gnu: rust-gio: Add 0.15.12.

* gnu/packages/crates-gtk.scm (rust-gio-0.15): New variable.
(rust-gio-0.14): Inherit from it.

gnu/packages/crates-gtk.scm

@@ -796,8 +796,48 @@
        (("rust-shell-words" ,rust-shell-words-0.1)
         ("rust-tempfile" ,rust-tempfile-3))))))
 
+(define-public rust-gio-0.15
+  (package
+    (name "rust-gio")
+    (version "0.15.12")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (crate-uri "gio" version))
+       (file-name (string-append name "-" version ".tar.gz"))
+       (sha256
+        (base32 "0fr8qiqjf9yzl8867kgqdsxpkcx2jrns3xwmlf0jfiid668brzb8"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(;; FIXME: error[E0277]: `Errors` doesn't implement `std::fmt::Display`
+       #:tests? #f
+       #:cargo-inputs
+       (("rust-bitflags" ,rust-bitflags-1)
+        ("rust-futures-channel" ,rust-futures-channel-0.3)
+        ("rust-futures-core" ,rust-futures-core-0.3)
+        ("rust-futures-io" ,rust-futures-io-0.3)
+        ("rust-futures-util" ,rust-futures-util-0.3)
+        ("rust-gio-sys" ,rust-gio-sys-0.15)
+        ("rust-glib" ,rust-glib-0.15)
+        ("rust-libc" ,rust-libc-0.2)
+        ("rust-once-cell" ,rust-once-cell-1)
+        ("rust-thiserror" ,rust-thiserror-1))
+       #:cargo-development-inputs
+       (("rust-futures-util" ,rust-futures-util-0.3)
+        ("rust-gir-format-check" ,rust-gir-format-check-0.1)
+        ("rust-serial-test" ,rust-serial-test-0.6))))
+    (native-inputs
+     (list pkg-config))
+    (inputs
+     (list glib))
+    (home-page "https://gtk-rs.org/")
+    (synopsis "Rust bindings for the Gio library")
+    (description "Rust bindings for the Gio library")
+    (license license:expat)))
+
 (define-public rust-gio-0.14
   (package
+    (inherit rust-gio-0.15)
     (name "rust-gio")
     (version "0.14.8")
     (source
@@ -823,13 +863,7 @@
         ("rust-thiserror" ,rust-thiserror-1))
        #:cargo-development-inputs
        (("rust-gir-format-check" ,rust-gir-format-check-0.1)
-        ("rust-serial-test" ,rust-serial-test-0.4))))
+        ("rust-serial-test" ,rust-serial-test-0.4))))))
-    (inputs
-     (list glib))
-    (home-page "https://gtk-rs.org/")
-    (synopsis "Rust bindings for the Gio library")
-    (description "Rust bindings for the Gio library")
-    (license license:expat)))
 
 (define-public rust-gio-0.9
   (package