Initial commit for fern

nix/rust-overlay/flake-module.nix

@@ -0,0 +1,18 @@
+{ inputs, ... }:
+let
+  overlays = [
+    (import inputs.rust-overlay)
+    (self: super: assert !(super ? rust-toolchain); rec {
+      rust-toolchain = super.rust-bin.fromRustupToolchainFile ../../rust-toolchain.toml;
+
+      # buildRustCrate/crate2nix depend on this.
+      rustc = rust-toolchain;
+      cargo = rust-toolchain;
+    })
+  ];
+in
+{
+  perSystem = { system, ... }: {
+    _module.args.pkgs = import inputs.nixpkgs { inherit system overlays; config = { }; };
+  };
+}