Feature Request: Style warning on 'suspicious' constant/symbol

[Wilfredo Velázquez-Rodríguez]

A user error I see come up here and there, especially among beginners, is something along the lines of:

(defun foo (&key a b c)
  (list c a b))

(let ((bar 0) (baz 1) (quux 2))
  (foo :a bar)
       :b baz
       :c quux)

Where clearly parens lining up was incorrect.
This occurs in any other places with progn-style behaviour, except notably, tagbody.

This class of bug would be eliminated if SBCL were to notice usage of a constant (here :b and :c) or a symbol (here baz) being in a position where their value is completely unused as in the above, and signal a style warning along the lines of

style-warning:
  Suspicious use of constant :B in noop position

I'm struggling to come up with a better name than 'noop' or 'unimpactful', but the general idea should be clear.
Additionally, in reference to tagbody, it would be useful to signal a style-warning on referenced tags, which would accomplish the equivalent 'fix' for the user-error in those cases.

Clearly this is low priority, but I believe it'd be very useful and I imagine it's relatively low effort to implement.

For posterity, this is as of SBCL 1.5.4

[Paul F. Dietz (paul-f-dietz)]

One would have to be sure that macros only very rarely expand to such code.

[Douglas Katzman (dougk)]

There's no syntax to say that tagbody tags are ignorable, so it would produce too many warnings.
I don't think this is worth doing.