Function classify_assertion 

pub fn classify_assertion(
    stmt: &AIRNode,
) -> Option<(TestAssertion, &AIRNode, Option<&AIRNode>)>

If stmt is an expect(actual).<assertion>(expected?) chain, classify it.

Returns (assertion, actual_expr, expected_expr_opt) where actual_expr is the argument to expect(...) and expected_expr_opt is the explicit argument to the assertion method (present for TestAssertion::Equal, absent for the nullary predicates). Returns None for any statement that is not an expect(...)-rooted assertion chain — the emitter falls back to its normal statement lowering for those.

A Bock method call recv.m(args) is lowered (by bock-air::lower) to Call { callee: FieldAccess(recv, m), args: [self=recv, ...args] } — the receiver is prepended as the implicit self argument. So an assertion expect(actual).to_equal(expected) is:

Call {
  callee: FieldAccess(object: Call{expect, [actual]}, field: to_equal),
  args: [ self = Call{expect, [actual]}, expected ],
}

The explicit expected is therefore args[1] (args[0] is the self copy).