gnu: Add rust-once-cell-1.2.

* gnu/packages/crates-io.scm (rust-once-cell-1.2): New variable.

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

gnu/packages/crates-io.scm

@@ -5457,6 +5457,34 @@ slices (@code{.find()}, @code{RevSlice}), strings and other things.
 Things in odds may move to more appropriate crates if we find them.")
     (license (list license:asl2.0 license:expat))))
 
+(define-public rust-once-cell-1.2
+  (package
+    (name "rust-once-cell")
+    (version "1.2.0")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (crate-uri "once-cell" version))
+       (file-name
+        (string-append name "-" version ".tar.gz"))
+       (sha256
+        (base32
+         "1vdz8xlg3r05w3wfjimnc347hgm54i5nrqf72r4mlp0fcdplh7w9"))))
+    (build-system cargo-build-system)
+    (arguments
+     `(#:skip-build? #t
+       #:cargo-inputs
+       (("rust-parking-lot" ,rust-parking-lot-0.9))
+       #:cargo-development-inputs
+       (("rust-crossbeam-utils" ,rust-crossbeam-utils-0.6)
+        ("rust-lazy-static" ,rust-lazy-static-1.3)
+        ("rust-regex" ,rust-regex-1.1))))
+    (home-page "https://github.com/matklad/once_cell")
+    (synopsis "Single assignment cells and lazy values")
+    (description
+     "Single assignment cells and lazy values.")
+    (license (list license:expat license:asl2.0))))
+
 (define-public rust-opaque-debug-0.2
   (package
     (name "rust-opaque-debug")