Test coverage for native code WIP

Part of: https://git.lix.systems/lix-project/lix/issues/1186

tests/coverage/args/meson.build

@@ -0,0 +1,36 @@
+# Early initialization of LLVM source-based coverage.
+#
+# Needs to run before defining any targets.
+
+add_project_arguments(
+  '-fprofile-instr-generate',
+  '-fcoverage-mapping',
+  # TODO: -mllvm -runtime-counter-relocation may fix problems with tests with
+  # nix run/fmt/etc that execvp, bypassing atexit. need to confirm that's real.
+  language : 'cpp',
+)
+# N.B. This is a link argument because it needs to link the LLVM profiling runtime, I believe.
+add_project_link_arguments(
+  '-fprofile-instr-generate',
+  language: 'cpp',
+)
+
+add_project_arguments(
+  '-Cinstrument-coverage',
+  language : 'rust',
+)
+
+coverage_profraw_dir = meson.project_build_root() / 'profraw'
+run_command('mkdir', '-p', coverage_profraw_dir, check : true)
+coverage_test_env = {
+  # TODO: may need %c, but that may only work if you set runtime counter relocation, need to find that out
+  #
+  # See: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html#running-the-instrumented-program
+  # %20m -> use 20 raw profiles and merge at runtime, so that our large number of
+  # invocations of lix in the test suite don't create unreasonable numbers of
+  # files. This, I think, also limits the test concurrency.
+  'LLVM_PROFILE_FILE': coverage_profraw_dir / '%20m.profraw'
+}
+
+llvm_profdata = find_program('llvm-profdata', required : true)
+llvm_cov = find_program('llvm-cov', required : true)