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/exec/projection_exec.cpp b/src/mongo/db/exec/projection_exec.cpp
index edf0386d042..230673dcc7e 100644
--- a/src/mongo/db/exec/projection_exec.cpp
+++ b/src/mongo/db/exec/projection_exec.cpp
@@ -475,7 +475,7 @@ void ProjectionExec::appendArray(BSONObjBuilder* bob, const BSONObj& array, bool
                 BSONObjBuilder subBob;
                 BSONObjIterator jt(elt.embeddedObject());
                 while (jt.more()) {
-                    append(&subBob, jt.next());
+                    append(&subBob, jt.next()).transitional_ignore();
                 }
                 bob->append(bob->numStr(index++), subBob.obj());
                 break;
@@ -518,7 +518,7 @@ Status ProjectionExec::append(BSONObjBuilder* bob,
         BSONObjBuilder subBob;
         BSONObjIterator it(elt.embeddedObject());
         while (it.more()) {
-            subfm.append(&subBob, it.next(), details, arrayOpType);
+            subfm.append(&subBob, it.next(), details, arrayOpType).transitional_ignore();
         }
         bob->append(elt.fieldName(), subBob.obj());
     } else {