nix: build with crane, dependencies cached separately

buildDepsOnly builds the dependencies of the host and the tests once
(keyed only on Cargo.toml/Cargo.lock); packages.default and checks.tests
reuse them, so a change to our crates no longer recompiles every crate.
checks.package builds the binary the host loop runs.

flake.nix

@@ -6,9 +6,10 @@
     flake-utils.url = "github:numtide/flake-utils";
     rust-overlay.url = "github:oxalica/rust-overlay";
     rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
+    crane.url = "github:ipetkov/crane";
   };
 
-  outputs = { nixpkgs, flake-utils, rust-overlay, ... }:
+  outputs = { nixpkgs, flake-utils, rust-overlay, crane, ... }:
     flake-utils.lib.eachSystem flake-utils.lib.allSystems (system:
       let
         pkgs = import nixpkgs {
@@ -20,13 +21,28 @@
           targets = [ "wasm32-wasip2" ];
           extensions = [ "rust-src" ];
         };
-        rustPlatform = pkgs.makeRustPlatform { cargo = rust; rustc = rust; };
-        sausage = rustPlatform.buildRustPackage {
+        # Crane builds the dependencies once (`cargoArtifacts`, keyed only on
+        # Cargo.toml/Cargo.lock), so a change to our own crates rebuilds
+        # just them.
+        craneLib = (crane.mkLib pkgs).overrideToolchain rust;
+        common = {
           pname = "sausage";
           version = "0.1.0";
           src = ./.;
-          cargoLock.lockFile = ./Cargo.lock;
-          cargoBuildFlags = [ "-p" "sausage" ];
+        };
+        # The test packages for `checks.tests`.
+        testPackages = "-p sausage -p sausage-actor -p agent";
+        cargoArtifacts = craneLib.buildDepsOnly (common // {
+          doCheck = false;
+          # The dependencies of the host binary and of the tests.
+          buildPhaseCargoCommand = ''
+            cargoWithProfile build --locked -p sausage
+            cargoWithProfile test --locked --no-run ${testPackages}
+          '';
+        });
+        sausage = craneLib.buildPackage (common // {
+          inherit cargoArtifacts;
+          cargoExtraArgs = "--locked -p sausage";
           doCheck = false;
           nativeBuildInputs = [ pkgs.makeWrapper ];
           # The `cargo` actor runs cargo at runtime. Build scripts need a C linker.
@@ -34,7 +50,7 @@
             wrapProgram $out/bin/sausage --prefix PATH : ${pkgs.lib.makeBinPath [ rust pkgs.stdenv.cc ]}
           '';
           meta.mainProgram = "sausage";
-        };
+        });
         # `nix run .#loop -- <world>` runs the newest `sausage run <world>`
         # (without a folder: bare `sausage`, i.e. ./world, created if needed)
         # from the public repo, again and again: after a crash or a broker
@@ -62,10 +78,12 @@
         # `nix flake check` runs every test of the host, the SDK, and the
         # agent. The tests build WASM actors with cargo, offline, from the
         # vendored crates of Cargo.lock.
-        checks.tests = sausage.overrideAttrs (old: {
-          pname = "sausage-tests";
-          doCheck = true;
-          cargoTestFlags = [ "-p" "sausage" "-p" "sausage-actor" "-p" "agent" ];
+        # The binary the host loop runs (`nix run`), built from the same
+        # cached dependencies as the tests.
+        checks.package = sausage;
+        checks.tests = craneLib.cargoTest (common // {
+          inherit cargoArtifacts;
+          cargoExtraArgs = "--locked ${testPackages}";
           preCheck = ''
             # Its own lock file pins crates that the vendored set may lack.
             rm -f cli/tests/actors/toolbox/Cargo.lock