Function new_resolver 

pub fn new_resolver(
    opts: Options,
    static_vars: Option<HashMap<String, String>>,
    tool_versions: Option<HashMap<String, String>>,
) -> Box<dyn Resolver>

Resolves all predefined, tool-version, and static variables up front and returns an immutable Resolver. Precedence, highest first: static variables, predefined variables, tool versions, then (via expand) the process environment.