diff --git a/library/alloc/src/vec/in_place_collect.rs b/library/alloc/src/vec/in_place_collect.rs index 6dc548fc8e9ae..282af8cc33fdd 100644 --- a/library/alloc/src/vec/in_place_collect.rs +++ b/library/alloc/src/vec/in_place_collect.rs @@ -1,18 +1,21 @@ //! 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::>()` -//! where `source` is an owning iterator obtained from [`Vec`], [`Box<[T]>`] (by conversion to `Vec`) +//! where `source` is an owning iterator obtained from [`Vec`], [`Box<[T]>`][box] (by conversion to `Vec`) //! or [`BinaryHeap`], 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. +//! have the same size and alignment, this is currently ensured via const eval instead of trait bounds +//! in the specialized [`SpecFromIter`] implementation. //! //! [`BinaryHeap`]: crate::collections::BinaryHeap -//! [`Box<[T]>`]: crate::boxed::Box +//! [box]: crate::boxed::Box //! -//! By extension some other collections which use `collect::Vec<_>()` internally in their +//! By extension some other collections which use `collect::>()` internally in their //! `FromIterator` implementation benefit from this too. //! //! Access to the underlying source goes through a further layer of indirection via the private @@ -27,10 +30,10 @@ //! # 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 `fold()` and co. take 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. +//! 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. //! @@ -49,7 +52,8 @@ //! All those drops in turn can panic which then must either leak the allocation or abort to avoid //! double-drops. //! -//! These tasks are handled by [`InPlaceDrop`] and [`vec::IntoIter::forget_allocation_drop_remaining()`] +//! This is handled by the [`InPlaceDrop`] guard for sink items (`U`) and by +//! [`vec::IntoIter::forget_allocation_drop_remaining()`] for remaining source items (`T`). //! //! [`vec::IntoIter::forget_allocation_drop_remaining()`]: super::IntoIter::forget_allocation_drop_remaining() //! @@ -57,10 +61,11 @@ //! //! 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 +//! [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] //! //! [#79308]: https://github.com/rust-lang/rust/issues/79308 +//! [induction variable]: https://en.wikipedia.org/wiki/Induction_variable //! //! 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. @@ -68,6 +73,8 @@ //! 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 //! @@ -76,7 +83,7 @@ //! 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). +//! enough room, thus breaking an invariant ([#85322]). //! //! [#85322]: https://github.com/rust-lang/rust/issues/85322 //! [`Peekable`]: core::iter::Peekable