gnu: Add rust-unreachable.

* gnu/packages/crates-io.scm (rust-unreachable): New variable.

gnu/packages/crates-io.scm

@@ -4118,6 +4118,28 @@ whitespace from a string.")
     (license (list license:asl2.0
                    license:expat))))
 
+(define-public rust-unreachable
+  (package
+    (name "rust-unreachable")
+    (version "1.0.0")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (crate-uri "unreachable" version))
+        (file-name (string-append name "-" version ".tar.gz"))
+        (sha256
+         (base32
+          "0mps2il4xy2mjqc3appas27hhn2xmvixc3bzzhfrjj74gy3i0a1q"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(#:cargo-inputs (("rust-void" ,rust-void))))
+    (home-page "https://github.com/reem/rust-unreachable")
+    (synopsis "Unreachable code optimization hint in rust")
+    (description
+     "This package provides an unreachable code optimization hint in rust.")
+    (license (list license:asl2.0
+                   license:expat))))
+
 (define-public rust-unsafe-any
   (package
     (name "rust-unsafe-any")