json: Re-export 'define-json-mapping' from (json) when available.

This prepares for the migration to Guile-JSON 4.3.0.

* guix/json.scm (define-as-needed): New macro.
(define-json-mapping): Use 'define-as-needed' instead of
'define-syntax-rule'.

guix/json.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -18,15 +18,33 @@
 
 (define-module (guix json)
   #:use-module (json)
-  #:use-module (srfi srfi-9)
-  #:export (define-json-mapping))
+  #:use-module (srfi srfi-9))
 
 ;;; Commentary:
 ;;;
 ;;; Helpers to map JSON objects to SRFI-9 records.  Taken from (guix swh).
+;;; This module is superseded by 'define-json-mapping' as found since version
+;;; 4.2.0 of Guile-JSON and will be removed once migration is complete.
 ;;;
 ;;; Code:
 
+(define-syntax define-as-needed
+  (lambda (s)
+    "Define the given syntax rule unless (json) already provides it."
+    (syntax-case s ()
+      ((_ (macro args ...) body ...)
+       (if (module-defined? (resolve-interface '(json))
+                            (syntax->datum #'macro))
+           #'(eval-when (expand load eval)
+               ;; Re-export MACRO from (json).
+               (module-re-export! (current-module) '(macro)))
+           #'(begin
+               ;; Using Guile-JSON < 4.2.0, so provide our own MACRO.
+               (define-syntax-rule (macro args ...)
+                 body ...)
+               (eval-when (expand load eval)
+                 (module-export! (current-module) '(macro)))))))))
+
 (define-syntax-rule (define-json-reader json->record ctor spec ...)
   "Define JSON->RECORD as a procedure that converts a JSON representation,
 read from a port, string, or hash table, into a record created by CTOR and
@@ -48,8 +66,11 @@ (define (json->record input)
                                                 (symbol->string 'field))))))
         (ctor (extract-field table spec) ...)))))
 
-(define-syntax-rule (define-json-mapping rtd ctor pred json->record
-                      (field getter spec ...) ...)
+;; For some reason we cannot just have colliding definitions of
+;; 'define-json-mapping' (that leads to a build failure in users of this
+;; module), hence the use of 'define-as-needed'.
+(define-as-needed (define-json-mapping rtd ctor pred json->record
+                    (field getter spec ...) ...)
   "Define RTD as a record type with the given FIELDs and GETTERs, à la SRFI-9,
 and define JSON->RECORD as a conversion from JSON to a record of this type."
   (begin