Add allow-legacy-syntax-objects? parameter

* module/ice-9/psyntax.scm (syntax?): Only recognize legacy syntax
  objects if the new allow-legacy-syntax-objects? parameter is true.
* module/ice-9/boot-9.scm (allow-legacy-syntax-objects?): New
  parameter.
* doc/ref/api-macros.texi (Syntax Transformer Helpers): Document the
  horrible situation with legacy syntax objects.
* NEWS: Add entry.

2 files changed, 10 insertions, 2 deletions

diff --git a/module/ice-9/boot-9.scm b/module/ice-9/boot-9.scm
index be890fa45..a70cd11ef 100644
--- a/module/ice-9/boot-9.scm
+++ b/module/ice-9/boot-9.scm
@@ -299,6 +299,9 @@ This is handy for tracing function calls, e.g.:
 (define (absolute-file-name? file-name) #t)
 (define (open-input-file str) (open-file str "r"))
 
+;; Temporary definition; replaced by a parameter later.
+(define (allow-legacy-syntax-objects?) #f)
+
 ;;; {and-map and or-map}
 ;;;
 ;;; (and-map fn lst) is like (and (fn (car lst)) (fn (cadr lst)) (fn...) ...)
@@ -1423,11 +1426,15 @@ CONV is not applied to the initial value."
 
 
 ;;; Once parameters have booted, define the default prompt tag as being
-;;; a parameter.
+;;; a parameter, and make allow-legacy-syntax-objects? a parameter.
 ;;;
 
 (set! default-prompt-tag (make-parameter (default-prompt-tag)))
 
+;; Because code compiled with Guile 2.2.0 embeds legacy syntax objects
+;; into its compiled macros, we have to default to true, sadly.
+(set! allow-legacy-syntax-objects? (make-parameter #t))
+
 
 
 ;;; {Languages}
diff --git a/module/ice-9/psyntax.scm b/module/ice-9/psyntax.scm
index a45e2a6cc..5696c4642 100644
--- a/module/ice-9/psyntax.scm
+++ b/module/ice-9/psyntax.scm
@@ -473,7 +473,8 @@
 
     (define (syntax-object? x)
       (or (syntax? x)
-          (and (vector? x)
+          (and (allow-legacy-syntax-objects?)
+               (vector? x)
                (= (vector-length x) 4)
                (eqv? (vector-ref x 0) 'syntax-object))))
     (define (make-syntax-object expression wrap module)