bug#60974: 30.0.50; byte-compile-preprocess mutates self evaluating forms in expanded macro bodies

[Stefan Monnier]

> Ah, right. Theother way I could think of a fix is setq-ing `form` to a
> shallow copy of the original form, with only the place(s) changed. This
> patch tries to do that by using `pcase-let` to destructure forms.

Hmm... yes, that's closer.  It's pretty ugly, tho.
Yet it's hard to make it less ugly here because the
`cconv-freevars-alist` really depends on `eq` equality.

Maybe we should pass `cif` to `cconv--convert-function` (or maybe even
move most of that `cconv--interactive-form-funs`-handling to that
function) and arrange for that function to do the
"non-side-effecting-equivalent-of" (setf (cadr (car bf)) form)?


        Stefan