expander: extend "illegal use of syntax" error

Show the compile-time value that is not a procedure. While
this runs some risk of exposing details that are meant
to be private to a macro/language, a macro/language can
use an applicable structure to provide a more specific
error message. Meanwhile, showing the value is likely to
help for someone who needs to debug a macro problem.

racket/src/expander/expand/main.rkt

@@ -259,7 +259,11 @@
     (dispatch-variable t s id ctx binding primitive? protected?)]
    [else
     ;; Some other compile-time value:
-    (raise-syntax-error #f "illegal use of syntax" s)]))
+    (raise-syntax-error #f "illegal use of syntax" s
+                        #f null
+                        (format "\n  value at phase ~s: ~e"
+                                (add1 (expand-context-phase ctx))
+                                t))]))
 
 ;; Call a core-form expander (e.g., `lambda`)
 (define (dispatch-core-form t s ctx)

racket/src/racket/src/startup.inc

@@ -19812,6 +19812,7 @@ static const char *startup_source =
 " #%app"
 " #%call-with-values"
 " make-pthread-parameter"
+" break-enabled-key"
 " fasl->s-exp/intern))))"
 "(define-values(phase-shift-id)(make-built-in-symbol! 'phase))"
 "(define-values(dest-phase-id)(make-built-in-symbol! 'dest-phase))"
@@ -43044,7 +43045,17 @@ static const char *startup_source =
 "(if(variable? t_0)"
 "(let-values()"
 "(dispatch-variable t_0 s_0 id_0 ctx_0 binding_0 primitive?_0 protected?_0))"
-"                                (let-values () (raise-syntax-error$1 #f \"illegal use of syntax\" s_0))))))))))))))))))"
+"(let-values()"
+"(raise-syntax-error$1"
+" #f"
+"                                   \"illegal use of syntax\""
+" s_0"
+" #f"
+" null"
+"(format"
+"                                    \"\\n  value at phase ~s: ~e\""
+"(add1(expand-context-phase ctx_0))"
+" t_0)))))))))))))))))))"
 "(define-values"
 "(dispatch-core-form)"
 "(lambda(t_0 s_0 ctx_0)"