Fix over-allocation in List::from_iter

crates/roc_std/src/roc_list.rs

@@ -549,7 +549,7 @@ impl<T: Clone> FromIterator<T> for RocList<T> {
     where
         I: IntoIterator<Item = T>,
     {
-        let mut iter = into.into_iter();
+        let iter = into.into_iter();
 
         if std::mem::size_of::<T>() == 0 {
             let count = iter.count();
@@ -566,25 +566,23 @@ impl<T: Clone> FromIterator<T> for RocList<T> {
             Self::with_capacity(init_capacity)
         };
 
-        loop {
-            let start = list.length;
-            let elements = list.elements.unwrap().as_ptr();
-            for i in start..list.capacity {
-                if let Some(new_elem) = iter.next() {
-                    unsafe {
-                        elements
-                            .add(i)
-                            .write(ptr::read(&ManuallyDrop::new(new_elem)));
-                    }
-                    list.length += 1;
-                } else {
-                    return list;
-                }
-            }
-
+        let mut elements = list.elements.unwrap().as_ptr();
+        for new_elem in iter {
             // If the size_hint didn't give us a max, we may need to grow. 1.5x seems to be good, based on:
             // https://archive.ph/Z2R8w and https://github.com/facebook/folly/blob/1f2706/folly/docs/FBVector.md
-            list.reserve(list.capacity / 2);
+            if list.length == list.capacity {
+                list.reserve(list.capacity / 2);
+                elements = list.elements.unwrap().as_ptr();
+            }
+
+            unsafe {
+                elements
+                    .add(list.length)
+                    .write(ptr::read(&ManuallyDrop::new(new_elem)));
+            }
+            list.length += 1;
         }
+
+        list
     }
 }

crates/roc_std/tests/test_roc_std.rs

@@ -162,6 +162,7 @@ mod test_roc_std {
         let from_slice = RocList::from_slice(&elems);
         let from_iter = RocList::from_iter(elems);
         assert_eq!(from_iter, from_slice);
+        assert_eq!(from_iter.capacity(), from_slice.capacity());
     }
 
     #[test]