1) add "-fprofile-instr-generate" to "dflags-ldc"
2) build
3) ldc-profdata merge default.profraw -output test.profdata
4) replace "-fprofile-instr-generate" with "-fprofile-instr-use=test.profdata"
5) rebuild

// code coverage
llvm-cov show ./test -instr-profile=test.profdata test.c