gnu: Add rust-downcast-rs-1.1.

* gnu/packages/crates-io.scm (rust-downcast-rs-1.1): New variable.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>

gnu/packages/crates-io.scm

@@ -2996,6 +2996,27 @@ from macros.")
     (description "Command line argument parsing.")
     (license (list license:expat license:unlicense))))
 
+(define-public rust-downcast-rs-1.1
+  (package
+    (name "rust-downcast-rs")
+    (version "1.1.1")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (crate-uri "downcast-rs" version))
+       (file-name
+        (string-append name "-" version ".tar.gz"))
+       (sha256
+        (base32
+         "1xhs2qj02k9m4mm5fgh19y88850y9jsnwwlblf2ffc91gjs6xfjj"))))
+    (build-system cargo-build-system)
+    (home-page "https://github.com/marcianx/downcast-rs")
+    (synopsis "Trait object downcasting support using only safe Rust")
+    (description
+     "Trait object downcasting support using only safe Rust.  It supports type
+parameters, associated types, and type constraints.")
+    (license (list license:expat license:asl2.0))))
+
 (define-public rust-dtoa-0.4
   (package
     (name "rust-dtoa")