The references resolving to one function: how many, whether the function references
itself, and which function the first reference came from, None when it came from
outside any function body (a variable initializer). Self-references are recorded apart
rather than counted.
Counts, for every function of the unit, the expressions that resolve to it, calls and
references used as values alike. type_of_expr gives the single declaration the type
checker selected, so overload selection, the qualified and using for forms and import
aliases are all attributed to the right function.
Whether a function’s single reference only enters it through a reference cycle: the
chain of single-reference sources loops back on itself, so there is no non-recursive
caller to inline into (mutually recursive helpers with no external caller).
Collects every function the unit binds as a user-defined operator, through a
using {f as +} for T entry of a file-level or contract-level directive. The HIR
already resolved those entries to function ids.