util-core: Make Future.raiseWithin take a thunk Throwable

Problem

Future.raiseWithin takes an Exception value as an argument which
is inconsistent with the other Future timer based API's and can
be costly since creating an exception often necessitates creating
a stacktrace.

Solution

Make the exception call-by-name.

JIRA Issues: CSL-7133

TBR=true

Differential Revision: https://phabricator.twitter.biz/D229559

CHANGELOG.rst

@@ -7,6 +7,9 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+* util-core: `c.t.u.Future.raiseWithin` methods now take the timeout exception as a call-by-name
+  parameter instead of a strict exception. ``PHAB_ID=D229559``
+
 18.10.0
 -------
 

util-core/src/main/scala/com/twitter/util/Future.scala

@@ -1595,15 +1595,15 @@ abstract class Future[+A] extends Awaitable[A] { self =>
    *
    * ''Note'': On timeout, the underlying future is interrupted.
    */
-  def raiseWithin(timeout: Duration, exc: Throwable)(implicit timer: Timer): Future[A] =
+  def raiseWithin(timeout: Duration, exc: => Throwable)(implicit timer: Timer): Future[A] =
     raiseWithin(timer, timeout, exc)
 
   /**
    * Returns a new Future that fails if this Future does not return in time.
    *
    * ''Note'': On timeout, the underlying future is interrupted.
    */
-  def raiseWithin(timer: Timer, timeout: Duration, exc: Throwable): Future[A] = {
+  def raiseWithin(timer: Timer, timeout: Duration, exc: => Throwable): Future[A] = {
     if (timeout == Duration.Top || isDefined)
       return this