Improve C++ layout test error messages

[dkorpel]

Instead of a default static assert message, give the values and locations
of the mismatches symbols.

cpp-layout-test failed:
/home/dennis/repos/dmd/generated/linux/release/64/cpp_layout_asserts.d(4782): Error: static assert:  "

Mangled name mismatch for `dmd.cxxfrontend.cppThunkMangleItanium`:
  D:   _ZN3dmd21cppThunkMangleItaniumEP15FuncDeclarationif at /home/dennis/repos/dmd/compiler/src/dmd/../dmd/cxxfrontend.d:68
  C++: _ZN3dmd21cppThunkMangleItaniumEP15FuncDeclarationi at mangle.h:25
Changes to dmd's extern(C++) types/functions must be reflected in compiler/include/*.h

Also important: pass -unittest in the build step so those checks are actually performed.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#23190"

[dkorpel]

Verified that the CI runs:

+ python3 -m pip install libclang
Defaulting to user installation because normal site-packages is not writeable
Collecting libclang
  Downloading libclang-18.1.1-py2.py3-none-manylinux2010_x86_64.whl (24.5 MB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 24.5/24.5 MB 56.2 MB/s eta 0:00:00
Installing collected packages: libclang
Successfully installed libclang-18.1.1
+ ./dmd/generated/build cpp-layout-test
(TEST) CPP-LAYOUT

The following operation failed:
Name: cpp-layout-test
Description: Check that compiler/include/dmd/*.h matches the D extern(C++) declarations
CommandFunction: Yes

-----------------------------------------------------------
cpp-layout-test failed:
/home/runner/work/dmd/dmd/dmd/generated/linux/release/64/cpp_layout_asserts.d(44): Error: static assert:  "

Offset mismatch for `Param.objfiles`:
  D field at offset 992 at /home/runner/work/dmd/dmd/dmd/compiler/src/dmd/../dmd/globals.d:261
  C++ field at offset 984 at globals.h:281
Changes to dmd's extern(C++) types/functions must be reflected in compiler/include/*.h
"
        static assert(actualOffset == expectedOffset,
        ^
/home/runner/work/dmd/dmd/dmd/generated/linux/release/64/cpp_layout_asserts.d(296):        instantiated from here: `checkField!(Param, "objfiles", 984, 32, "globals.h:281")`
    checkField!(Param, "objfiles", 984, 32, "globals.h:281");
    ^

DETAILS:


Error: Process completed with exit code 1.

[thewilsonator]

Mangled name mismatch for `dmd.cxxfrontend.cppThunkMangleItanium`:
  D:   _ZN3dmd21cppThunkMangleItaniumEP15FuncDeclarationif at /home/dennis/repos/dmd/compiler/src/dmd/../dmd/cxxfrontend.d:68
  C++: _ZN3dmd21cppThunkMangleItaniumEP15FuncDeclarationi at mangle.h:25

can you demangle those symbols with druntime and c++filt (if available)? would make it a lot easier for those who don't spend a lot of time looking at mangled symbols.

[dkorpel]

The problem is that it's currently a compile time only test using static assert, and druntime's demangler only implements extern(D) demangling (it hooks to __cxa_demangle for C++). In theory, just visiting both source locations should give you the demangled signatures.

[thewilsonator]

test using static assert

if that fails surely it will print on stderr, which you can pipe to c++filt? Anyway, it can be done later.