alloc/vec/
spec_from_iter.rs

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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    #[track_caller]
33    default fn from_iter(iterator: I) -> Self {
34        SpecFromIterNested::from_iter(iterator)
35    }
36}
37
38impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
39    #[track_caller]
40    fn from_iter(iterator: IntoIter<T>) -> Self {
41        // A common case is passing a vector into a function which immediately
42        // re-collects into a vector. We can short circuit this if the IntoIter
43        // has not been advanced at all.
44        // When it has been advanced We can also reuse the memory and move the data to the front.
45        // But we only do so when the resulting Vec wouldn't have more unused capacity
46        // than creating it through the generic FromIterator implementation would. That limitation
47        // is not strictly necessary as Vec's allocation behavior is intentionally unspecified.
48        // But it is a conservative choice.
49        let has_advanced = iterator.buf != iterator.ptr;
50        if !has_advanced || iterator.len() >= iterator.cap / 2 {
51            unsafe {
52                let it = ManuallyDrop::new(iterator);
53                if has_advanced {
54                    ptr::copy(it.ptr.as_ptr(), it.buf.as_ptr(), it.len());
55                }
56                return Vec::from_parts(it.buf, it.len(), it.cap);
57            }
58        }
59
60        let mut vec = Vec::new();
61        // must delegate to spec_extend() since extend() itself delegates
62        // to spec_from for empty Vecs
63        vec.spec_extend(iterator);
64        vec
65    }
66}
alloc/vec/spec_from_iter.rs

alloc/vec/
spec_from_iter.rs
