builder: add runtime tools and preserve the host Cargo configuration

flake.lock

@@ -33,6 +33,29 @@
         "type": "github"
       }
     },
+    "kache": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "rust-overlay": "rust-overlay",
+        "treefmt-nix": "treefmt-nix"
+      },
+      "locked": {
+        "lastModified": 1792081631,
+        "narHash": "sha256-thzKe50UE6CKYYfKNrVyNdgQdAqr0bLRYOZzvWwlxW8=",
+        "owner": "kunobi-ninja",
+        "repo": "kache",
+        "rev": "96052b4842239bb216f6c4812054203c21550788",
+        "type": "github"
+      },
+      "original": {
+        "owner": "kunobi-ninja",
+        "ref": "stable",
+        "repo": "kache",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1791637130,
@@ -53,11 +76,33 @@
       "inputs": {
         "crane": "crane",
         "flake-utils": "flake-utils",
+        "kache": "kache",
         "nixpkgs": "nixpkgs",
-        "rust-overlay": "rust-overlay"
+        "rust-overlay": "rust-overlay_2"
       }
     },
     "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "kache",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1791101754,
+        "narHash": "sha256-y/GF+9B0t0TZ0nQiSRMqDXpr76yT7sdDbS/3GNLhzkE=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "dbc715a4b7c0ace63b9769a032d1dd34cd89e5bd",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "rust-overlay_2": {
       "inputs": {
         "nixpkgs": [
           "nixpkgs"
@@ -91,6 +136,27 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "kache",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1784369104,
+        "narHash": "sha256-47cxbcZODibHv3rELFQ9vZly0vUNkND/atn/U7HLeb0=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "df3c0640565d04a0261253cdd89fce78ec50168a",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -7,9 +7,13 @@
     rust-overlay.url = "github:oxalica/rust-overlay";
     rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
     crane.url = "github:ipetkov/crane";
+    kache = {
+      url = "github:kunobi-ninja/kache/stable";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
-  outputs = { nixpkgs, flake-utils, rust-overlay, crane, ... }:
+  outputs = { nixpkgs, flake-utils, rust-overlay, crane, kache, ... }:
     flake-utils.lib.eachSystem flake-utils.lib.allSystems (system:
       let
         pkgs = import nixpkgs {
@@ -35,7 +39,8 @@
         moldEnv = pkgs.lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
           ${moldVar} = "-C link-arg=-fuse-ld=mold";
         };
-        runtimePath = pkgs.lib.makeBinPath ([ rust pkgs.stdenv.cc ] ++ mold);
+        builderTools = [ kache.packages.${system}.default pkgs.bubblewrap pkgs.nushell pkgs.wasm-tools ];
+        runtimePath = pkgs.lib.makeBinPath ([ rust pkgs.stdenv.cc ] ++ builderTools ++ mold);
         # A WASI clang with wasi-libc, so that build scripts (the `cc` crate)
         # can compile C for wasm32-wasip2. Its headers are WASI 0.1: C code
         # that needs sockets does not build.
@@ -204,10 +209,7 @@
         apps.loop = { type = "app"; program = "${loop}/bin/sausage-loop"; };
 
         devShells.default = pkgs.mkShell (wasiCcEnv // moldEnv // {
-          packages = [ rust pkgs.wasmtime pkgs.wasm-tools ] ++ mold;
-          shellHook = ''
-            export CARGO_HOME="$PWD/.cargo-home"
-          '';
+          packages = [ rust pkgs.wasmtime ] ++ builderTools ++ mold;
         });
       });
 }