Add weeder to local dev tool chain.

Makefile

@@ -151,7 +151,11 @@ ghcid:
 
 # Used by CI
 .PHONY: lint-all
-lint-all: formatc hlint-check-all check-local-nix-derivations treefmt-check
+lint-all: formatc hlint-check-all check-local-nix-derivations treefmt-check # weeder (does not work on CI yet)
+
+.PHONY: weeder
+weeder:
+	time weeder -N || echo -e '\n\n*** make sure you have added this to your cabal.project.local and make clean before building wire-server:\n\npackage *\n  ghc-options: -fwrite-ide-info\n\n'
 
 .PHONY: hlint-check-all
 hlint-check-all:

nix/haskell-pins.nix

@@ -266,6 +266,16 @@ let
         "warp" = "warp";
       };
     };
+
+    # https://github.com/ocharles/weeder/pull/165
+    weeder = {
+      src = pkgs.fetchFromGitHub {
+        owner = "fisx";
+        repo = "weeder";
+        rev = "027571e39a25aa0cfa6a7c7effcb868757527a2c";
+        hash = "sha256-E35PVxi/4iJFfWts3td52KKZKQt4dj9KFP3SvWG77Cc=";
+      };
+    };
   };
   hackagePins = {
     # Major re-write upstream, we should get rid of this dependency rather than

nix/wire-server.nix

@@ -420,6 +420,7 @@ let
     pkgs.cfssl
     pkgs.awscli2
     (hlib.justStaticExecutables pkgs.haskellPackages.cabal-fmt)
+    (hlib.justStaticExecutables pkgs.haskellPackages.weeder)
   ] ++ pkgs.lib.optionals pkgs.stdenv.isLinux [
     pkgs.skopeo
   ];

weeder.toml

@@ -0,0 +1,6 @@
+# weeder intro and further reading: https://github.com/ocharles/weeder?tab=readme-ov-file#weeder
+roots = ["^Main.main$", "^Paths_.*\\..*"]
+type-class-roots = true  # `root-instances` is more precise, but requires more config maintenance.
+
+# TODO: unused-types = true
+# TODO: type-class-roots = false, and see how bad it gets