it's when i call

mk_scheduler()

, so graph construction time. here's the full trace for a non-root rule named

compile_cpp_sources_to_objects

.

Compiler

and

CppObjOutputDir

are defined as root rules in a function

create_cpp_rules

, which also adds the rule

compile_cpp_sources_to_objects

.

E         (CppObjects, (Select(Compiler), Select(CppSourceSnapshot), Select(CppObjOutputDir)), compile_cpp_sources_to_objects):
                     E           no matches for Select(Compiler) with subject types: Linker, CppDylibOutputDir, CppLinkRequest, PathGlobs, CppSources, CppObjOutputDir
                     E           no matches for Select(CppObjOutputDir) with subject types: CppDylibOutputDir, Compiler, CppLinkRequest, CppSources, PathGlobs, Linker