocators.
+    ///
+    /// The returned block is suitable for passing to the
+    /// `alloc`/`realloc` methods of this allocator.
+    ///
+    /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
+    /// must be considered "currently allocated" and must be
+    /// acceptable input to methods such as `realloc` or `dealloc`,
+    /// *even if* `T` is a zero-sized type. In other words, if your
+    /// `Alloc` implementation overrides this method in a manner
+    /// that can return a zero-sized `ptr`, then all reallocation and
+    /// deallocation methods need to be similarly overridden to accept
+    /// such values as input.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `T` does not meet allocator's size or alignment constraints.
+    ///
+    /// For zero-sized `T`, may return either of `Ok` or `Err`, but
+    /// will *not* yield undefined behavior.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// allocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    fn alloc_one<T>(&mut self) -> Result<Unique<T>, AllocErr>
+        where Self: Sized
+    {
+        let k = Layout::new::<T>();
+        if k.size() > 0 {
+            unsafe { self.alloc(k).map(|p|Unique::new(*p as *mut T)) }
+        } else {
+            Err(AllocErr::invalid_input("zero-sized type invalid for alloc_one"))
+        }
+    }
+
+    /// Deallocates a block suitable for holding an instance of `T`.
+    ///
+    /// The given block must have been produced by this allocator,
+    /// and must be suitable for storing a `T` (in terms of alignment
+    /// as well as minimum and maximum size); otherwise yields
+    /// undefined behavior.
+    ///
+    /// Captures a common usage pattern for allocators.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure both:
+    ///
+    /// * `ptr` must denote a block of memory currently allocated via this allocator
+    ///
+    /// * the layout of `T` must *fit* that block of memory.
+    unsafe fn dealloc_one<T>(&mut self, ptr: Unique<T>)
+        where Self: Sized
+    {
+        let raw_ptr = ptr.as_ptr() as *mut u8;
+        let k = Layout::new::<T>();
+        if k.size() > 0 {
+            self.dealloc(raw_ptr, k);
+        }
+    }
+
+    /// Allocates a block suitable for holding `n` instances of `T`.
+    ///
+    /// Captures a common usage pattern for allocators.
+    ///
+    /// The returned block is suitable for passing to the
+    /// `alloc`/`realloc` methods of this allocator.
+    ///
+    /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
+    /// must be considered "currently allocated" and must be
+    /// acceptable input to methods such as `realloc` or `dealloc`,
+    /// *even if* `T` is a zero-sized type. In other words, if your
+    /// `Alloc` implementation overrides this method in a manner
+    /// that can return a zero-sized `ptr`, then all reallocation and
+    /// deallocation methods need to be similarly overridden to accept
+    /// such values as input.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `[T; n]` does not meet allocator's size or alignment
+    /// constraints.
+    ///
+    /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
+    /// `Err`, but will *not* yield undefined behavior.
+    ///
+    /// Always returns `Err` on arithmetic overflow.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// allocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    fn alloc_array<T>(&mut self, n: usize) -> Result<Unique<T>, AllocErr>
+        where Self: Sized
+    {
+        match Layout::array::<T>(n) {
+            Some(ref layout) if layout.size() > 0 => {
+                unsafe {
+                    self.alloc(layout.clone())
+                        .map(|p| {
+                            Unique::new(p as *mut T)
+                        })
+                }
+            }
+            _ => Err(AllocErr::invalid_input("invalid layout for alloc_array")),
+        }
+    }
+
+    /// Reallocates a block previously suitable for holding `n_old`
+    /// instances of `T`, returning a block suitable for holding
+    /// `n_new` instances of `T`.
+    ///
+    /// Captures a common usage pattern for allocators.
+    ///
+    /// The returned block is suitable for passing to the
+    /// `alloc`/`realloc` methods of this allocator.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure all of the following:
+    ///
+    /// * `ptr` must be currently allocated via this allocator,
+    ///
+    /// * the layout of `[T; n_old]` must *fit* that block of memory.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either memory is exhausted or
+    /// `[T; n_new]` does not meet allocator's size or alignment
+    /// constraints.
+    ///
+    /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
+    /// `Err`, but will *not* yield undefined behavior.
+    ///
+    /// Always returns `Err` on arithmetic overflow.
+    ///
+    /// Clients wishing to abort computation in response to an
+    /// reallocation error are encouraged to call the allocator's `oom`
+    /// method, rather than directly invoking `panic!` or similar.
+    unsafe fn realloc_array<T>(&mut self,
+                               ptr: Unique<T>,
+                               n_old: usize,
+                               n_new: usize) -> Result<Unique<T>, AllocErr>
+        where Self: Sized
+    {
+        match (Layout::array::<T>(n_old), Layout::array::<T>(n_new), ptr.as_ptr()) {
+            (Some(ref k_old), Some(ref k_new), ptr) if k_old.size() > 0 && k_new.size() > 0 => {
+                self.realloc(ptr as *mut u8, k_old.clone(), k_new.clone())
+                    .map(|p|Unique::new(p as *mut T))
+            }
+            _ => {
+                Err(AllocErr::invalid_input("invalid layout for realloc_array"))
+            }
+        }
+    }
+
+    /// Deallocates a block suitable for holding `n` instances of `T`.
+    ///
+    /// Captures a common usage pattern for allocators.
+    ///
+    /// # Unsafety
+    ///
+    /// This function is unsafe because undefined behavior can result
+    /// if the caller does not ensure both:
+    ///
+    /// * `ptr` must denote a block of memory currently allocated via this allocator
+    ///
+    /// * the layout of `[T; n]` must *fit* that block of memory.
+    ///
+    /// # Errors
+    ///
+    /// Returning `Err` indicates that either `[T; n]` or the given
+    /// memory block does not meet allocator's size or alignment
+    /// constraints.
+    ///
+    /// Always returns `Err` on arithmetic overflow.
+    unsafe fn dealloc_array<T>(&mut self, ptr: Unique<T>, n: usize) -> Result<(), AllocErr>
+        where Self: Sized
+    {
+        let raw_ptr = ptr.as_ptr() as *mut u8;
+        match Layout::array::<T>(n) {
+            Some(ref k) if k.size() > 0 => {
+                Ok(self.dealloc(raw_ptr, k.clone()))
+            }
+            _ => {
+                Err(AllocErr::invalid_input("invalid layout for dealloc_array"))
+            }
+        }
+    }
+}
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 5252dabc127..ca52943ea97 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -143,6 +143,10 @@ extern crate std_unicode;
 #[macro_use]
 mod macros;
 
+// Allocator trait and helper struct definitions
+
+pub mod allocator;
+
 // Heaps provided for low-level allocation strategies
 
 pub mod heap;