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`.

3 files changed, 13 insertions, 0 deletions

diff --git a/src/mongo/platform/compiler.h b/src/mongo/platform/compiler.h
index 59d7de5dbfb..af3e0b20dc3 100644
--- a/src/mongo/platform/compiler.h
+++ b/src/mongo/platform/compiler.h
@@ -143,8 +143,16 @@
  *    adversarial inputs, but for some reason the compiler cannot figure this out on its own, for
  *    example after a call to a function that never returns but cannot be labeled with
  *    MONGO_COMPILER_NORETURN. In almost all cases MONGO_UNREACHABLE is preferred.
+ *
+ *
+ * MONGO_WARN_UNUSED_RESULT_CLASS
+ *
+ *    Tells the compiler that a class defines a type for which checking results is necessary.  Types
+ *    thus defined turn functions returning them into "must check results" style functions.  Preview
+ *    of the `[[nodiscard]]` C++17 attribute.
  */
 
+
 #if defined(_MSC_VER)
 #include "mongo/platform/compiler_msvc.h"
 #elif defined(__GNUC__)
diff --git a/src/mongo/platform/compiler_gcc.h b/src/mongo/platform/compiler_gcc.h
index cf3b239db35..cf5c6960a86 100644
--- a/src/mongo/platform/compiler_gcc.h
+++ b/src/mongo/platform/compiler_gcc.h
@@ -42,9 +42,12 @@
 // worth it.
 #define MONGO_COMPILER_COLD_FUNCTION
 #define MONGO_COMPILER_NORETURN __attribute__((__noreturn__))
+// MONGO_WARN_UNUSED_RESULT is only supported in the semantics we want for classes in Clang, not in GCC.
+#define MONGO_WARN_UNUSED_RESULT_CLASS
 #else
 #define MONGO_COMPILER_COLD_FUNCTION __attribute__((__cold__))
 #define MONGO_COMPILER_NORETURN __attribute__((__noreturn__, __cold__))
+#define MONGO_WARN_UNUSED_RESULT_CLASS
 #endif
 
 #define MONGO_COMPILER_VARIABLE_UNUSED __attribute__((__unused__))
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