Optimize more for the empty case

src/lib.rs

@@ -935,12 +935,6 @@ impl<T> ThinVec<T> {
         }
     }
 
-    unsafe fn deallocate(&mut self) {
-        if self.has_allocation() {
-            dealloc(self.ptr() as *mut u8, layout::<T>(self.capacity()))
-        }
-    }
-
     /// Resize the buffer and update its capacity, without changing the length.
     /// Unsafe because it can cause length to be greater than capacity.
     unsafe fn reallocate(&mut self, new_cap: usize) {
@@ -1078,10 +1072,25 @@ impl<T: PartialEq> ThinVec<T> {
 }
 
 impl<T> Drop for ThinVec<T> {
+    #[inline]
     fn drop(&mut self) {
-        unsafe {
-            ptr::drop_in_place(&mut self[..]);
-            self.deallocate();
+        #[cold]
+        #[inline(never)]
+        fn drop_non_singleton<T>(this: &mut ThinVec<T>) {
+            unsafe {
+                ptr::drop_in_place(&mut this[..]);
+
+                #[cfg(feature = "gecko-ffi")]
+                if this.ptr.as_ref().uses_stack_allocated_buffer() {
+                    return;
+                }
+
+                dealloc(this.ptr() as *mut u8, layout::<T>(this.capacity()))
+            }
+        }
+
+        if !self.is_singleton() {
+            drop_non_singleton(self);
         }
     }
 }
@@ -1125,7 +1134,10 @@ impl<T> Extend<T> for ThinVec<T> {
         I: IntoIterator<Item = T>,
     {
         let iter = iter.into_iter();
-        self.reserve(iter.size_hint().0);
+        let hint = iter.size_hint().0;
+        if hint > 0 {
+            self.reserve(hint);
+        }
         for x in iter {
             self.push(x);
         }
@@ -1318,20 +1330,33 @@ impl<T> Clone for ThinVec<T>
 where
     T: Clone,
 {
+    #[inline]
     fn clone(&self) -> ThinVec<T> {
-        let len = self.len();
-        let mut new_vec = ThinVec::<T>::with_capacity(len);
-        let mut data_raw = new_vec.data_raw();
-        for x in self.iter() {
+        #[cold]
+        #[inline(never)]
+        fn clone_non_singleton<T: Clone>(this: &ThinVec<T>) -> ThinVec<T> {
+            let len = this.len();
+            let mut new_vec = ThinVec::<T>::with_capacity(len);
+            let mut data_raw = new_vec.data_raw();
+            for x in this.iter() {
+                unsafe {
+                    ptr::write(data_raw, x.clone());
+                    data_raw = data_raw.add(1);
+                }
+            }
             unsafe {
-                ptr::write(data_raw, x.clone());
-                data_raw = data_raw.add(1);
+                // `this` is not the singleton, but `new_vec` will be if
+                // `this` is empty.
+                new_vec.set_len(len); // could be the singleton
             }
+            new_vec
         }
-        unsafe {
-            new_vec.set_len(len); // could be the singleton
+
+        if self.is_singleton() {
+            ThinVec::new()
+        } else {
+            clone_non_singleton(self)
         }
-        new_vec
     }
 }
 
@@ -1394,11 +1419,20 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
 }
 
 impl<T> Drop for IntoIter<T> {
+    #[inline]
     fn drop(&mut self) {
-        unsafe {
-            let mut vec = mem::replace(&mut self.vec, ThinVec::new());
-            ptr::drop_in_place(&mut vec[self.start..]);
-            vec.set_len(0) // could be the singleton
+        #[cold]
+        #[inline(never)]
+        fn drop_non_singleton<T>(this: &mut IntoIter<T>) {
+            unsafe {
+                let mut vec = mem::replace(&mut this.vec, ThinVec::new());
+                ptr::drop_in_place(&mut vec[this.start..]);
+                vec.set_len_non_singleton(0)
+            }
+        }
+
+        if !self.vec.is_singleton() {
+            drop_non_singleton(self);
         }
     }
 }
@@ -1795,6 +1829,27 @@ mod tests {
             assert_eq!(v.capacity(), 0);
             assert_eq!(&v[..], &[]);
         }
+
+        {
+            let v = ThinVec::<i32>::new();
+            let v = v.clone();
+
+            assert_eq!(v.len(), 0);
+            assert_eq!(v.capacity(), 0);
+            assert_eq!(&v[..], &[]);
+        }
+    }
+
+    #[test]
+    fn test_clone() {
+        let mut v = ThinVec::<i32>::new();
+        assert!(v.is_singleton());
+        v.push(0);
+        v.pop();
+        assert!(!v.is_singleton());
+
+        let v2 = v.clone();
+        assert!(v2.is_singleton());
     }
 }