refactor(rust): update the way the rust.srcToolchain default works and also some defaultText

rust/default.nix

@@ -29,7 +29,7 @@
         manifest = (lib.importTOML "${src}/Cargo.toml").package;
 
         craneLib =
-          if builtins.pathExists cfg.srcToolchain
+          if cfg.srcToolchain != null && builtins.pathExists cfg.srcToolchain
           then
             (inputs.crane.mkLib (pkgs.extend inputs.rust-overlay.overlays.default)).overrideToolchain (
               p: p.rust-bin.fromRustupToolchainFile cfg.srcToolchain
@@ -61,18 +61,26 @@
             description = "A path to the filtered directory of the root of the crate.";
             type = lib.types.path;
             default = craneLib.cleanCargoSource cfg.src;
-            defaultText = lib.literalExpression "config.krantz.rust.craneLib cfg.src";
+            defaultText = lib.literalExpression "config.krantz.rust.craneLib.cleanCargoSource cfg.src";
             example = lib.literalExpression "./.";
           };
           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 = "${
+            type = lib.types.nullOr lib.types.path;
+            default =
               if cfg.src != null
-              then cfg.src
-              else "/dev/null"
-            }/rust-toolchain.toml";
-            defaultText = lib.literalExpression "cfg.src + \"/rust-toolchain.toml\"";
+              then cfg.src + "/rust-toolchain.toml"
+              else null;
+            defaultText =
+              lib.literalExpression
+              /*
+              nix
+              */
+              ''
+                if cfg.src != null
+                then cfg.src + "/rust-toolchain.toml"
+                else null;
+              '';
             example = lib.literalExpression "./rust-toolchain";
           };