Make wording around 0-cost casts more precise

src/libstd/ffi/c_str.rs

@@ -1026,9 +1026,9 @@ impl CStr {
     /// The returned slice will **not** contain the trailing nul terminator that this C
     /// string has.
     ///
-    /// > **Note**: This method is currently implemented as a 0-cost cast, but
-    /// > it is planned to alter its definition in the future to perform the
-    /// > length calculation whenever this method is called.
+    /// > **Note**: This method is currently implemented as a constant-time
+    /// > cast, but it is planned to alter its definition in the future to
+    /// > perform the length calculation whenever this method is called.
     ///
     /// # Examples
     ///
@@ -1077,9 +1077,9 @@ impl CStr {
     /// it will return an error with details of where UTF-8 validation failed.
     ///
     /// > **Note**: This method is currently implemented to check for validity
-    /// > after a 0-cost cast, but it is planned to alter its definition in the
-    /// > future to perform the length calculation in addition to the UTF-8
-    /// > check whenever this method is called.
+    /// > after a constant-time cast, but it is planned to alter its definition
+    /// > in the future to perform the length calculation in addition to the
+    /// > UTF-8 check whenever this method is called.
     ///
     /// [`&str`]: ../primitive.str.html
     ///
@@ -1110,9 +1110,9 @@ impl CStr {
     /// with the result.
     ///
     /// > **Note**: This method is currently implemented to check for validity
-    /// > after a 0-cost cast, but it is planned to alter its definition in the
-    /// > future to perform the length calculation in addition to the UTF-8
-    /// > check whenever this method is called.
+    /// > after a constant-time cast, but it is planned to alter its definition
+    /// > in the future to perform the length calculation in addition to the
+    /// > UTF-8 check whenever this method is called.
     ///
     /// [`Cow`]: ../borrow/enum.Cow.html
     /// [`Borrowed`]: ../borrow/enum.Cow.html#variant.Borrowed