improve nix flake

Also fix the comment in `Cargo.toml` about the rust-version stuff.

Cargo.toml

@@ -9,10 +9,10 @@ readme = "README.md"
 version = "0.7.0-alpha"
 edition = "2021"
 
-# When changing this, make sure to update the `flake.lock` file by running
-# `nix flake update`. If you don't have Nix installed or otherwise don't know
-# how to do this, ping `@charles:computer.surgery` or `@dusk:gaze.systems` in
-# the matrix room.
+# When changing this, make sure to update the hash near the text "THE
+# rust-version HASH" in `flake.nix`. If you don't have Nix installed or
+# otherwise don't know how to do this, ping `@charles:computer.surgery` or
+# `@dusk:gaze.systems` in the matrix room.
 rust-version = "1.70.0"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

flake.nix

@@ -43,53 +43,64 @@
         sha256 = "sha256-gdYqng0y9iHYzYPAdkC/ka3DRny3La/S5G8ASj0Ayyc=";
       };
 
-      # The system's RocksDB
-      ROCKSDB_INCLUDE_DIR = "${pkgs.rocksdb}/include";
-      ROCKSDB_LIB_DIR = "${pkgs.rocksdb}/lib";
+      mkToolchain = fenix.packages.${system}.combine;
+
+      buildToolchain = mkToolchain (with toolchain; [
+        cargo
+        rustc
+      ]);
+
+      devToolchain = mkToolchain (with toolchain; [
+        cargo
+        clippy
+        rust-src
+        rustc
+
+        # Always use nightly rustfmt because most of its options are unstable
+        fenix.packages.${system}.latest.rustfmt
+      ]);
+
+      builder =
+        ((crane.mkLib pkgs).overrideToolchain buildToolchain).buildPackage;
 
-      # Shared between the package and the devShell
       nativeBuildInputs = (with pkgs.rustPlatform; [
         bindgenHook
       ]);
 
-      builder =
-        ((crane.mkLib pkgs).overrideToolchain toolchain.toolchain).buildPackage;
+      env = {
+        ROCKSDB_INCLUDE_DIR = "${pkgs.rocksdb}/include";
+        ROCKSDB_LIB_DIR = "${pkgs.rocksdb}/lib";
+      };
     in
     {
       packages.default = builder {
         src = ./.;
 
+        # This is redundant with CI
+        doCheck = false;
+
         inherit
-          stdenv
+          env
           nativeBuildInputs
-          ROCKSDB_INCLUDE_DIR
-          ROCKSDB_LIB_DIR;
+          stdenv;
+
+        meta.mainProgram = cargoToml.package.name;
       };
 
       devShells.default = (pkgs.mkShell.override { inherit stdenv; }) {
-        # Rust Analyzer needs to be able to find the path to default crate
-        # sources, and it can read this environment variable to do so
-        RUST_SRC_PATH = "${toolchain.rust-src}/lib/rustlib/src/rust/library";
-
-        inherit
-          ROCKSDB_INCLUDE_DIR
-          ROCKSDB_LIB_DIR;
+        env = env // {
+          # Rust Analyzer needs to be able to find the path to default crate
+          # sources, and it can read this environment variable to do so. The
+          # `rust-src` component is required in order for this to work.
+          RUST_SRC_PATH = "${devToolchain}/lib/rustlib/src/rust/library";
+        };
 
         # Development tools
-        nativeBuildInputs = nativeBuildInputs ++ (with toolchain; [
-          cargo
-          clippy
-          rust-src
-          rustc
-          rustfmt
-        ]) ++ (with pkgs; [
+        nativeBuildInputs = nativeBuildInputs ++ [
+          devToolchain
+        ] ++ (with pkgs; [
           engage
         ]);
       };
-
-      checks = {
-        packagesDefault = self.packages.${system}.default;
-        devShellsDefault = self.devShells.${system}.default;
-      };
     });
 }