MSVC CMake generation optimization (13.6s -> 5.7s)

[mattiasljungstrom]

The following CMake changes reduce project generation on Win11 with MSVC 2022 from 13.6s to 5.7s. (Using a deskop system with Ryzen 3900X / M.2 SSD).

Motivation:

• Tests in cxx14.cmake are using Clang/GCC style flags, these are all invalid on MSVC.
• The flag SUPPORTS_USER_DEFINED_LITERALS is only used for tests that are disabled on WIN32.
• The flag FMT_HAS_VARIANT is not used at all from what I can see. (?)
• check_cxx_compiler_flag(-fno-exceptions) is always invalid on MSVC.
• check_cxx_compiler_flag(-fno-delete-null-pointer-checks) is always invalid on MSVC.

With FMT_TEST=OFF, project generation takes 4.3s and is almost instant when included as sub-project.

Note, correct check on MSVC would be like this:

check_cxx_compiler_flag("/std:c++20" has_std_20_flag)

But it might be better to use CMake checks instead of testing flags, like 'CMAKE_CXX_COMPILE_FEATURES' (available from CMake 3.1).

[mattiasljungstrom]

this style also works on MSVC: (note ':' instead of '=')

check_cxx_compiler_flag(-std:c++20 has_std_20_flag)

[vitaut]

Thanks for the PR.

Instead of patching every invocation of check_cxx_compiler_flag let's move it to one place, a new fmt_check_cxx_compiler_flag function, that is a noop for MSVC.

[vitaut]

The flag FMT_HAS_VARIANT is not used at all from what I can see.

Let's remove the check completely then.

[mattiasljungstrom]

Pushed an update with your change requests.

Note: it's possible to run Clang under MSVC, in which case some clang/gcc flags are supported. But not the "-std=c++XX" flags. These still use the MSVC format instead.

[vitaut]

Unrelated, please revert.

[vitaut]

Let's make it a function instead of a macro unless there is a strong reason not to and use normal naming convention, i.e. flag and result for arguments.

[vitaut]

Let's move this to test/compile-error-test/CMakeLists.txt and make unconditional. The other usage of SUPPORTS_USER_DEFINED_LITERALS us unnecessary and can be removed.

[mattiasljungstrom]

Pushed a new update based on your feedback.

[phprus]

CXX_STANDARD_FLAG is not defined in test/compile-error-test/CMakeLists.txt

Suggestion:
+ "-DCXX_STANDARD_FLAG=${CXX_STANDARD_FLAG}"

[vitaut]

Thank you