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add Vec::push_within_capacity - fallible, does not allocate

This commit is contained in:
The8472 2021-09-20 20:22:14 +02:00 committed by The 8472
parent 75b7e52e92
commit 46396e847d

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