The `TyCtxt::hir_get_fn_id_for_return_block()` function was too broad,
as it will return positively even when given part of an expression that can
be used as a return value. A new `potential_return_of_enclosing_body()`
utility function has been made to represent the fact that an expression
might be directly returned from its enclosing body.
Samuel Tardieu
89f7a4301d