Make macro-aux safe for other things together with syntax-case

If you set the macro-aux of a macro outside of (chibi syntax-case), it
would previously case `syntax` to think that it was a pattern variable
and try to substitute it, even if the macro-aux was being used for
something else.

This patch fixes that by wrapping pattern variable values in an extra
typed box and checking that it has the right type before deciding that
it’s actually a pattern variable.

lib/chibi/syntax-case.scm

@@ -61,6 +61,10 @@
     ((letrec-syntax ((keyword transformer) ...) . body)
      (%letrec-syntax ((keyword (make-transformer transformer)) ...) . body))))
 
+(define-record-type Pattern-Cell
+  (make-pattern-cell val) pattern-cell?
+  (val pattern-cell-value))
+
 (define-syntax define-pattern-variable
   (er-macro-transformer
    (lambda (expr rename compare)
@@ -68,7 +72,7 @@
            (binding (cddr expr)))
        (let ((cell (env-cell (current-usage-environment) id)))
          (if cell
-             (macro-aux-set! (cdr cell) binding)))
+             (macro-aux-set! (cdr cell) (make-pattern-cell binding))))
        (rename '(begin))))))
 
 (define (make-pattern-variable pvar)
@@ -76,8 +80,13 @@
     (error "reference to pattern variable outside syntax" pvar)))
 
 (define (pattern-variable x)
-  (let ((cell (env-cell (current-usage-environment) x)))
-    (and cell (macro? (cdr cell)) (macro-aux (cdr cell)))))
+  (and-let*
+      ((cell (env-cell (current-usage-environment) x))
+       (cell-ref (cdr cell))
+       ((macro? cell-ref))
+       (aux (macro-aux cell-ref))
+       ((pattern-cell? aux)))
+    (pattern-cell-value aux)))
 
 (define (rename id)
   ((current-renamer) id))

lib/chibi/syntax-case.sld

@@ -16,6 +16,8 @@
                 make-variable-transformer)
           (only (meta) environment)
           (srfi 1)
+          (srfi 2)
+          (srfi 9)
           (srfi 11)
           (srfi 39))
   (include "syntax-case.scm"))