gexp: Add 'delayed-object'.

* guix/gexp.scm (<delayed-object>): New record type.
(delayed-object): New macro.
(delayed-object-compiler): New gexp compiler.
* tests/gexp.scm ("delayed-object"): New test.
* doc/guix.texi (G-Expressions): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -148,6 +148,7 @@ Copyright @copyright{} 2025 Edouard Klein@*
 Copyright @copyright{} 2025 Rodion Goritskov@*
 Copyright @copyright{} 2025 dan@*
 Copyright @copyright{} 2025 Noé Lopez@*
+Copyright @copyright{} 2026 David Elsing@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -12982,6 +12983,15 @@ The example above returns an object that corresponds to the i686 build
 of Coreutils, regardless of the current value of @code{%current-system}.
 @end defmac
 
+@defmac delayed object exp
+This macro delays the evaluation of @var{exp} until the returned object
+is lowered to a derivation or store item.
+
+Its intended use case is to prevent the use of a non-delayed top-level
+variable of another module, e.g. when using a gexp in the @code{source}
+field of a @var{package}.
+@end defmac
+
 @anchor{gexp-input}
 @deffn {Procedure} gexp-input @var{obj} [@var{output}] [#:native? #f]
 Return a @dfn{gexp input} record for the given @var{output} of file-like

guix/gexp.scm

@@ -6,6 +6,7 @@
 ;;; Copyright © 2020 Maxim Cournoyer <maxim@guixotic.coop>
 ;;; Copyright © 2021, 2022 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2025 Tomas Volf <~@wolfsden.cz>
+;;; Copyright © 2026 David Elsing <david.elsing@posteo.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -96,6 +97,10 @@
             with-parameters
             parameterized?
 
+            delayed-object
+            delayed-object?
+            delayed-object-promise
+
             load-path-expression
             gexp-modules
 
@@ -778,6 +783,21 @@ x86_64-linux when COREUTILS is lowered."
                            (obj                   ;store item
                             obj)))))))))
 
+;; Object which evaluates its promise when it is lowered.
+(define-record-type <delayed-object>
+  (%delayed-object promise)
+  delayed-object?
+  (promise delayed-object-promise))
+
+(define-syntax-rule (delayed-object body ...)
+  "Delays the evaluation of BODY to lowering time of the return object."
+  (%delayed-object (delay (begin body ...))))
+
+(define-gexp-compiler (delayed-object-compiler (object <delayed-object>)
+                                               system target)
+  (with-monad %store-monad
+    (return (force (delayed-object-promise object)))))
+
 
 ;;;
 ;;; Inputs & outputs.

tests/gexp.scm

@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014-2025 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021-2022 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2026 David Elsing <david.elsing@posteo.net>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -518,6 +519,20 @@
     (return (and (eq? drv0 result0)
                  (eq? drv1 result1)))))
 
+(test-assertm "delayed-object"
+  (let ((evaluated #f))
+    (mlet* %store-monad ((drv (package->derivation coreutils))
+                         (obj -> (delayed-object
+                                  (begin
+                                    (set! evaluated #t)
+                                    coreutils)))
+                         (first-evaluated -> evaluated)
+                         (result (lower-object obj)))
+      (return (and (string=? (derivation-file-name drv)
+                             (derivation-file-name result))
+                   (not first-evaluated)
+                   evaluated)))))
+
 (test-assert "with-parameters + file-append"
   (let* ((system (match (%current-system)
                    ("aarch64-linux" "x86_64-linux")