From 5ece63ce763acec5e3f8c691ef0575517ae331fc Mon Sep 17 00:00:00 2001
From: DaniPopes <57450786+DaniPopes@users.noreply.github.com>
Date: Tue, 10 Dec 2024 21:28:28 +0100
Subject: [PATCH] Implement spare_capacity_mut
Mirrors the implementation from `Vec::spare_capacity_mut`.
This is useful to write directly into the buffer after `SmallVec::with_capacity`,
removing the overhead of the `SmallVec` model entirely when doing so.
---
src/lib.rs | 16 ++++++++++++++++
src/tests.rs | 31 ++++++++++++++++++++++++++++++-
2 files changed, 46 insertions(+), 1 deletion(-)
diff --git a/src/lib.rs b/src/lib.rs
index 803e50e..1fbdc7b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1413,6 +1413,22 @@ impl<T, const N: usize> SmallVec<T, N> {
}
}
+ /// Returns the remaining spare capacity of the vector as a slice of
+ /// `MaybeUninit<T>`.
+ ///
+ /// The returned slice can be used to fill the vector with data (e.g. by
+ /// reading from a file) before marking the data as initialized using the
+ /// [`set_len`](Self::set_len) method.
+ #[inline]
+ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
+ unsafe {
+ core::slice::from_raw_parts_mut(
+ self.as_mut_ptr().add(self.len()) as *mut MaybeUninit<T>,
+ self.capacity() - self.len(),
+ )
+ }
+ }
+
/// Creates a `SmallVec` directly from the raw components of another `SmallVec`.
///
/// # Safety
diff --git a/src/tests.rs b/src/tests.rs
index 3186fb5..2f0a1f2 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -1136,7 +1136,9 @@ fn collect_from_iter() {
impl<I: Iterator> Iterator for IterNoHint<I> {
type Item = I::Item;
- fn next(&mut self) -> Option<Self::Item> { self.0.next() }
+ fn next(&mut self) -> Option<Self::Item> {
+ self.0.next()
+ }
// no implementation of size_hint means it returns (0, None) - which forces from_iter to
// grow the allocated space iteratively.
@@ -1148,3 +1150,30 @@ fn collect_from_iter() {
let _y: SmallVec<u8, 1> = SmallVec::from_iter(iter);
}
+
+#[test]
+fn test_spare_capacity_mut() {
+ let mut v: SmallVec<u8, 2> = SmallVec::new();
+ assert!(!v.spilled());
+ let spare = v.spare_capacity_mut();
+ assert_eq!(spare.len(), 2);
+ assert_eq!(spare.as_ptr().cast::<u8>(), v.as_ptr());
+
+ v.push(1);
+ assert!(!v.spilled());
+ let spare = v.spare_capacity_mut();
+ assert_eq!(spare.len(), 1);
+ assert_eq!(spare.as_ptr().cast::<u8>(), unsafe { v.as_ptr().add(1) });
+
+ v.push(2);
+ assert!(!v.spilled());
+ let spare = v.spare_capacity_mut();
+ assert_eq!(spare.len(), 0);
+ assert_eq!(spare.as_ptr().cast::<u8>(), unsafe { v.as_ptr().add(2) });
+
+ v.push(3);
+ assert!(v.spilled());
+ let spare = v.spare_capacity_mut();
+ assert!(spare.len() >= 1);
+ assert_eq!(spare.as_ptr().cast::<u8>(), unsafe { v.as_ptr().add(3) });
+}