Show message about --update-input being replaced by nix flake update

Change-Id: I6ee23874cb09f51d788521273076a25ba8764859

src/nix/flake.cc

@@ -87,11 +87,13 @@ public:
         expectArgs({
             .label="inputs",
             .optional=true,
-            .handler={[&](std::string inputToUpdate){
+            .handler={[&](std::vector<std::string> inputsToUpdate) {
+                for (const auto & inputToUpdate : inputsToUpdate) {
                     auto inputPath = flake::parseInputPath(inputToUpdate);
                     if (lockFlags.inputUpdates.contains(inputPath))
                         warn("Input '%s' was specified multiple times. You may have done this by accident.");
                     lockFlags.inputUpdates.insert(inputPath);
+                }
             }},
             .completer = {[&](AddCompletions & completions, size_t, std::string_view prefix) {
                 completeFlakeInputPath(completions, getEvalState(), getFlakeRefsForCompletion(), prefix);
@@ -132,6 +134,15 @@ struct CmdFlakeLock : FlakeCommand
 
     CmdFlakeLock()
     {
+        addFlag({
+            .longName="update-input",
+            .description="Replaced with `nix flake update input...`",
+            .labels={"input-path"},
+            .handler={[&](std::string inputToUpdate){
+                throw UsageError("`nix flake lock --update-input %1%` has been replaced by `nix flake update %1%`", inputToUpdate);
+            }}
+        });
+
         /* Remove flags that don't make sense. */
         removeFlag("no-write-lock-file");
     }