Set variables declared through let* as ignorable

I am trying to write a macro that would generate code that looks like:

(defun test ()
    (let* ((_ (exp1))
           (_ (exp2))
           ...
           (_ (expn)))))

The (expn) calls might be called purely due to their side effects, and it is possible that their return value (stored in _) will never be used. This leads SBCL to emit an style warning: The variable _ is defined but never used.

How can I prevent this warning? I tried declaring the variable as ignorable (through (declare (ignorable _))) in various places in the code but that doesn't work.

This is hard to do and is a somewhat unclear part of the CL standard. In particular in a case like

(let ((foo 1)
      (foo 2))
  (declare ... involving foo ...)
  ...)

Which foo does the declaration apply to? Or does it apply to all of them?

There have been discussions about this and I forget the conclusion. I am fairly sure different implementations differ. In the case where you are generating this let* form with a macro then the answer is to not do that, but instead generate a set of nested let forms with appropriate declarations.