flake: move the pre-commit definition to its own file

It's a good hundred LOC, and wasn't coupled to the actual flake logic at
all.

Change-Id: Iebb4667b3197dbd8cb2b019014e99fa651848832

2 files changed, 100 insertions, 82 deletions

diff --git a/flake.nix b/flake.nix
index 1c090cadf..6594961ff 100644
--- a/flake.nix
+++ b/flake.nix
@@ -286,89 +286,8 @@
           system:
           let
             pkgs = nixpkgsFor.${system}.native;
-            # Import pre-commit bypassing the flake because flakes don't let
-            # you have overlays. Also their implementation forces an
-            # unnecessary reimport of nixpkgs for our use cases.
-            tools = import (pre-commit-hooks + "/nix/call-tools.nix") pkgs;
-            pre-commit-run = pkgs.callPackage (pre-commit-hooks + "/nix/run.nix") {
-              inherit tools;
-              isFlakes = true;
-              # unused!
-              gitignore-nix-src = builtins.throw "gitignore-nix-src is unused";
-            };
           in
-          pre-commit-run {
-            src = self;
-            hooks = {
-              no-commit-to-branch = {
-                enable = true;
-                settings.branch = [ "main" ];
-              };
-              check-case-conflicts.enable = true;
-              check-executables-have-shebangs = {
-                enable = true;
-                stages = [ "commit" ];
-              };
-              check-shebang-scripts-are-executable = {
-                enable = true;
-                stages = [ "commit" ];
-              };
-              check-symlinks = {
-                enable = true;
-                excludes = [ "^tests/functional/lang/symlink-resolution/broken$" ];
-              };
-              check-merge-conflicts.enable = true;
-              end-of-file-fixer = {
-                enable = true;
-                excludes = [
-                  "\\.drv$"
-                  "^tests/functional/lang/"
-                ];
-              };
-              mixed-line-endings = {
-                enable = true;
-                excludes = [ "^tests/functional/lang/" ];
-              };
-              release-notes = {
-                enable = true;
-                package = pkgs.build-release-notes;
-                files = "^doc/manual/rl-next(-dev)?";
-                pass_filenames = false;
-                entry = ''
-                  ${lib.getExe pkgs.build-release-notes} doc/manual/rl-next doc/manual/rl-next-dev
-                '';
-              };
-              check-headers = {
-                enable = true;
-                package = pkgs.check-headers;
-                files = "^src/";
-                types = [
-                  "c++"
-                  "file"
-                  "header"
-                ];
-                # generated files; these will never actually be seen by this
-                # check, and are left here as documentation
-                excludes = [
-                  "(parser|lexer)-tab\\.hh$"
-                  "\\.gen\\.hh$"
-                ];
-                entry = lib.getExe pkgs.check-headers;
-              };
-              # TODO: Once the test suite is nicer, clean up and start
-              # enforcing trailing whitespace on tests that don't explicitly
-              # check for it.
-              trim-trailing-whitespace = {
-                enable = true;
-                stages = [ "commit" ];
-                excludes = [ "^tests/functional/lang/" ];
-              };
-              treefmt = {
-                enable = true;
-                settings.formatters = [ pkgs.nixfmt ];
-              };
-            };
-          }
+          import ./misc/pre-commit.nix { inherit self pkgs pre-commit-hooks; }
         );
       };
 
diff --git a/misc/pre-commit.nix b/misc/pre-commit.nix
new file mode 100644
index 000000000..b287f3cec
--- /dev/null
+++ b/misc/pre-commit.nix
@@ -0,0 +1,99 @@
+{
+  /**
+    Path to Lix's source, normally the flake's "self" argument
+  */
+  self ? pkgs.lib.cleanSource ./.,
+  /**
+    Already instantiated Nixpkgs
+  */
+  pkgs,
+  /**
+    pre-commit-hooks source path, normally from the flake input
+  */
+  pre-commit-hooks,
+}:
+let
+  inherit (pkgs) lib;
+  # Import pre-commit bypassing the flake because flakes don't let
+  # you have overlays. Also their implementation forces an
+  # unnecessary reimport of nixpkgs for our use cases.
+  tools = import (pre-commit-hooks + "/nix/call-tools.nix") pkgs;
+  pre-commit-run = pkgs.callPackage (pre-commit-hooks + "/nix/run.nix") {
+    inherit tools;
+    isFlakes = true;
+    # unused!
+    gitignore-nix-src = builtins.throw "gitignore-nix-src is unused";
+  };
+in
+pre-commit-run {
+  src = self;
+  hooks = {
+    no-commit-to-branch = {
+      enable = true;
+      settings.branch = [ "main" ];
+    };
+    check-case-conflicts.enable = true;
+    check-executables-have-shebangs = {
+      enable = true;
+      stages = [ "commit" ];
+    };
+    check-shebang-scripts-are-executable = {
+      enable = true;
+      stages = [ "commit" ];
+    };
+    check-symlinks = {
+      enable = true;
+      excludes = [ "^tests/functional/lang/symlink-resolution/broken$" ];
+    };
+    check-merge-conflicts.enable = true;
+    end-of-file-fixer = {
+      enable = true;
+      excludes = [
+        "\\.drv$"
+        "^tests/functional/lang/"
+      ];
+    };
+    mixed-line-endings = {
+      enable = true;
+      excludes = [ "^tests/functional/lang/" ];
+    };
+    release-notes = {
+      enable = true;
+      package = pkgs.build-release-notes;
+      files = "^doc/manual/rl-next(-dev)?";
+      pass_filenames = false;
+      entry = ''
+        ${lib.getExe pkgs.build-release-notes} doc/manual/rl-next doc/manual/rl-next-dev
+      '';
+    };
+    check-headers = {
+      enable = true;
+      package = pkgs.check-headers;
+      files = "^src/";
+      types = [
+        "c++"
+        "file"
+        "header"
+      ];
+      # generated files; these will never actually be seen by this
+      # check, and are left here as documentation
+      excludes = [
+        "(parser|lexer)-tab\\.hh$"
+        "\\.gen\\.hh$"
+      ];
+      entry = lib.getExe pkgs.check-headers;
+    };
+    # TODO: Once the test suite is nicer, clean up and start
+    # enforcing trailing whitespace on tests that don't explicitly
+    # check for it.
+    trim-trailing-whitespace = {
+      enable = true;
+      stages = [ "commit" ];
+      excludes = [ "^tests/functional/lang/" ];
+    };
+    treefmt = {
+      enable = true;
+      settings.formatters = [ pkgs.nixfmt ];
+    };
+  };
+}