Guarantee vec.clear/truncate is O(1) for trivial types

src/liballoc/vec.rs

@@ -685,6 +685,7 @@ impl<T> Vec<T> {
     /// [`drain`]: #method.drain
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn truncate(&mut self, len: usize) {
+        if mem::needs_drop::<T>() {
             let current_len = self.len;
             unsafe {
                 let mut ptr = self.as_mut_ptr().add(self.len);
@@ -701,6 +702,9 @@ impl<T> Vec<T> {
                     ptr::drop_in_place(ptr);
                 }
             }
+        } else if len <= self.len {
+            self.len = len;
+        }
     }
 
     /// Extracts a slice containing the entire vector.