[feat] package: reduce rebuilds

Only copy Rust sources to reduce rebuilds if only nix stuff (or CI)
changed.

.cargo/config

@@ -0,0 +1 @@
+config.toml

default.nix

@@ -6,13 +6,30 @@
   cargoToml = readToml ./Cargo.toml;
   pname = cargoToml.package.name;
   inherit (cargoToml.package) version description;
+  src = lib.cleanSourceWith {
+    src = ./.;
+    name = "${pname}-source";
+    # Adapted from <https://github.com/ipetkov/crane/blob/master/lib/filterCargoSources.nix>
+    # no need to pull in crane for just this
+    filter = orig_path: type: let
+      path = toString orig_path;
+      base = baseNameOf path;
+      parentDir = baseNameOf (dirOf path);
+      matchesSuffix = lib.any (suffix: lib.hasSuffix suffix base) [
+        # Rust sources
+        ".rs"
+        # TOML files are often used to configure cargo based tools (e.g. .cargo/config.toml)
+        ".toml"
+      ];
+      isCargoLock = base == "Cargo.lock";
+      # .cargo/config.toml is captured above
+      isOldStyleCargoConfig = parentDir == ".cargo" && base == "config";
+    in
+      type == "directory" || matchesSuffix || isCargoLock || isOldStyleCargoConfig;
+  };
 in
   rustPlatform.buildRustPackage {
-    inherit pname version;
+    inherit pname version src;
-    src = builtins.path {
-      path = ./.;
-      name = "${pname}-source";
-    };
     cargoLock.lockFile = ./Cargo.lock;
     useNextest = true;