[kammce]

Adds libhal_add_compile_checks(), for testing code that must fail to compile: misuse rejected by the type system, and dangling references reported by Clang's lifetime safety analysis.

libhal_add_compile_checks(libhal
    FILES
        tests/compile_checks/lifetime_rejects.cpp
        tests/compile_checks/lifetime_analysis.cpp
)

Each file is compiled with Clang's -verify mode and marks the diagnostics it must produce:

// expected-error@+1 {{is returned}}
return sensor(bus);

The build fails if an expected diagnostic is missing or an unexpected one appears. Known gaps can stay in a file without annotations; once Clang starts catching them, the build fails and they become checked cases.

How

• The function adds an object library, <target>_compile_checks, built with LIBHAL_CXX_FLAGS, linked to the target, with clang-tidy off.
• -verify is added by a compiler launcher, LibhalVerifyLauncher.cmake (run with cmake -P), because a launcher wraps only the compile step. As a compile option it would also reach the module dependency scanner, which doesn't see the expected-* comments and fails.
• Clang 23+ only; other compilers print a status line and skip the checks.
• Clang writes no object file when the expected diagnostics include errors, so after a passing check the launcher creates an empty stand-in object (never linked). A passing check is rebuilt only when the file, a header it includes (Clang still writes the dependency file) or an imported module changes. A failing check gets no stand-in, so it keeps failing on later builds.

Testing

macOS 27, LLVM 23.1.2: libhal v5's 36 compile-check cases, converted to four files (libhal#245), pass. Removing an expectation, or expecting an error on a known gap, fails the build with Clang's message. The launcher passes arguments through unchanged and returns the compiler's exit code. Incremental builds: no change rebuilds no checks; editing fixtures.hpp or lifetime.cppm rebuilds all four; editing one check file rebuilds only that one; a broken check fails again on the next build until fixed.