refactor(deque): make truncate safe

Author: peter-jerry-ye

deque/deque.mbt

@@ -861,8 +861,8 @@ pub fn shrink_to_fit[A](self : T[A]) -> Unit {
 /// Shortens the deque in-place, keeping the first `len` elements and dropping
 /// the rest.
 ///
-/// If `len` is greater than or equal to the deque's current length, this has no
-/// effect; if `len` is negative, the function will panic.
+/// If `len` is greater than or equal to the deque's current length or negative, 
+/// this has no effect
 ///
 /// Parameters:
 ///
@@ -879,8 +879,7 @@ pub fn shrink_to_fit[A](self : T[A]) -> Unit {
 /// }
 /// ```
 pub fn truncate[A](self : T[A], len : Int) -> Unit {
-  guard len < self.len else { return }
-  guard len >= 0
+  guard len >= 0 && len < self.len else { return }
   if len == 0 {
     self.clear()
     return

deque/deque_test.mbt

@@ -832,12 +832,8 @@ test "deque truncate" {
   // Current layout: [X, X, X, 4, 5, X]
   dq.truncate(2)
   @json.inspect!(dq.as_views(), content=[[4, 5], []])
-}
-
-///|
-test "panic deque truncate" {
-  let dq = @deque.of([1, 2, 3, 4, 5])
   dq.truncate(-1)
+  @json.inspect!(dq.as_views(), content=[[4, 5], []])
 }
 
 ///|