Add Separate CMake Target for C++20 Modules

[MathewBensonCode]

In the same vein as there is the fmt::fmt-header-only, fmt::fmt and fmt::fmt_c targets, I propose the addition of a new target fmt::fmt-module which will be for the compilation of the FMT_MODULE library option.

The new target will have the properties requried for Compiling, Installing and using the C++20 functionality

The add_module_library CMake function is factored out in favour of simply specifying the targets specifically in a cleaner and simpler CMake syntax.

The FMT_MODULE option is defaulted to true for versions of CMake newer than 3.28.

Closes #4684

[vitaut]

Adding a module target is a good idea but there are too many unrelated refactors in this PR. Please keep add_module_library and avoid unrelated changes (you can submit separate PR(s) for those).

[MathewBensonCode]

I have tried to make the requested changes. I have restored the Removed functions to the CMakeLists.txt file.

However, I marked the add_module_library as deprecated and unused.

Kindly advise if the change is agreeable.

[ClausKlein]

bash-5.3$ cmake -G Ninja -S . -B build --fresh -D CMAKE_CXX_STANDARD=17
-- CMake version: 4.2.1
-- The CXX compiler identification is AppleClang 16.0.0.16000026
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Ninja Version: 1.13.0.git.kitware.jobserver-pipe-1

-- Using CXX Modules by Default with Ninja                           <<<<<<<<<<<<<< he?
-- {fmt} version: 12.1.1
-- Build type: Release
-- Performing Test HAS_NULLPTR_WARNING
-- Performing Test HAS_NULLPTR_WARNING - Success
-- Target 'doc' disabled because mkdocs not found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- FMT_PEDANTIC: OFF
-- The C compiler identification is AppleClang 16.0.0.16000026
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done (2.7s)
CMake Error in CMakeLists.txt:
  The target named "fmt-module" has C++ sources that may use modules, but the
  compiler does not provide a way to discover the import graph dependencies.
  See the cmake-cxxmodules(7) manual for details.  Use the
  CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning.


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.
bash-5.3$ 

[ClausKlein]

you may have a look at ClausKlein#1

[MathewBensonCode]

you may have a look at ClausKlein#1

@ClausKlein I have just seen this and noted the changes needed in the test/CMakeLists.txt file with regards to the FMT_MODULE flag. Thanks.

[ClausKlein]

you may have a look at ClausKlein#1

@ClausKlein I have just seen this and noted the changes needed in the test/CMakeLists.txt file with regards to the FMT_MODULE flag. Thanks.

I was wondering, I could not open a PR against your repo?

[MathewBensonCode]

you may have a look at ClausKlein#1

@ClausKlein I have just seen this and noted the changes needed in the test/CMakeLists.txt file with regards to the FMT_MODULE flag. Thanks.

I was wondering, I could not open a PR against your repo?

I don't know why this didn't work, but I have incorporated the commit in the workflow.

the tests for FMT_MODULE are still excluded, but the ordinary tests should now work. This can be solved in another pull request and issue.

[ClausKlein]

The CMakeFiles are still unreadable, the control flow is not clear, and the supported version range historic!

@MathewBensonCode I Ask to respect the cmake format style if possible. It is not clean yet, but mostly indented by 2 spaces and space after controls: if, else, endif, ...

This Files should be formatted with gersemi; then the formatting battle may end!

@vitaut Check your CI, most often on GitHub the most resent cmake version is available, Today the range is CMake version: 3.31.6...4.2.3.

Since 3.28 there are lot of fixes and improvements for CXX_MODULE handling in CMake

And starting with CMake v4.0, a lot of policies changed, you should notice this.
On CI no newer compiler and no newer OS runner is used.

The world changes, don't stay back!

[MathewBensonCode]

The CMakeFiles are still unreadable, the control flow is not clear, and the supported version range historic!

@MathewBensonCode I Ask to respect the cmake format style if possible. It is not clean yet, but mostly indented by 2 spaces and space after controls: if, else, endif, ...

This Files should be formatted with gersemi; then the formatting battle may end!

@vitaut Check your CI, most often on GitHub the most resent cmake version is available, Today the range is CMake version: 3.31.6...4.2.3.

9> ### Since 3.28 there are lot of fixes and improvements for CXX_MODULE handling in CMake

And starting with CMake v4.0, a lot of policies changed, you should notice this. On CI no newer compiler and no newer OS runner is used.

The world changes, don't stay back!

@ClausKlein, thank you for the time you have taken to provide your reviews and comments. I will try and ensure that the code I write has better formatting. It is not a battle, it is usually a lower priority as I want to focus on correctness more.

On this matter of the version range, I believe that you are wrong. kindly refer to the documentation for the cmake_minimum_required for the use of the maximum policy.

We don't just write the latest version in the command and call it a day. It is meant to be a guarantee of the code we have written. This means we would have to write all the CMake code in this repo to that standard, test it everywhere and only after all that make that statement about the policies that our code supports.

That is a big undertaking that will require many other issues and pull requests.

The scope and goal of this pull request is simply to add a "modern" target but also to ensure that it doesn't break any expected behavior.

This repo is used in many varied environments, modern and legacy, so a lot of care is needed when making changes.

I have had to revert several good and modern ideas in this pull request to keep to the scope. I intend to bring those in separate issues and pull requests later.

So I appreciate and understand your desire to see modernization, I am pursuing the same goals and I'm certain @vitaut is too.

[vitaut]

Merged, thank you!

[vitaut]

@ClausKlein please stop spamming the repo or you will be blocked. Improvements to CMake configs are fine but they should be split into independent PRs that tackle one thing at a time.