fix: don't error if src and srcToolchain is not defined. Just default to nixpkgs toolchain instead.

rust/default.nix

@@ -52,7 +52,8 @@
 
           src = lib.mkOption {
             description = "The path to the root of the crate.";
-            type = lib.types.path;
+            type = lib.types.nullOr lib.types.path;
+            default = null;
             example = lib.literalExpression "./.";
           };
           srcFiltered = lib.mkOption {
@@ -65,7 +66,7 @@
           srcToolchain = lib.mkOption {
             description = "A path to the rust-toolchain(.toml) file for the repo. Will use the toolchain from nixpkgs if the file does not exist.";
             type = lib.types.path;
-            default = "${cfg.src}/rust-toolchain.toml";
+            default = "${if cfg.src != null then cfg.src else "/dev/null"}/rust-toolchain.toml";
             defaultText = lib.literalExpression "cfg.src + \"/rust-toolchain.toml\"";
             example = lib.literalExpression "./rust-toolchain";
           };