add Vec::push_within_capacity - fallible, does not allocate
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@ -1773,6 +1773,50 @@ impl<T, A: Allocator> Vec<T, A> {
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}
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}
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/// Appends an element if there is sufficient spare capacity, otherwise the element is returned.
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///
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/// Unlike [`push`] method will not reallocate when there's insufficient capacity.
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/// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
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///
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/// [`push`]: Vec::push
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/// [`reserve`]: Vec::reserve
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/// [`try_reserve`]: Vec::try_reserve
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///
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/// # Examples
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///
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/// A manual, panic-free alternative to FromIterator
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///
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/// ```
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/// #![feature(vec_push_within_capacity, try_reserve)]
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///
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/// use std::collections::TryReserveError;
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/// fn from_iter<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
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/// let mut vec = Vec::new();
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/// for value in iter {
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/// if let Err(value) = vec.push_within_capacity(value) {
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/// vec.try_reserve(1)?;
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/// // this cannot fail, the previous line either returned or added at least 1 free slot
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/// let _ = vec.push_within_capacity(value);
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/// }
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/// }
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/// Ok(vec)
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/// }
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/// # from_iter(0..100).expect("please insert more memory");
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/// ```
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#[inline]
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#[unstable(feature = "vec_push_within_capacity", issue = "none")]
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pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
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if self.len == self.buf.capacity() {
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return Err(value);
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}
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unsafe {
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let end = self.as_mut_ptr().add(self.len);
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ptr::write(end, value);
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self.len += 1;
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}
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Ok(())
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}
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/// Removes the last element from a vector and returns it, or [`None`] if it
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/// is empty.
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///
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