Filter source in Nix build to prevent spurious rebuilds

flake.lock

@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "nix-filter": {
+      "locked": {
+        "lastModified": 1619956448,
+        "narHash": "sha256-4OZjkK1vot9vdg+8jj9+hm4c6mSEc9/OJDQzNvLHBKE=",
+        "owner": "numtide",
+        "repo": "nix-filter",
+        "rev": "cf06efe613a06f1505f187712a307c05b3901884",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "ref": "master",
+        "repo": "nix-filter",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1618886496,
@@ -18,6 +34,7 @@
     },
     "root": {
       "inputs": {
+        "nix-filter": "nix-filter",
         "nixpkgs": "nixpkgs"
       }
     }

flake.nix

@@ -1,16 +1,18 @@
 {
+
   description = "Tinyquiz – an open source online quiz platform";
 
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  inputs.nix-filter.url = "github:numtide/nix-filter/master";
 
-  outputs = { self, nixpkgs }:
+  outputs = { self, nixpkgs, nix-filter }:
   let
     pkgs = nixpkgs.legacyPackages.x86_64-linux;
     buildCmd = name:
       (pkgs.buildGoPackage {
         pname = "tinyquiz-${name}";
         version = "dev";
-        src = ./.;
+        src = with nix-filter.lib; filter { root = ./.; include = [ (inDirectory "cmd") (inDirectory "pkg") (inDirectory "ui") "go.mod" "go.sum" ]; };
         goDeps = ./deps.nix;
         preBuild = "go generate vkane.cz/tinyquiz/...";
         checkPhase = "go test -race vkane.cz/tinyquiz/...";