warn on unknown files in output dir

this helps you clean up files if you delete a source file but don't
delete the corresponding output target

compiler.rkt

@@ -7,7 +7,7 @@
 (require racket/bool racket/file racket/match racket/set
          syntax/parse/define (for-syntax racket/base racket/syntax))
 
-(provide define-rule define-rule*
+(provide define-rule define-rule* rule-name rule-inputs rule-output
          generate-operations execute-rule execute-operations generate/execute)
 
 ;; A compiler rule

render.rkt

@@ -334,7 +334,17 @@
             (ir-doc->page xrefs-repo config)
             (write-string out)))
       output-rule))
-  (append intermediate-rules output-rules img-rules static-rules (list scss hashtag-page)))
+  (define all-rules (append intermediate-rules output-rules img-rules static-rules (list scss hashtag-page)))
+  (define existing-output-files
+    (parameterize ([current-directory output-dir])
+      (for/set ([file (in-directory #f)] #:when (file-exists? file))
+        (build-path output-dir file))))
+  (define accounted-output-files
+    (for/set ([rule (in-list all-rules)]) (rule-output rule)))
+  (define unknown-output-files (set-subtract existing-output-files accounted-output-files))
+  (for ([f (in-set unknown-output-files)])
+    (printf "WARN: unknown file in output dir: ~a\n" f))
+  all-rules)
 
 (module+ main
   (require racket/cmdline)