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, 0 deletions

diff --git a/src/mongo/platform/compiler_msvc.h b/src/mongo/platform/compiler_msvc.h
index c47372e04bd..e9e2bfa86ef 100644
--- a/src/mongo/platform/compiler_msvc.h
+++ b/src/mongo/platform/compiler_msvc.h
@@ -50,6 +50,8 @@
 #define MONGO_COMPILER_API_EXPORT __declspec(dllexport)
 #define MONGO_COMPILER_API_IMPORT __declspec(dllimport)
 
+#define MONGO_WARN_UNUSED_RESULT_CLASS
+
 #ifdef _M_IX86
 // 32-bit x86 supports multiple of calling conventions.  We build supporting the cdecl convention
 // (most common).  By labeling our exported and imported functions as such, we do a small favor to