ci/treefmt: add markdown-code-runner

This was run as a test in `doc/tests/check-nix-code-blocks.nix` before,
but its DX can be improved: By including it in `treefmt` we get better
error reporting and auto-fixing, as well as running it on *all* markdown
files (including READMEs etc.) for free.

ci/default.nix

@@ -75,6 +75,23 @@ let
           includes = [ "*" ];
           priority = 1;
         };
+
+        # TODO: Upstream this into treefmt-nix eventually:
+        #   https://github.com/numtide/treefmt-nix/issues/387
+        settings.formatter.markdown-code-runner = {
+          command = pkgs.lib.getExe pkgs.markdown-code-runner;
+          options =
+            let
+              config = pkgs.writers.writeTOML "markdown-code-runner-config" {
+                presets.nixfmt = {
+                  language = "nix";
+                  command = [ (pkgs.lib.getExe pkgs.nixfmt) ];
+                };
+              };
+            in
+            [ "--config=${config}" ];
+          includes = [ "*.md" ];
+        };
       };
       fs = pkgs.lib.fileset;
       nixFilesSrc = fs.toSource {

doc/doc-support/package.nix

@@ -137,7 +137,6 @@ stdenvNoCC.mkDerivation (
 
       tests = {
         manpage-urls = callPackage ../tests/manpage-urls.nix { };
-        check-nix-code-blocks = callPackage ../tests/check-nix-code-blocks.nix { };
       };
     };
   }

doc/tests/check-nix-code-blocks.nix

@@ -1,32 +0,0 @@
-{
-  runCommand,
-  markdown-code-runner,
-  nixfmt-rfc-style,
-}:
-
-runCommand "manual_check-nix-code-blocks"
-  {
-    nativeBuildInputs = [
-      markdown-code-runner
-      nixfmt-rfc-style
-    ];
-  }
-  ''
-    set +e
-
-    mdcr --check --config ${./mdcr-config.toml} ${./..}
-
-    if [ $? -ne 0 ]; then
-      cat <<EOF
-    Error: `mdcr` command failed. Please make sure the Nix code snippets in Markdown files are correctly formatted.
-
-    Run this command from the Nixpkgs repository root for automatic formatting:
-
-        mdcr --log debug --config ${toString ./..}/tests/mdcr-config.toml ${toString ./..}
-
-    EOF
-      exit 1
-    fi
-
-    touch "$out"
-  ''

doc/tests/mdcr-config.toml

@@ -1,3 +0,0 @@
-[presets.nixfmt]
-language = "nix"
-command = ["nixfmt"]