nix: pure Rust check/fmt/clippy/test (#2525)

Runs the tests in a Nix build sandbox.

.github/workflows/nix_tests.yaml

@@ -38,4 +38,4 @@ jobs:
       env:
         HF_TOKEN: ${{ secrets.HF_TOKEN }}
     - name: Rust tests.
-      run: nix develop .#test --command cargo test
+      run: nix build .#checks.$(nix eval --impure --raw --expr 'builtins.currentSystem').rust -L

flake.nix

@@ -69,7 +69,29 @@
         server = pkgs.python3.pkgs.callPackage ./nix/server.nix { inherit nix-filter; };
       in
       {
+        checks = {
+          rust = with pkgs; rustPlatform.buildRustPackage {
+            name = "rust-checks";
+            src = ./.;
+            cargoLock = {
+              lockFile = ./Cargo.lock;
+            };
+            buildInputs = [ openssl.dev ];
+            nativeBuildInputs = [ clippy pkg-config protobuf python3 rustfmt ];
+            buildPhase = ''
+              cargo check
+            '';
+            checkPhase = ''
+              cargo fmt -- --check
+              cargo test -j $NIX_BUILD_CORES
+              cargo clippy
+            '';
+            installPhase = "touch $out";
+          } ;
+        };
+
         formatter = pkgs.nixfmt-rfc-style;
+
         devShells = with pkgs; rec {
           default = pure;