gnu: Add rust-futures-sink-preview.

* gnu/packages/crates-io.scm (rust-futures-sink-preview): New variable.

gnu/packages/crates-io.scm

@@ -1124,6 +1124,30 @@ the computation on the threads themselves.")
     (license (list license:asl2.0
                    license:expat))))
 
+(define-public rust-futures-sink-preview
+  (package
+    (name "rust-futures-sink-preview")
+    (version "0.3.0-alpha.17")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (crate-uri "futures-sink-preview" version))
+        (file-name (string-append name "-" version ".tar.gz"))
+        (sha256
+         (base32
+          "1r4d0gy73hdxkh5g1lrhl1kjnwp6mywjgcj70v0z78b921da42a3"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(#:cargo-inputs
+       (("rust-futures-core-preview" ,rust-futures-core-preview))))
+    (home-page "https://rust-lang-nursery.github.io/futures-rs/")
+    (synopsis  "Asynchronous `Sink` trait for the futures-rs library")
+    (description
+     "This package provides the asynchronous @code{Sink} trait for the
+futures-rs library.")
+    (license (list license:asl2.0
+                   license:expat))))
+
 (define-public rust-gcc
   (package
     (inherit rust-cc)