flake.nix

{
  description = "You're own Michel";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs = {
        nixpkgs.follows = "nixpkgs";
        flake-utils.follows = "flake-utils";
      };
    };
  };

  outputs = { self, nixpkgs, flake-utils, rust-overlay }:
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          overlays = [ (import rust-overlay) ];
          pkgs = import nixpkgs {
            inherit system overlays;
          };
          # 👇 new! note that it refers to the path ./rust-toolchain.toml
          rustToolchain = pkgs.pkgsBuildHost.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;

          buildInputs = with pkgs; [ rustToolchain trunk ];

        in
        with pkgs;
        {
          devShells.default = mkShell {
            # 👇 we can just use `rustToolchain` here:
            inherit buildInputs;
          };
        }
      );
}