Function apply_graph 

pub fn apply_graph(
    project_root: Option<&Path>,
    overlay: &HashMap<PathBuf, String>,
    parsed: &mut Vec<ParsedFile>,
    tests: bool,
    errors: &mut ErrorSink,
)

Resolve the package graph of the project at project_root and fold the dependencies’ files in parsed into the compile. A project with no dependencies is untouched.

tests says whether this build checks suites: when it doesn’t (a deployable build), suites reach nothing, so a unit only a suite uses — every [dev-dependencies] unit — is never compiled (#2240).