> You can even promise a func is pure without even implementing it in the same translation unit!

Actually that's the main point, as if it's in the same TU, the compiler can probably figure out a function is pure.

often but not always, for example a function using asm() to access, say, some AES intrinsics is pure (const even) since the output only depends on input, but oftentimes compiler by itself will not declare a function with an asm() block pure or const

Well, fair point, but - compilers shouldn't be that lazy :-)