shell: Introduce treefmt

Introduces treefmt with a simple nixfmt-rfc-style configuration to
format all Nix files.

This is only practically usable with the following commit that formats
all files accordingly.

ci/default.nix

@@ -21,9 +21,42 @@ let
     config = { };
     overlays = [ ];
   };
+
+  fmt =
+    let
+      treefmtNixSrc = fetchTarball {
+        # Master at 2025-02-12
+        url = "https://github.com/numtide/treefmt-nix/archive/4f09b473c936d41582dd744e19f34ec27592c5fd.tar.gz";
+        sha256 = "051vh6raskrxw5k6jncm8zbk9fhbzgm1gxpq9gm5xw1b6wgbgcna";
+      };
+      treefmtEval = (import treefmtNixSrc).evalModule pkgs {
+        # Important: The auto-rebase script uses `git filter-branch --tree-filter`,
+        # which creates trees within the Git repository under `.git-rewrite/t`,
+        # notably without having a `.git` themselves.
+        # So if this projectRootFile were the default `.git/config`,
+        # having the auto-rebase script use treefmt on such a tree would make it
+        # format all files in the _parent_ Git tree as well.
+        projectRootFile = ".git-blame-ignore-revs";
+
+        # Be a bit more verbose by default, so we can see progress happening
+        settings.verbose = 1;
+
+        # By default it's info, which is too noisy since we have many unmatched files
+        settings.on-unmatched = "debug";
+
+        # This uses nixfmt-rfc-style underneath,
+        # the default formatter for Nix code.
+        # See https://github.com/NixOS/nixfmt
+        programs.nixfmt.enable = true;
+      };
+    in
+    {
+      shell = treefmtEval.config.build.devShell;
+    };
+
 in
 {
-  inherit pkgs;
+  inherit pkgs fmt;
   requestReviews = pkgs.callPackage ./request-reviews { };
   codeownersValidator = pkgs.callPackage ./codeowners-validator { };
   eval = pkgs.callPackage ./eval { };

shell.nix

@@ -16,7 +16,7 @@
   nixpkgs ? null,
 }:
 let
-  inherit (import ./ci { inherit nixpkgs system; }) pkgs;
+  inherit (import ./ci { inherit nixpkgs system; }) pkgs fmt;
 
   # For `nix-shell -A hello`
   curPkgs = builtins.removeAttrs (import ./. { inherit system; }) [
@@ -28,10 +28,10 @@ let
 in
 curPkgs
 // pkgs.mkShellNoCC {
+  inputsFrom = [
+    fmt.shell
+  ];
   packages = with pkgs; [
-    # The default formatter for Nix code
-    # See https://github.com/NixOS/nixfmt
-    nixfmt-rfc-style
     # Helper to review Nixpkgs PRs
     # See CONTRIBUTING.md
     nixpkgs-review