version: add `isReleaseBranch`

docs/release-notes/rl-2311.adoc

@@ -16,6 +16,13 @@ but could theoretically break
 if you have very complex bash expressions in a session variable.
 Please report any issues you experience.
 
+* The `.release` file in the Home Manager source tree
+has been supplanted by `release.json`,
+which contains more information about the branch.
+If you have any external code reading this file,
+please migrate to using `release.json` instead.
+The `.release` file will be removed in 24.05.
+
 [[sec-release-23.11-state-version-changes]]
 === State Version Changes
 

modules/home-environment.nix

@@ -483,8 +483,8 @@ in
 
     warnings =
       let
-        hmRelease = fileContents ../.release;
-        nixpkgsRelease = pkgs.lib.trivial.release;
+        hmRelease = config.home.version.release;
+        nixpkgsRelease = lib.trivial.release;
         releaseMismatch =
           config.home.enableNixpkgsReleaseCheck
           && hmRelease != nixpkgsRelease;

modules/misc/version.nix

@@ -2,7 +2,9 @@
 
 with lib;
 
-{
+let releaseInfo = lib.importJSON ../../release.json;
+
+in {
   options = {
     home.stateVersion = mkOption {
       type = types.enum [
@@ -51,11 +53,22 @@ with lib;
         internal = true;
         readOnly = true;
         type = types.str;
-        default = fileContents ../../.release;
+        default = releaseInfo.release;
         example = "22.11";
         description = "The Home Manager release.";
       };
 
+      isReleaseBranch = mkOption {
+        internal = true;
+        readOnly = true;
+        type = types.bool;
+        default = releaseInfo.isReleaseBranch;
+        description = ''
+          Whether the Home Manager version is from a versioned
+          release branch.
+        '';
+      };
+
       revision = mkOption {
         internal = true;
         type = types.nullOr types.str;

release.json

@@ -0,0 +1,4 @@
+{
+  "release": "23.11",
+  "isReleaseBranch": false
+}