1use core::mem::ManuallyDrop;
2use core::ptr::{self};
3
4use super::{IntoIter, SpecExtend, SpecFromIterNested, Vec};
5
6/// Specialization trait used for Vec::from_iter
7///
8/// ## The delegation graph:
9///
10/// ```text
11/// +-------------+
12/// |FromIterator |
13/// +-+-----------+
14/// |
15/// v
16/// +-+---------------------------------+ +---------------------+
17/// |SpecFromIter +---->+SpecFromIterNested |
18/// |where I: | | |where I: |
19/// | Iterator (default)------------+ | | Iterator (default) |
20/// | vec::IntoIter | | | TrustedLen |
21/// | InPlaceCollect--(fallback to)-+ | +---------------------+
22/// +-----------------------------------+
23/// ```
24pub(super) trait SpecFromIter<T, I> {
25 fn from_iter(iter: I) -> Self;
26}
27
28impl<T, I> SpecFromIter<T, I> for Vec<T>
29where
30 I: Iterator<Item = T>,
31{
32 default fn from_iter(iterator: I) -> Self {
33 SpecFromIterNested::from_iter(iterator)
34 }
35}
36
37impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
38 fn from_iter(iterator: IntoIter<T>) -> Self {
39 // A common case is passing a vector into a function which immediately
40 // re-collects into a vector. We can short circuit this if the IntoIter
41 // has not been advanced at all.
42 // When it has been advanced We can also reuse the memory and move the data to the front.
43 // But we only do so when the resulting Vec wouldn't have more unused capacity
44 // than creating it through the generic FromIterator implementation would. That limitation
45 // is not strictly necessary as Vec's allocation behavior is intentionally unspecified.
46 // But it is a conservative choice.
47 let has_advanced = iterator.buf != iterator.ptr;
48 if !has_advanced || iterator.len() >= iterator.cap / 2 {
49 unsafe {
50 let it = ManuallyDrop::new(iterator);
51 if has_advanced {
52 ptr::copy(it.ptr.as_ptr(), it.buf.as_ptr(), it.len());
53 }
54 return Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap);
55 }
56 }
57
58 let mut vec = Vec::new();
59 // must delegate to spec_extend() since extend() itself delegates
60 // to spec_from for empty Vecs
61 vec.spec_extend(iterator);
62 vec
63 }
64}
65