generalize the support for mouseover tooltips so that

the computation of the string doesn't have to happen
during macro expansion, but can happen only when check syntax
is in the picture

pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/annotate.rkt

@@ -25,10 +25,10 @@
   (when defs
     (send defs syncheck:color-range source start finish style-name)))
 
-;; add-mouse-over : syntax[original] string -> void
+;; add-mouse-over : syntax[original] (or/c string (-> string?)) -> void
 ;; registers the range in the editor so that a mouse over
 ;; this area shows up in the status line.
-(define (add-mouse-over stx str)
+(define (add-mouse-over stx str/proc)
   (define source (find-source-editor stx))
   (define defs-text (current-annotations))
   (when (and defs-text 
@@ -37,12 +37,19 @@
              (syntax-span stx))
     (define pos-left (- (syntax-position stx) 1))
     (define pos-right (+ pos-left (syntax-span stx)))
-    (send defs-text syncheck:add-mouse-over-status
-          source pos-left pos-right str)))
+    (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)))
 
-(define (add-mouse-over/loc source pos-left pos-right str)
+(define (add-mouse-over/loc source pos-left pos-right str/proc)
   (define defs-text (current-annotations))
   (when defs-text
+    (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)))
+
+(define (add-mouse-over/loc/proc defs-text source pos-left pos-right str/proc)
+  (define str
+    (cond
+      [(procedure? str/proc) (str/proc)]
+      [else str/proc]))
+  (when (string? str)
     (send defs-text syncheck:add-mouse-over-status
           source pos-left pos-right str)))
 

pkgs/drracket-pkgs/drracket-tool-lib/drracket/private/syncheck/traversals.rkt

@@ -545,7 +545,10 @@
             prop)])))
 
 (define mouse-over-tooltip-prop?
-  (vector/c #:flat? #t syntax? exact-nonnegative-integer? exact-nonnegative-integer? string?))
+  (vector/c #:flat? #t syntax? exact-nonnegative-integer? exact-nonnegative-integer?
+            (or/c string?
+                  ;; don't check the range here, since we want a predicate, not really a contract
+                  (-> any))))
 (define (add-mouse-over-tooltips stx)
   (let loop ([prop (syntax-property stx 'mouse-over-tooltips)])
     (cond

pkgs/drracket-pkgs/drracket/scribblings/tools/tools.scrbl

@@ -753,12 +753,18 @@ After putting this code in the DrRacket window, mouse over the words ``big'' and
 The value of the @racket['mouse-over-tooltips] property is expected to be 
 to be a tree of @racket[cons] pairs (in any configuration) whose leaves
 are either ignored or are vectors of the shape
-@racketblock[(vector/c syntax? exact-nonnegative-integer? exact-nonnegative-integer? string?)]
+@racketblock[(vector/c syntax?
+                       exact-nonnegative-integer?
+                       exact-nonnegative-integer?
+                       (or/c string? (-> string?)))]
 Each vector's content indicates where to show a tooltip. The first three components are
 a syntax object whose @racket[syntax-source] field indicates which file the tooltip goes in,
 the start and end position in the editor where mouseovers will show the tooltip, 
-and the content of the tooltip. For example, here's a macro that shows the span of itself 
-in a tooltip on mouseover:
+and the content of the tooltip. If the tooltip content is a procedure, this procedure
+is called by Check Syntax to compute the string used for the tooltip, as Check Syntax
+traverses the syntax objects looking for properties.
+
+For example, here's a macro that shows the span of itself in a tooltip on mouseover:
 @codeblock{
 #lang racket
 (define-syntax (char-span stx)
@@ -777,7 +783,7 @@ in a tooltip on mouseover:
 
 (char-span (+ 1 2))}
 
-Finally, Check Syntax only draws arrows between identifiers that are @racket[syntax-original?]
+Finally, Check Syntax draws arrows only between identifiers that are @racket[syntax-original?]
 or that have the @racket[syntax-property] @racket['original-for-check-syntax]
 set to @racket[#t].