library/alloc/src/vec/spec_extend.rs

   1 use crate::alloc::Allocator;
   2 use core::iter::TrustedLen;
   3 use core::ptr::{self};
   4 use core::slice::{self};
   5
   6 use super::{IntoIter, SetLenOnDrop, Vec};
   7
   8 // Specialization trait used for Vec::extend
   9 pub(super) trait SpecExtend<T, I> {
  10     fn spec_extend(&mut self, iter: I);
  11 }
  12
  13 impl<T, I, A: Allocator> SpecExtend<T, I> for Vec<T, A>
  14 where
  15     I: Iterator<Item = T>,
  16 {
  17     default fn spec_extend(&mut self, iter: I) {
  18         self.extend_desugared(iter)
  19     }
  20 }
  21
  22 impl<T, I, A: Allocator> SpecExtend<T, I> for Vec<T, A>
  23 where
  24     I: TrustedLen<Item = T>,
  25 {
  26     default fn spec_extend(&mut self, iterator: I) {
  27         // This is the case for a TrustedLen iterator.
  28         let (low, high) = iterator.size_hint();
  29         if let Some(additional) = high {
  30             debug_assert_eq!(
  31                 low,
  32                 additional,
  33                 "TrustedLen iterator's size hint is not exact: {:?}",
  34                 (low, high)
  35             );
  36             self.reserve(additional);
  37             unsafe {
  38                 let mut ptr = self.as_mut_ptr().add(self.len());
  39                 let mut local_len = SetLenOnDrop::new(&mut self.len);
  40                 iterator.for_each(move |element| {
  41                     ptr::write(ptr, element);
  42                     ptr = ptr.offset(1);
  43                     // Since the loop executes user code which can panic we have to bump the pointer
  44                     // after each step.
  45                     // NB can't overflow since we would have had to alloc the address space
  46                     local_len.increment_len(1);
  47                 });
  48             }
  49         } else {
  50             // Per TrustedLen contract a `None` upper bound means that the iterator length
  51             // truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
  52             // Since the other branch already panics eagerly (via `reserve()`) we do the same here.
  53             // This avoids additional codegen for a fallback code path which would eventually
  54             // panic anyway.
  55             panic!("capacity overflow");
  56         }
  57     }
  58 }
  59
  60 impl<T, A: Allocator> SpecExtend<T, IntoIter<T>> for Vec<T, A> {
  61     fn spec_extend(&mut self, mut iterator: IntoIter<T>) {
  62         unsafe {
  63             self.append_elements(iterator.as_slice() as _);
  64         }
  65         iterator.ptr = iterator.end;
  66     }
  67 }
  68
  69 impl<'a, T: 'a, I, A: Allocator + 'a> SpecExtend<&'a T, I> for Vec<T, A>
  70 where
  71     I: Iterator<Item = &'a T>,
  72     T: Clone,
  73 {
  74     default fn spec_extend(&mut self, iterator: I) {
  75         self.spec_extend(iterator.cloned())
  76     }
  77 }
  78
  79 impl<'a, T: 'a, A: Allocator + 'a> SpecExtend<&'a T, slice::Iter<'a, T>> for Vec<T, A>
  80 where
  81     T: Copy,
  82 {
  83     fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
  84         let slice = iterator.as_slice();
  85         unsafe { self.append_elements(slice) };
  86     }
  87 }