/// - `decl`: the HIR declaration of the closure
/// - `body`: the body of the closure
/// - `expected_sig`: the expected signature (if any). Note that
/// this is missing a binder: that is, there may be late-bound
/// regions with depth 1, which are bound then by the closure.
/// - `decl`: the HIR declaration of the closure
/// - `body`: the body of the closure
/// - `expected_sig`: the expected signature (if any). Note that
/// this is missing a binder: that is, there may be late-bound
/// regions with depth 1, which are bound then by the closure.