109 lines
4.8 KiB
Rust
109 lines
4.8 KiB
Rust
use core::iter::{InPlaceIterable, SourceIter};
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use core::mem::{self, ManuallyDrop};
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use core::ptr::{self};
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use super::{AsIntoIter, InPlaceDrop, SpecFromIter, SpecFromIterNested, Vec};
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/// Specialization marker for collecting an iterator pipeline into a Vec while reusing the
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/// source allocation, i.e. executing the pipeline in place.
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///
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/// The SourceIter parent trait is necessary for the specializing function to access the allocation
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/// which is to be reused. But it is not sufficient for the specialization to be valid. See
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/// additional bounds on the impl.
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#[rustc_unsafe_specialization_marker]
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pub(super) trait SourceIterMarker: SourceIter<Source: AsIntoIter> {}
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// The std-internal SourceIter/InPlaceIterable traits are only implemented by chains of
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// Adapter<Adapter<Adapter<IntoIter>>> (all owned by core/std). Additional bounds
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// on the adapter implementations (beyond `impl<I: Trait> Trait for Adapter<I>`) only depend on other
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// traits already marked as specialization traits (Copy, TrustedRandomAccess, FusedIterator).
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// I.e. the marker does not depend on lifetimes of user-supplied types. Modulo the Copy hole, which
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// several other specializations already depend on.
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impl<T> SourceIterMarker for T where T: SourceIter<Source: AsIntoIter> + InPlaceIterable {}
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impl<T, I> SpecFromIter<T, I> for Vec<T>
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where
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I: Iterator<Item = T> + SourceIterMarker,
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{
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default fn from_iter(mut iterator: I) -> Self {
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// Additional requirements which cannot expressed via trait bounds. We rely on const eval
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// instead:
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// a) no ZSTs as there would be no allocation to reuse and pointer arithmetic would panic
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// b) size match as required by Alloc contract
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// c) alignments match as required by Alloc contract
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if mem::size_of::<T>() == 0
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|| mem::size_of::<T>()
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!= mem::size_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()
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|| mem::align_of::<T>()
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!= mem::align_of::<<<I as SourceIter>::Source as AsIntoIter>::Item>()
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{
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// fallback to more generic implementations
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return SpecFromIterNested::from_iter(iterator);
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}
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let (src_buf, src_ptr, dst_buf, dst_end, cap) = unsafe {
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let inner = iterator.as_inner().as_into_iter();
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(
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inner.buf.as_ptr(),
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inner.ptr,
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inner.buf.as_ptr() as *mut T,
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inner.end as *const T,
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inner.cap,
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)
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};
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// use try-fold since
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// - it vectorizes better for some iterator adapters
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// - unlike most internal iteration methods, it only takes a &mut self
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// - it lets us thread the write pointer through its innards and get it back in the end
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let sink = InPlaceDrop { inner: dst_buf, dst: dst_buf };
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let sink = iterator
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.try_fold::<_, _, Result<_, !>>(sink, write_in_place_with_drop(dst_end))
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.unwrap();
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// iteration succeeded, don't drop head
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let dst = ManuallyDrop::new(sink).dst;
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let src = unsafe { iterator.as_inner().as_into_iter() };
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// check if SourceIter contract was upheld
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// caveat: if they weren't we may not even make it to this point
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debug_assert_eq!(src_buf, src.buf.as_ptr());
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// check InPlaceIterable contract. This is only possible if the iterator advanced the
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// source pointer at all. If it uses unchecked access via TrustedRandomAccess
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// then the source pointer will stay in its initial position and we can't use it as reference
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if src.ptr != src_ptr {
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debug_assert!(
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dst as *const _ <= src.ptr,
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"InPlaceIterable contract violation, write pointer advanced beyond read pointer"
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);
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}
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// drop any remaining values at the tail of the source
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// but prevent drop of the allocation itself once IntoIter goes out of scope
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// if the drop panics then we also leak any elements collected into dst_buf
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src.forget_allocation_drop_remaining();
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let vec = unsafe {
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let len = dst.offset_from(dst_buf) as usize;
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Vec::from_raw_parts(dst_buf, len, cap)
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};
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vec
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}
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}
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fn write_in_place_with_drop<T>(
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src_end: *const T,
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) -> impl FnMut(InPlaceDrop<T>, T) -> Result<InPlaceDrop<T>, !> {
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move |mut sink, item| {
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unsafe {
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// the InPlaceIterable contract cannot be verified precisely here since
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// try_fold has an exclusive reference to the source pointer
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// all we can do is check if it's still in range
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debug_assert!(sink.dst as *const _ <= src_end, "InPlaceIterable contract violation");
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ptr::write(sink.dst, item);
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sink.dst = sink.dst.add(1);
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}
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Ok(sink)
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}
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}
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