Function trace_dependency 

pub fn trace_dependency(
    graph: &RetainedModuleGraph,
    root: &Path,
    workspace_roots: &[&Path],
    ignore_patterns: &IgnorePatternSet,
    package_name: &str,
    script_used_packages: &FxHashSet<String>,
) -> DependencyTrace

Trace where a dependency is used.

workspace_roots are the roots of the workspaces, so the peer-dependency credit runs one closure per workspace, as the unused-dependency check does. ignore_patterns are the configured ignorePatterns. A workspace package.json that they match does not turn on an optional peer, as in the unused-dependency check.