Function resolve_program 

pub fn resolve_program(program: &OsStr, paths: Option<OsString>) -> OsString

Resolve a bare program name against paths, a PATH-style list, using the same rules as command_exists: (including PATHEXT on Windows).

Anything that is not a bare name (./tool, C:\bin\tool.exe) and any name that is not found is returned unchanged, so spawning it reports the ordinary not-found error.