Yuhong Zhang:

 We define MONGO_COMPILER_ALWAYS_INLINE as

#define MONGO_COMPILER_ALWAYS_INLINE MONGO_COMPILER_ALWAYS_INLINE_
#if defined(_MSC_VER)
#define MONGO_COMPILER_ALWAYS_INLINE_ __forceinline
#elif MONGO_COMPILER_HAS_ATTRIBUTE(gnu::always_inline)
#define MONGO_COMPILER_ALWAYS_INLINE_ [[gnu::always_inline]]
#else
#define MONGO_COMPILER_ALWAYS_INLINE_
#endif

For a non-member function, gcc requires specifying inline after the attribute [[gnu::always_inline]] or it gives an error
error: 'always_inline' function might not be inlinable unless also declared 'inline' [-Werror=attributes]
For MSVC, specifying both __forceinline and inline gives you a warning (then treated as error by us)
warning C4141: 'inline': used more than once
Our current definition of the macro forces you to not use inline or instead you need to suppress the MSVC warning explicitly.