Added failing test for #353 and fixed bug (#354)

Author: josephg

src/lib.rs

@@ -961,10 +961,11 @@ impl<T, const N: usize> SmallVec<T, N> {
         // SAFETY: see above
         debug_assert!(self.spilled());
         let len = self.len();
-        let (ptr, cap) = self.raw.heap;
+        let cap = self.raw.heap.1;
         if len == cap {
             self.reserve(1);
         }
+        let ptr = self.raw.heap.0;
         ptr.as_ptr().add(len).write(value);
         self.set_len(len + 1)
     }

src/tests.rs

@@ -1128,3 +1128,23 @@ fn max_insert() {
     let mut sv: SmallVec<i32, 2> = smallvec![0];
     sv.insert(usize::MAX, 0);
 }
+
+#[test]
+fn collect_from_iter() {
+    // Regression test for https://github.com/servo/rust-smallvec/issues/353
+    struct IterNoHint<I: Iterator>(I);
+
+    impl<I: Iterator> Iterator for IterNoHint<I> {
+        type Item = I::Item;
+        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.
+    }
+
+    // A length of 3 is fine to trigger this bug under valgrind, but making the vector 1 million
+    // elements makes it crash - which is much easier to detect.
+    let iter = IterNoHint(std::iter::repeat(1u8).take(1_000_000));
+
+    let _y: SmallVec<u8, 1> = SmallVec::from_iter(iter);
+}