Use cargo crane in Nix flake and set up cachix action (#539)

flake.nix

@@ -5,49 +5,99 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
-    rust-overlay.url = "github:oxalica/rust-overlay";
+    crane.url = "github:ipetkov/crane";
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =
     {
       self,
       nixpkgs,
+      crane,
       flake-utils,
-      rust-overlay,
+      fenix,
       ...
     }:
     flake-utils.lib.eachDefaultSystem (
       system:
       let
-        # We only need the nightly overlay in the devShell because .rs files are formatted with nightly.
-        overlays = [ (import rust-overlay) ];
-        pkgs = import nixpkgs { inherit system overlays; };
-        rustNightly = pkgs.rust-bin.nightly."2025-08-31".default;
-      in
-      with pkgs;
-      {
-        packages.default = rustPlatform.buildRustPackage {
-          pname = "iamb";
-          version = self.shortRev or self.dirtyShortRev;
-          src = ./.;
-          cargoLock = {
-            lockFile = ./Cargo.lock;
-          };
-          nativeBuildInputs = [ pkg-config ];
-          buildInputs = [ openssl ];
+        pkgs = nixpkgs.legacyPackages.${system};
+        inherit (pkgs) lib;
+
+        rustToolchain = fenix.packages.${system}.fromToolchainFile {
+          file = ./rust-toolchain.toml;
+          # When the file changes, this hash must be updated.
+          sha256 = "sha256-Hn2uaQzRLidAWpfmRwSRdImifGUCAb9HeAqTYFXWeQk=";
         };
 
-        devShell = mkShell {
-          buildInputs = [
-            (rustNightly.override {
-              extensions = [
-                "rust-src"
-                "rust-analyzer-preview"
-                "rustfmt"
-                "clippy"
-              ];
-            })
-            pkg-config
+        # Nightly toolchain for rustfmt (pinned to current flake lock)
+        # Note that the github CI uses "current nightly" for formatting, it 's not pinned.
+        rustNightly = fenix.packages.${system}.latest.toolchain;
+        rustNightlyFmt = fenix.packages.${system}.latest.rustfmt;
+
+        craneLib = (crane.mkLib pkgs).overrideToolchain rustToolchain;
+        craneLibNightly = (crane.mkLib pkgs).overrideToolchain rustNightly;
+
+        src = lib.fileset.toSource {
+          root = ./.;
+          fileset = lib.fileset.unions [
+            (craneLib.fileset.commonCargoSources ./.)
+            ./src/windows/welcome.md
+          ];
+        };
+
+        commonArgs = {
+          inherit src;
+          strictDeps = true;
+          pname = "iamb";
+          version = self.shortRev or self.dirtyShortRev;
+        };
+
+        # Build *just* the cargo dependencies, so we can reuse
+        # all of that work (e.g. via cachix) when running in CI
+        cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+
+        # Build the actual crate
+        iamb = craneLib.buildPackage (commonArgs // {
+          inherit cargoArtifacts;
+        });
+      in
+      {
+        checks = {
+          # Build the crate as part of `nix flake check`
+          inherit iamb;
+
+          iamb-clippy = craneLib.cargoClippy (commonArgs // {
+            inherit cargoArtifacts;
+            cargoClippyExtraArgs = "--all-targets -- --deny warnings";
+          });
+
+          iamb-fmt = craneLibNightly.cargoFmt {
+            inherit src;
+          };
+
+          iamb-nextest = craneLib.cargoNextest (commonArgs // {
+            inherit cargoArtifacts;
+            partitions = 1;
+            partitionType = "count";
+          });
+        };
+
+        packages.default = iamb;
+
+        apps.default = flake-utils.lib.mkApp {
+          drv = iamb;
+        };
+
+        devShells.default = craneLib.devShell {
+          # Inherit inputs from checks
+          checks = self.checks.${system};
+
+          packages = with pkgs; [
+            rustNightlyFmt
             cargo-tarpaulin
             cargo-watch
             sqlite