Check for required C++ features during initial build (std::format, etc)

[mikekasprzak]

Per the discussion in #3472, this is an alternative approach of informing users that their C++ compiler and standard library doesn't support certain required features. It was born out of my challenges getting mainline Mir built and running on Pop!_OS (Ubuntu 22.04 LTS) instead of Ubuntu 24.04 LTS.

Admittedly it's a "big ask" of any project to track used language features, but C++ is an unusual case, for how closely your compilers are tied to your Linux distro.

To make managing the language feature list serve a dual purpose, and in lieu of a coding style guide, I proposing treating these additions to the CMakeLists.txt file as a master list of recommended C++ features for all contributors. That doesn't mean adding every C++20 feature to the list, just the ones you want contributors to use.

Under the hood, I wrote some macros that wrap CMAKE_CXX_COMPILE_FEATURES and check_cxx_symbol_exists in a way that should be both forward and backward compatible with different versions of cmake. It relies on C++20 feature testing, using the exact names from the specification. You can find a handy list of C++ feature macro names here:

https://en.cppreference.com/w/cpp/utility/feature_test

NOTE: cmake refers to C++ as cxx, but C++ feature testing refers to it as cpp.

Usage looks something like this.

CMakeLists.txt

...

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

include (cmake/RequireCXXFeature.cmake)
# Reference: https://en.cppreference.com/w/cpp/utility/feature_test

# Required C++ features
require_cxx_feature(__cpp_constexpr)

# Required C++ standard library features
require_cxx_feature(__cpp_lib_format)

...

If your tools meets the requirements, nothing out of the ordinary will occur. If you don't meet them, you will get an output similar to this:

mike@DevL14:~/Work/mir$ cmake -B build
-- The C compiler identification is GNU 11.4.0
-- The CXX compiler identification is GNU 11.4.0
...
-- Looking for __cpp_constexpr
-- Looking for __cpp_constexpr - found
-- Looking for __cpp_lib_format
-- Looking for __cpp_lib_format - not found
CMake Error at cmake/RequireCXXFeature.cmake:28 (message):
  Required C++ feature __cpp_lib_format not found!

  See the feature test documentation, or
  https://en.cppreference.com/w/cpp/feature_test for details.

Call Stack (most recent call first):
  CMakeLists.txt:73 (require_cxx_feature)


-- Configuring incomplete, errors occurred!
mike@DevL14:~/Work/mir$ 

This is a proof-of-concept, but it should be fully functional for the use case described. The next step would be to dig into the source code and populate the list of recommended C++ features.

Polyfilling features

For completeness, I included cmake macros for both require_cxx_feature and check_cxx_feature. In practice require_cxx_feature is probably all that's needed, but in case you wanted to polyfill a C++ feature or standard library, check_cxx_feature can be used instead.

...
include (cmake/CheckCXXFeature.cmake)
include (cmake/RequireCXXFeature.cmake)
# Reference: https://en.cppreference.com/w/cpp/utility/feature_test

# Required C++ features
require_cxx_feature(__cpp_constexpr)

# Required C++ standard library features
check_cxx_feature(__cpp_lib_format HAS_LIB_FORMAT)
if (NOT DEFINED "${HAS_LIB_FORMAT}")
  # polyfill config goes here
endif()
...

Nitty gritty

require_cxx_feature and check_cxx_feature are a simpler interface to the standard cmake module CheckCXXSymbolExists. The compilers define the language feature symbols (i.e. __cpp_FEATURENAME), and C++20 requires that the standard library provide a header called version defining all library features (i.e. __cpp_lib_FEATURENAME). GCC 11 for example supports many C++20 features, but the standard library shipped alongside it was incomplete. The version header exists, but per the examples above, the format header and symbol __cpp_lib_format do not.

CheckCXXSymbolExists stores the variable given in the final argument in the cmake cache. Once cached, the check will not be done again until the cache is purged. require_cxx_feature purges that cache entry on failure, so the test will be run again when cmake is run again. Once it succeeds, like CheckCXXSmybolExists it will not be checked again until the cmake cache is purged.

[RAOF]

I'm fairly happy with this approach.

I don't think we'll keep the required feature list up to date (and this is missing a bunch of features we use, like operator <=> and std::binary_semaphore), but when people run into problems and pop up in the bugtracker we can reactively encode that knowledge.

[RAOF]

This change seems incidental. Is this a required part of the PR, or a remnant of a previous test?

[RAOF]

Likewise, is this required?

[RAOF]

Probably don't need to leave commented-out remnants of testing 😉

[RAOF]

Also commented out remnant of testing?

[RAOF]

And here!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.97%. Comparing base (5557be6) to head (93f0fa9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3479      +/-   ##
==========================================
+ Coverage   76.95%   76.97%   +0.01%     
==========================================
  Files        1080     1080              
  Lines       69380    69380              
==========================================
+ Hits        53394    53404      +10     
+ Misses      15986    15976      -10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.