From 1038b13dbb7f696d8752a9cea408e51985544133 Mon Sep 17 00:00:00 2001
From: Josh Triplett <josh@joshtriplett.org>
Date: Sat, 3 Oct 2026 00:23:54 -0700
Subject: [PATCH] When collecting into a ThinVec, pass the size hint to
 `with_capacity`

This ensures that collecting from an iterator with a good size hint will
avoid creating a ThinVec with excess capacity.

In particular, collecting 1-3 items into a ThinVec will no longer create
a ThinVec with capacity 4.
---
 src/lib.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/lib.rs b/src/lib.rs
index 3cf70d3..97fb92b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2371,7 +2371,8 @@ impl<T> Default for ThinVec<T> {
 impl<T> FromIterator<T> for ThinVec<T> {
     #[inline]
     fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> ThinVec<T> {
-        let mut vec = ThinVec::new();
+        let iter = iter.into_iter();
+        let mut vec = ThinVec::with_capacity(iter.size_hint().0);
         vec.extend(iter);
         vec
     }
@@ -3600,6 +3601,15 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_collect_capacity() {
+        for i in 0..20 {
+            let v = (0..i).into_iter().collect::<ThinVec<usize>>();
+            assert_eq!(v.len(), i);
+            assert_eq!(v.capacity(), i);
+        }
+    }
+
     #[test]
     fn test_clone() {
         let mut v = ThinVec::<i32>::new();