nix flake update add deprecation warnings.

This builds on NixOS#8817, to add additional UX help for people with existing
muscle memory (or shell history) with --update-input and tries to gently
guide them towards the newly evolved CLI UI.

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

src/libcmd/installables.cc

@@ -47,6 +47,16 @@ MixFlakeOptions::MixFlakeOptions()
 {
     auto category = "Common flake-related options";
 
+    addFlag({
+        .longName = "recreate-lock-file",
+        .description = "Recreate the flake's lock file from scratch.",
+        .category = category,
+        .handler = {[&]() {
+            lockFlags.recreateLockFile = true;
+            warn("'--recreate-lock-file' is deprecated and will be removed in a future version; use 'nix flake update' instead.");
+        }}
+    });
+
     addFlag({
         .longName = "no-update-lock-file",
         .description = "Do not allow any updates to the flake's lock file.",
@@ -79,6 +89,20 @@ MixFlakeOptions::MixFlakeOptions()
         .handler = {&lockFlags.commitLockFile, true}
     });
 
+    addFlag({
+        .longName = "update-input",
+        .description = "Update a specific flake input (ignoring its previous entry in the lock file).",
+        .category = category,
+        .labels = {"input-path"},
+        .handler = {[&](std::string s) {
+            warn("'--update-input' is a deprecated alias for 'flake update' and will be removed in a future version.");
+            lockFlags.inputUpdates.insert(flake::parseInputPath(s));
+        }},
+        .completer = {[&](AddCompletions & completions, size_t, std::string_view prefix) {
+            completeFlakeInputPath(completions, getEvalState(), getFlakeRefsForCompletion(), prefix);
+        }}
+    });
+
     addFlag({
         .longName = "override-input",
         .description = "Override a specific flake input (e.g. `dwarffs/nixpkgs`). This implies `--no-write-lock-file`.",

src/nix/flake.cc

@@ -89,7 +89,13 @@ struct CmdFlakeUpdate : FlakeCommand
             .label="inputs",
             .optional=true,
             .handler={[&](std::string inputToUpdate){
-                auto inputPath = flake::parseInputPath(inputToUpdate);
+                InputPath inputPath;
+                try {
+                    inputPath = flake::parseInputPath(inputToUpdate);
+                } catch (Error & e) {
+                    warn("Invalid flake input '%s'. To update a specific flake, use 'nix flake update --flake %s' instead.", inputToUpdate, inputToUpdate);
+                    throw e;
+                }
                 if (lockFlags.inputUpdates.contains(inputPath))
                     warn("Input '%s' was specified multiple times. You may have done this by accident.");
                 lockFlags.inputUpdates.insert(inputPath);