clean both flake and non-flake API

shell.nix

@@ -1,17 +1,26 @@
-{ pkgs ? import <nixpkgs> { } }:
+{ pkgs ? import <nixpkgs> { }
+, fenix ? import (fetchTarball "https://github.com/nix-community/fenix/archive/main.tar.gz") { }
+, withNotmuch ? false
+, withGpg ? false
+, withOpenSsl ? false
+}:
 
 let
   inherit (pkgs) lib;
-
-  fenix = import (fetchTarball "https://github.com/soywod/fenix/archive/main.tar.gz") { };
-  mkToolchain = import ./rust-toolchain.nix { inherit lib fenix; };
-  rust = mkToolchain.fromFile;
+  mkRustToolchain = import ./rust-toolchain.nix { inherit lib fenix; };
+  rust = mkRustToolchain.fromFile;
 in
 
 pkgs.mkShell {
-  buildInputs = with pkgs; [
-    nixd
-    nixpkgs-fmt
-    rust
-  ];
+  buildInputs = [ ]
+    # Nix language
+    ++ [ pkgs.nixd pkgs.nixpkgs-fmt ]
+
+    # Rust
+    ++ [ rust ]
+
+    # Cargo features
+    ++ lib.optional withNotmuch pkgs.notmuch
+    ++ lib.optional withGpg pkgs.gpgme
+    ++ lib.optional withOpenSsl pkgs.openssl;
 }