buildRustCrate: support rust editions

In combination with carnix we can now build crates that require a
specific edition of rust features. A few crates started requiring that
already and having this in nixpkgs is just logical.

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -71,7 +71,7 @@ let crate = crate_ // (lib.attrByPath [ crate_.crateName ] (attr: {}) crateOverr
     processedAttrs = [
       "src" "buildInputs" "crateBin" "crateLib" "libName" "libPath"
       "buildDependencies" "dependencies" "features"
-      "crateName" "version" "build" "authors" "colors"
+      "crateName" "version" "build" "authors" "colors" "edition"
     ];
     extraDerivationAttrs = lib.filterAttrs (n: v: ! lib.elem n processedAttrs) crate;
     buildInputs_ = buildInputs;
@@ -136,7 +136,9 @@ stdenv.mkDerivation (rec {
         (crate.type or ["lib"]);
     colors = lib.attrByPath [ "colors" ] "always" crate;
     extraLinkFlags = builtins.concatStringsSep " " (crate.extraLinkFlags or []);
-    extraRustcOpts = (if crate ? extraRustcOpts then crate.extraRustcOpts else []) ++ extraRustcOpts_;
+    edition = crate.edition or null;
+    extraRustcOpts = (if crate ? extraRustcOpts then crate.extraRustcOpts else []) ++ extraRustcOpts_ ++ (lib.optional (edition != null) "--edition ${edition}");
+
     configurePhase = configureCrate {
       inherit crateName buildDependencies completeDeps completeBuildDeps
               crateFeatures libName build workspace_member release libPath crateVersion