cmake: Check user-defined APPEND_*FLAGS variables early #32367

1. 
   hebasto commented at 4:35 pm on April 28, 2025: member

   For the purpose of checks performed by the build system, we strive to handle user-defined APPEND_*FLAGS in the same way as flags provided by other standard means. In particular, they are considered when the try_compile() command is used.

   However, these flags are not considered during enable_language() command invocation due to design limitations, which might cause issues, such as #32323.

   This PR addresses the issue by introducing an additional compiler check that does consider the user-defined APPEND_*FLAGS.

   Fixes #32323:

    0$ cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init"
    1-- The CXX compiler identification is GNU 14.2.0
    2-- Detecting CXX compiler ABI info
    3-- Detecting CXX compiler ABI info - done
    4-- Check for working CXX compiler: /usr/bin/c++ - skipped
    5-- Detecting CXX compile features
    6-- Detecting CXX compile features - done
    7-- Performing Test CXX_COMPILER_WORKS
    8-- Performing Test CXX_COMPILER_WORKS - Failed
    9CMake Error at cmake/module/EnableLanguage.cmake:42 (message):
   10  The CXX compiler is not able to compile a simple test program.
   11
   12  Check that the "APPEND_*FLAGS" variables are set correctly.
   13
   14
   15
   16Call Stack (most recent call first):
   17  CMakeLists.txt:71 (bitcoincore_enable_language)
   18
   19
   20-- Configuring incomplete, errors occurred!
   21hebasto@TS-P340:~/dev/bitcoin$ cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init=zero"
   22-- Performing Test CXX_COMPILER_WORKS
   23-- Performing Test CXX_COMPILER_WORKS - Success
   24...
   

2. cmake: Wrap `enable_language()` command in macro

   This change introduces the `bitcoincore_enable_language()` macro to wrap
   CMake’s `enable_language()` command, applying Bitcoin Core-specific
   settings and checks.

   6d55894a02
3. cmake: Move `APPEND_*` variables logic to `bitcoincore_enable_language`

   This change is required for the following commit.

   8287a7670b
4. hebasto added the label Build system on Apr 28, 2025
5. 
   DrahtBot commented at 4:35 pm on April 28, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32367.

   Reviews

   See the guideline for information on the review process.

   Type	Reviewers
   ACK	janb84

   If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

   Conflicts

   No conflicts as of last run.

6. hebasto marked this as a draft on Apr 28, 2025
7. hebasto force-pushed on Apr 28, 2025
8. hebasto marked this as ready for review on Apr 28, 2025
9. cmake: Check user-defined `APPEND_*` variables early

   This change improves usability in cases where the user provides
   `APPEND_*` variables that are incompatible with the compiler for some
   reason.

   eb2a943504
10. hebasto force-pushed on Apr 28, 2025
11. 
    janb84 commented at 9:16 am on April 29, 2025: contributor

    ACK eb2a943

    • PR fixes #32323 (reproduced on main, checked it was solved by this PR) ✅
    • Code review looks good to me. Adds a new macro with additional checks and some optimisations.✅ (There is one reference to enable_languages() in secp256k1 but that is separate and not relevant to this PR )

    Main:

    Configure with faulty flag: cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init"

    expected config BREAK

    Configure with correct flag: cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init=zero" not expected: STILL BREAKS

    This PR:

    Configure with faulty flag cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init" expected BREAK

    Configure with correct flag cmake -B build -DAPPEND_CXXFLAGS="-ftrivial-auto-var-init=zero" config WORKS

Contributors
hebasto DrahtBot janb84

Labels
Build system