9111b8ae97
Add `TrustedRandomAccess` specialization for `Vec::extend()` This should do roughly the same as the `TrustedLen` specialization but result in less IR by using `__iterator_get_unchecked` instead of `Iterator::for_each` Conflicting specializations are manually prioritized by grouping them under yet another helper trait.
119 lines
3.7 KiB
Rust
119 lines
3.7 KiB
Rust
use crate::alloc::Allocator;
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use core::iter::{TrustedLen, TrustedRandomAccess};
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use core::ptr::{self};
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use core::slice::{self};
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use super::{IntoIter, SetLenOnDrop, Vec};
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// Specialization trait used for Vec::extend
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pub(super) trait SpecExtend<T, I> {
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fn spec_extend(&mut self, iter: I);
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}
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impl<T, I, A: Allocator> SpecExtend<T, I> for Vec<T, A>
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where
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I: Iterator<Item = T>,
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{
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default fn spec_extend(&mut self, iter: I) {
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SpecExtendInner::spec_extend(self, iter);
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}
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}
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impl<T, A: Allocator> SpecExtend<T, IntoIter<T>> for Vec<T, A> {
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fn spec_extend(&mut self, mut iterator: IntoIter<T>) {
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unsafe {
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self.append_elements(iterator.as_slice() as _);
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}
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iterator.ptr = iterator.end;
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}
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}
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impl<'a, T: 'a, I, A: Allocator + 'a> SpecExtend<&'a T, I> for Vec<T, A>
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where
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I: Iterator<Item = &'a T>,
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T: Clone,
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{
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default fn spec_extend(&mut self, iterator: I) {
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SpecExtend::spec_extend(self, iterator.cloned())
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}
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}
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impl<'a, T: 'a, A: Allocator + 'a> SpecExtend<&'a T, slice::Iter<'a, T>> for Vec<T, A>
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where
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T: Copy,
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{
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fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
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let slice = iterator.as_slice();
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unsafe { self.append_elements(slice) };
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}
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}
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// Helper trait to disambiguate overlapping specializations
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trait SpecExtendInner<T, I> {
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fn spec_extend(&mut self, iter: I);
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}
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impl<T, I, A: Allocator> SpecExtendInner<T, I> for Vec<T, A>
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where
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I: Iterator<Item = T>,
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{
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default fn spec_extend(&mut self, iter: I) {
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self.extend_desugared(iter)
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}
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}
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impl<T, I, A: Allocator> SpecExtendInner<T, I> for Vec<T, A>
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where
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I: TrustedLen<Item = T>,
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{
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default fn spec_extend(&mut self, iterator: I) {
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// This is the case for a TrustedLen iterator.
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let (low, high) = iterator.size_hint();
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if let Some(additional) = high {
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debug_assert_eq!(
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low,
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additional,
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"TrustedLen iterator's size hint is not exact: {:?}",
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(low, high)
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);
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self.reserve(additional);
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unsafe {
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let mut ptr = self.as_mut_ptr().add(self.len());
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let mut local_len = SetLenOnDrop::new(&mut self.len);
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iterator.for_each(move |element| {
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ptr::write(ptr, element);
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ptr = ptr.offset(1);
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// NB can't overflow since we would have had to alloc the address space
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local_len.increment_len(1);
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});
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}
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} else {
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// Per TrustedLen contract a `None` upper bound means that the iterator length
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// truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
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// Since the other branch already panics eagerly (via `reserve()`) we do the same here.
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// This avoids additional codegen for a fallback code path which would eventually
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// panic anyway.
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panic!("capacity overflow");
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}
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}
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}
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impl<T, I, A: Allocator> SpecExtendInner<T, I> for Vec<T, A>
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where
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I: TrustedLen<Item = T> + TrustedRandomAccess,
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{
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default fn spec_extend(&mut self, mut iterator: I) {
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let size = iterator.size();
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self.reserve(size);
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// SAFETY: reserve ensured that there is sufficient capacity for the additional items.
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// The loop upholds the TRA requirements by accessing each element only once.
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unsafe {
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let sink = self.as_mut_ptr().add(self.len());
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for i in 0..size {
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ptr::write(sink.add(i), iterator.__iterator_get_unchecked(i));
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self.set_len(self.len() + 1);
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}
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}
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}
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}
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