gnu: Add rust-darling-0.12.

* gnu/packages/crates-io.scm (rust-darling-0.12): New variable.
(rust-darling-0.10): Inherit from above.

gnu/packages/crates-io.scm

@@ -11113,8 +11113,34 @@ crate (implementation detail).")
     (description "Daemonize is a Rust library for writing system deaemons.")
     (license (list license:expat license:asl2.0))))
 
+(define-public rust-darling-0.12
+  (package
+    (name "rust-darling")
+    (version "0.12.4")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (crate-uri "darling" version))
+       (file-name
+        (string-append name "-" version ".tar.gz"))
+       (sha256
+        (base32 "0g4bkxpwl6i5jav5qqzny39yd5a1fdlh5l0mj020njza6ksl6b2z"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(#:skip-build? #t
+       #:cargo-inputs
+       (("rust-darling-core" ,rust-darling-core-0.12)
+        ("rust-darling-macro" ,rust-darling-macro-0.12))))
+    (home-page "https://github.com/TedDriggs/darling")
+    (synopsis "Proc-macro library for reading attributes in custom derives")
+    (description
+     "This package provides a proc-macro library for reading attributes
+into structs when implementing custom derives.")
+    (license license:expat)))
+
 (define-public rust-darling-0.10
   (package
+    (inherit rust-darling-0.12)
     (name "rust-darling")
     (version "0.10.2")
     (source
@@ -11126,7 +11152,6 @@ crate (implementation detail).")
        (sha256
         (base32
          "0n7qsp6854wm3y1q1lvylhv15zvc87ibbac1nyfmcdbyv1snww0d"))))
-    (build-system cargo-build-system)
     (arguments
      `(#:cargo-inputs
        (("rust-darling-core" ,rust-darling-core-0.10)
@@ -11134,13 +11159,7 @@ crate (implementation detail).")
        #:cargo-development-inputs
        (("rust-proc-macro2" ,rust-proc-macro2-1)
         ("rust-quote" ,rust-quote-1)
-        ("rust-syn" ,rust-syn-1))))
+        ("rust-syn" ,rust-syn-1))))))
-    (home-page "https://github.com/TedDriggs/darling")
-    (synopsis "Proc-macro library for reading attributes in custom derives")
-    (description
-     "This package provides a proc-macro library for reading attributes
-into structs when implementing custom derives.")
-    (license license:expat)))
 
 (define-public rust-darling-core-0.12
   (package