compiler: warn on misleading references in load-time.

If code does something like this, we produce a warning:

  (let ((var 42))
    (load-time (+ var 3)))

A load-time form occurs in an empty lexical environment,
so this code appears right but actually cannot work;
the appearance is a mere trompe d'oeil.

* share/txr/stdlib/compiler.tl (misleading-ref-check): New
function.
(compiler comp-load-time-lit): Apply misleading-ref-check.

1 files changed, 9 insertions, 0 deletions

diff --git a/share/txr/stdlib/compiler.tl b/share/txr/stdlib/compiler.tl
index 8af7384f..d734fccf 100644
--- a/share/txr/stdlib/compiler.tl
+++ b/share/txr/stdlib/compiler.tl
@@ -1146,6 +1146,14 @@
                    (end ,bfrag.oreg))
                  bfrag.fvars bfrag.ffuns)))))
 
+(defun misleading-ref-check (frag env form)
+  (each ((v frag.fvars))
+    (when env.(lookup-var v)
+      (compile-warning form "cannot refer to lexical variable ~s" v)))
+  (each ((f frag.ffuns))
+    (when env.(lookup-fun f)
+      (compile-warning form "cannot refer to lexical function ~s" f))))
+
 (defmeth compiler comp-load-time-lit (me oreg env form)
   (mac-param-bind form (op loaded-p exp) form
     (if loaded-p
@@ -1159,6 +1167,7 @@
                                      (mov ,dreg ,exp.oreg))
                                    exp.fvars
                                    exp.ffuns))))
+          (misleading-ref-check exp env form)
           me.(free-treg oreg)
           (push lt-frag me.lt-frags)
           (new (frag dreg nil)))))))