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, 2 insertions, 2 deletions

diff --git a/src/mongo/db/matcher/expression_algo.cpp b/src/mongo/db/matcher/expression_algo.cpp
index 12fe0224258..292bf1f590a 100644
--- a/src/mongo/db/matcher/expression_algo.cpp
+++ b/src/mongo/db/matcher/expression_algo.cpp
@@ -152,7 +152,7 @@ bool _isSubsetOf(const MatchExpression* lhs, const ComparisonMatchExpression* rh
         for (BSONElement elem : ime->getEqualities()) {
             // Each element in the $in-array represents an equality predicate.
             EqualityMatchExpression equality;
-            equality.init(lhs->path(), elem);
+            equality.init(lhs->path(), elem).transitional_ignore();
             equality.setCollator(ime->getCollator());
             if (!_isSubsetOf(&equality, rhs)) {
                 return false;
@@ -283,7 +283,7 @@ void applyRenamesToExpression(MatchExpression* expr, const StringMap<std::string
         auto it = renames.find(expr->path());
         if (it != renames.end()) {
             LeafMatchExpression* leafExpr = checked_cast<LeafMatchExpression*>(expr);
-            leafExpr->setPath(it->second);
+            leafExpr->setPath(it->second).transitional_ignore();
         }
     }