Module alloc::vec::in_place_collect
source · Expand description
Inplace iterate-and-collect specialization for Vec
Note: This documents Vec internals, some of the following sections explain implementation details and are best read together with the source of this module.
The specialization in this module applies to iterators in the shape of
source.adapter().adapter().adapter().collect::<Vec<U>>()
where source is an owning iterator obtained from Vec<T>, Box<[T]> (by conversion to Vec)
or BinaryHeap<T>, the adapters each consume one or more items per step
(represented by InPlaceIterable), provide transitive access to source (via SourceIter)
and thus the underlying allocation. And finally the layouts of T and U must
have the same size and alignment, this is currently ensured via const eval instead of trait bounds
in the specialized SpecFromIter implementation.
By extension some other collections which use collect::<Vec<_>>() internally in their
FromIterator implementation benefit from this too.
Access to the underlying source goes through a further layer of indirection via the private
trait AsVecIntoIter to hide the implementation detail that other collections may use
vec::IntoIter internally.
In-place iteration depends on the interaction of several unsafe traits, implementation details of multiple parts in the iterator pipeline and often requires holistic reasoning across multiple structs since iterators are executed cooperatively rather than having a central evaluator/visitor struct executing all iterator components.
Reading from and writing to the same allocation
By its nature collecting in place means that the reader and writer side of the iterator
use the same allocation. Since try_fold() (used in SpecInPlaceCollect) takes a
reference to the iterator for the duration of the iteration that means we can’t interleave
the step of reading a value and getting a reference to write to. Instead raw pointers must be
used on the reader and writer side.
That writes never clobber a yet-to-be-read item is ensured by the InPlaceIterable requirements.
Layout constraints
Allocator requires that allocate() and deallocate() have matching alignment and size.
Additionally this specialization doesn’t make sense for ZSTs as there is no reallocation to
avoid and it would make pointer arithmetic more difficult.
Drop- and panic-safety
Iteration can panic, requiring dropping the already written parts but also the remainder of the source. Iteration can also leave some source items unconsumed which must be dropped. All those drops in turn can panic which then must either leak the allocation or abort to avoid double-drops.
This is handled by the InPlaceDrop guard for sink items (U) and by
vec::IntoIter::forget_allocation_drop_remaining() for remaining source items (T).
If dropping any remaining source item (T) panics then InPlaceDstBufDrop will handle dropping
the already collected sink items (U) and freeing the allocation.
O(1) collect
The main iteration itself is further specialized when the iterator implements
TrustedRandomAccessNoCoerce to let the optimizer see that it is a counted loop with a single
induction variable. This can turn some iterators into a noop, i.e. it reduces them from O(n) to
O(1). This particular optimization is quite fickle and doesn’t always work, see #79308
Since unchecked accesses through that trait do not advance the read pointer of IntoIter
this would interact unsoundly with the requirements about dropping the tail described above.
But since the normal Drop implementation of IntoIter would suffer from the same problem it
is only correct for TrustedRandomAccessNoCoerce to be implemented when the items don’t
have a destructor. Thus that implicit requirement also makes the specialization safe to use for
in-place collection.
Note that this safety concern is about the correctness of impl Drop for IntoIter,
not the guarantees of InPlaceIterable.
Adapter implementations
The invariants for adapters are documented in SourceIter and InPlaceIterable, but
getting them right can be rather subtle for multiple, sometimes non-local reasons.
For example InPlaceIterable would be valid to implement for Peekable, except
that it is stateful, cloneable and IntoIter’s clone implementation shortens the underlying
allocation which means if the iterator has been peeked and then gets cloned there no longer is
enough room, thus breaking an invariant (#85322).
Examples
Some cases that are optimized by this specialization, more can be found in the Vec
benchmarks:
/// Converts a usize vec into an isize one.
pub fn cast(vec: Vec<usize>) -> Vec<isize> {
// Does not allocate, free or panic. On optlevel>=2 it does not loop.
// Of course this particular case could and should be written with `into_raw_parts` and
// `from_raw_parts` instead.
vec.into_iter().map(|u| u as isize).collect()
}/// Drops remaining items in `src` and if the layouts of `T` and `U` match it
/// returns an empty Vec backed by the original allocation. Otherwise it returns a new
/// empty vec.
pub fn recycle_allocation<T, U>(src: Vec<T>) -> Vec<U> {
src.into_iter().filter_map(|_| None).collect()
}let vec = vec![13usize; 1024];
let _ = vec.into_iter()
.enumerate()
.filter_map(|(idx, val)| if idx % 2 == 0 { Some(val+idx) } else {None})
.collect::<Vec<_>>();
// is equivalent to the following, but doesn't require bounds checks
let mut vec = vec![13usize; 1024];
let mut write_idx = 0;
for idx in 0..vec.len() {
if idx % 2 == 0 {
vec[write_idx] = vec[idx] + idx;
write_idx += 1;
}
}
vec.truncate(write_idx);Traits
- Internal helper trait for in-place iteration specialization.
- Specialization marker for collecting an iterator pipeline into a Vec while reusing the source allocation, i.e. executing the pipeline in place.
- Helper trait to hold specialized implementations of the in-place iterate-collect loop