| use core::mem::ManuallyDrop; |
| use core::ptr::{self}; |
| |
| use super::{IntoIter, SpecExtend, SpecFromIterNested, Vec}; |
| |
| /// Specialization trait used for Vec::from_iter |
| /// |
| /// ## The delegation graph: |
| /// |
| /// ```text |
| /// +-------------+ |
| /// |FromIterator | |
| /// +-+-----------+ |
| /// | |
| /// v |
| /// +-+---------------------------------+ +---------------------+ |
| /// |SpecFromIter +---->+SpecFromIterNested | |
| /// |where I: | | |where I: | |
| /// | Iterator (default)------------+ | | Iterator (default) | |
| /// | vec::IntoIter | | | TrustedLen | |
| /// | InPlaceCollect--(fallback to)-+ | +---------------------+ |
| /// +-----------------------------------+ |
| /// ``` |
| pub(super) trait SpecFromIter<T, I> { |
| fn from_iter(iter: I) -> Self; |
| } |
| |
| impl<T, I> SpecFromIter<T, I> for Vec<T> |
| where |
| I: Iterator<Item = T>, |
| { |
| default fn from_iter(iterator: I) -> Self { |
| SpecFromIterNested::from_iter(iterator) |
| } |
| } |
| |
| impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> { |
| fn from_iter(iterator: IntoIter<T>) -> Self { |
| // A common case is passing a vector into a function which immediately |
| // re-collects into a vector. We can short circuit this if the IntoIter |
| // has not been advanced at all. |
| // When it has been advanced We can also reuse the memory and move the data to the front. |
| // But we only do so when the resulting Vec wouldn't have more unused capacity |
| // than creating it through the generic FromIterator implementation would. That limitation |
| // is not strictly necessary as Vec's allocation behavior is intentionally unspecified. |
| // But it is a conservative choice. |
| let has_advanced = iterator.buf != iterator.ptr; |
| if !has_advanced || iterator.len() >= iterator.cap / 2 { |
| unsafe { |
| let it = ManuallyDrop::new(iterator); |
| if has_advanced { |
| ptr::copy(it.ptr.as_ptr(), it.buf.as_ptr(), it.len()); |
| } |
| return Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap); |
| } |
| } |
| |
| let mut vec = Vec::new(); |
| // must delegate to spec_extend() since extend() itself delegates |
| // to spec_from for empty Vecs |
| vec.spec_extend(iterator); |
| vec |
| } |
| } |