pre-commit check for pragma once and ///@file

This is in our style guide, we can cheaply enforce it, let's do it.

```
$ pre-commit
check-case-conflicts.....................................................Passed
check-executables-have-shebangs..........................................Passed
check-headers............................................................Failed
- hook id: check-headers
- exit code: 1

Missing pattern @file in file src/libexpr/value.hh

We found some header files that don't conform to the style guide.

The Lix style guide requests that header files:
- Begin with `#pragma once` so they only get parsed once
- Contain a doxygen comment (`/**` or `///`) containing `@file`, for
  example, `///@file`, which will make doxygen generate docs for them.

  When adding that, consider also adding a `@brief` with a sentence
  explaining what the header is for.

For more details: https://wiki.lix.systems/link/3#bkmrk-header-files

check-merge-conflicts....................................................Passed
check-shebang-scripts-are-executable.....................................Passed
check-symlinks.......................................(no files to check)Skipped
end-of-file-fixer........................................................Passed
mixed-line-endings.......................................................Passed
no-commit-to-branch......................................................Passed
release-notes........................................(no files to check)Skipped
treefmt..................................................................Passed
trim-trailing-whitespace.................................................Passed
```

Fixes: https://git.lix.systems/lix-project/lix/issues/233
Change-Id: I77150b9298c844ffedd0f85cc5250ae9208502e3

1 files changed, 18 insertions, 0 deletions

diff --git a/flake.nix b/flake.nix
index 56dd4bfc3..8eb5582b0 100644
--- a/flake.nix
+++ b/flake.nix
@@ -157,6 +157,7 @@
           nixUnstable = prev.nixUnstable;
 
           build-release-notes = final.buildPackages.callPackage ./maintainers/build-release-notes.nix { };
+          check-headers = final.buildPackages.callPackage ./maintainers/check-headers.nix { };
           clangbuildanalyzer = final.buildPackages.callPackage ./misc/clangbuildanalyzer.nix { };
 
           default-busybox-sandbox-shell = final.busybox.override {
@@ -353,6 +354,23 @@
                   ${lib.getExe pkgs.build-release-notes} doc/manual/rl-next doc/manual/rl-next-dev
                 '';
               };
+              check-headers = {
+                enable = true;
+                package = pkgs.check-headers;
+                files = "^src/";
+                types = [
+                  "c++"
+                  "file"
+                  "header"
+                ];
+                # generated files; these will never actually be seen by this
+                # check, and are left here as documentation
+                excludes = [
+                  "(parser|lexer)-tab\\.hh$"
+                  "\\.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.