micros/walker: fix let binding

contrib/walker/example.lisp

@@ -255,3 +255,8 @@
       :initially (print foo)
       :collect x :into foo
       :finally (print foo))
+
+(let ((a 0)
+      b)
+  a
+  b)

contrib/walker/tests/test-cases.lisp

@@ -3755,3 +3755,27 @@
         :DO (MICROS/WALKER::F MICROS/WALKER::N) (MICROS/WALKER::G MICROS/WALKER::N))
   (2))
  ((1 7) (1 6) (2)))
+((MICROS/WALKER:COLLECT-HIGHLIGHT-PATHS
+  (LET ((MICROS/WALKER::A 0) MICROS/WALKER::B)
+    MICROS/WALKER::A
+    MICROS/WALKER::B)
+  (3))
+ ((3) (1 1)))
+((MICROS/WALKER:COLLECT-HIGHLIGHT-PATHS
+  (LET ((MICROS/WALKER::A 0) MICROS/WALKER::B)
+    MICROS/WALKER::A
+    MICROS/WALKER::B)
+  (2))
+ ((2) (0 0 1)))
+((MICROS/WALKER:COLLECT-HIGHLIGHT-PATHS
+  (LET ((MICROS/WALKER::A 0) MICROS/WALKER::B)
+    MICROS/WALKER::A
+    MICROS/WALKER::B)
+  (1 1))
+ ((3) (1 1)))
+((MICROS/WALKER:COLLECT-HIGHLIGHT-PATHS
+  (LET ((MICROS/WALKER::A 0) MICROS/WALKER::B)
+    MICROS/WALKER::A
+    MICROS/WALKER::B)
+  (0 0 1))
+ ((2) (0 0 1)))

contrib/walker/walker.lisp

@@ -535,11 +535,14 @@
   (walk-lambda-form walker form env path))
 
 (defmethod walk-let-binding-form ((walker walker) binding-form env path)
-  (with-walker-bindings (var value) binding-form
+  (with-walker-bindings (var value)
+      (if (consp binding-form)
+          binding-form
+          (list binding-form nil))
     (assert-type var 'variable-symbol)
     (let ((value (walk walker value env (cons 1 path))))
       (make-instance 'let-binding-form
-                     :path (cons 0 path)
+                     :path (if (consp binding-form) (cons 0 path) path)
                      :binding (make-instance 'lexical-variable-binding
                                              :name var
                                              :value value)