nixfmt: support the new official 1.0 release of nixfmt

- Deprecate nixfmt-classic when used with v1.0+
- Warn when `hooks.nixfmt` is used with older `nixfmt` versions

Once 25.11 lands, we can deprecate `nixfmt-rfc-style`.

Author: sandydoo

modules/hooks.nix

@@ -959,7 +959,7 @@ in
         };
       };
       nixfmt = mkOption {
-        description = "Deprecated nixfmt hook. Use nixfmt-classic or nixfmt-rfc-style instead.";
+        description = "nixfmt hook";
         visible = false;
         type = types.submodule {
           imports = [ hookModule ];
@@ -2143,10 +2143,19 @@ in
     lib.optional cfg.hooks.rome.enable ''
       The hook `hooks.rome` has been renamed to `hooks.biome`.
     ''
-    ++ lib.optional cfg.hooks.nixfmt.enable ''
+    ++ lib.optional (cfg.hooks.nixfmt.enable && lib.versionOlder cfg.hooks.nixfmt.package.version "1.0") ''
       The hook `hooks.nixfmt` has been renamed to `hooks.nixfmt-classic`.
 
       The new RFC 166-style nixfmt is available as `hooks.nixfmt-rfc-style`.
+    ''
+    ++ lib.optional (cfg.hooks.nixfmt-classic.enable && lib.versionAtLeast cfg.hooks.nixfmt-classic.package.version "1.0") ''
+      The hook `hooks.nixfmt-classic` is using an incompatible version of `nixfmt`.
+
+        Found: ${cfg.hooks.nixfmt-classic.package.version}.
+        Expected: < v1.0
+
+      `hooks.nixfmt-classic` supports versions of `nixfmt` up to `v1.0`.
+      For `nixfmt` `v1.0` and newer, switch to `hooks.nixfmt`.
     '';
 
   # PLEASE keep this sorted alphabetically.
@@ -3402,8 +3411,8 @@ lib.escapeShellArgs (lib.concatMap (ext: [ "--ghc-opt" "-X${ext}" ]) hooks.fourm
         };
       nixfmt =
         {
-          name = "nixfmt-deprecated";
-          description = "Deprecated Nix code prettifier. Use nixfmt-classic.";
+          name = "nixfmt";
+          description = "Official Nix code formatter.";
           package = tools.nixfmt;
           entry = "${hooks.nixfmt.package}/bin/nixfmt ${lib.optionalString (hooks.nixfmt.settings.width != null) "--width=${toString hooks.nixfmt.settings.width}"}";
           files = "\\.nix$";

nix/tools.nix

@@ -231,8 +231,16 @@ in
   # Disable tests as these take way to long on our infra.
   julia-bin = julia-bin.overrideAttrs (_: _: { doInstallCheck = false; });
 
-  # nixfmt was renamed to nixfmt-classic in 24.05.
-  # nixfmt may be replaced by nixfmt-rfc-style in the future.
-  nixfmt = if nixfmt-classic == null then nixfmt else nixfmt-classic;
+  # nixfmt 1.0 is now the official Nix formatter as of 25.11.
+  #
+  # In 24.05, the `nixfmt` package was deprecated and replaced with two separate packages:
+  #   - nixfmt-classic
+  #   - nixfmt-rfc-style
+  #
+  # Remove this block in 26.05
+  nixfmt =
+    if lib.versionOlder nixfmt.version "1.0" && nixfmt-classic != null
+    then nixfmt-classic
+    else nixfmt;
   inherit nixfmt-classic nixfmt-rfc-style;
 }