SERVER-27244 Status usage compile-time facilities.

There are numerous places in the codebase where `mongo::Status` or
`mongo::StatusWith< T >` objects are returned and never checked.
Many of these are innocuous, but many of them are potentially
severe bugs.  This change introduces facilities to permit
compile-time warning of unchecked `Status` and `StatusWith` usage
on clang compilers.  It introduces an `ignore` function which is
useful to state that a specific "ignored status" case was
intentional.  It not presently an error, in clang builds, to
forget to check a `Status` -- this will come in a later commit.

This also introduces a `transitional_ignore` function, which allows
for easy continual auditing of the codebase for current "whitelisted"
unchecked-status instances.  All present "ignored status" cases
have been marked `transitional_ignore`.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mongo/dbtests/plan_ranking.cpp b/src/mongo/dbtests/plan_ranking.cpp
index c376bd605c3..a1750c90154 100644
--- a/src/mongo/dbtests/plan_ranking.cpp
+++ b/src/mongo/dbtests/plan_ranking.cpp
@@ -135,7 +135,7 @@ public:
         // This is what sets a backup plan, should we test for it.
         PlanYieldPolicy yieldPolicy(PlanExecutor::NO_YIELD,
                                     _opCtx.getServiceContext()->getFastClockSource());
-        _mps->pickBestPlan(&yieldPolicy);
+        _mps->pickBestPlan(&yieldPolicy).transitional_ignore();
         ASSERT(_mps->bestPlanChosen());
 
         size_t bestPlanIdx = _mps->bestPlanIdx();