Add optional Nix development environment

.envrc

@@ -0,0 +1 @@
+use flake

README.md

@@ -217,3 +217,22 @@ wrapper that skips gracefully if the tool isn't installed).
 [BSD-3-Clause](LICENSE) for the port code (© 2026 Reuben Brooks). The vendored
 Shen kernel under `kernel/` is © 2010–2022 Mark Tarver and retains its own
 BSD-3-Clause license (`kernel/LICENSE.txt`).
+## Optional Nix environment
+
+Nix is optional; the normal shen-rust build and launcher commands continue to work
+with tools installed by any method. For a pinned development toolchain:
+
+```sh
+nix develop
+```
+
+The flake also exports `packages.toolchain` for composition by
+[Bifrost](https://github.com/pyrex41/bifrost):
+
+```sh
+nix shell .#toolchain
+```
+
+If direnv is installed, `direnv allow` opts this checkout into the same dev
+shell automatically. Nothing activates until that explicit authorization, and
+Nix is never required at runtime.

crates/shen-rust/src/primitives.rs

@@ -401,12 +401,11 @@ fn register_core(interp: &mut Interp) {
         ))),
     });
     interp.register_native("string->n", 1, |_, args| match args[0].as_str() {
-        Some(s) => {
-            s.chars()
-                .next()
-                .map(|c| Value::int(c as i64))
-                .ok_or_else(|| ShenError::new("string->n: empty string"))
-        }
+        Some(s) => s
+            .chars()
+            .next()
+            .map(|c| Value::int(c as i64))
+            .ok_or_else(|| ShenError::new("string->n: empty string")),
         None => Err(ShenError::new(format!(
             "string->n: not a string: {:?}",
             args[0]

crates/shen-rust/tests/boot_kernel.rs

@@ -93,7 +93,10 @@ fn shen_batteries_features_query_reports_sha_host() {
     let value = eval(&mut interp, "(shen.x.features.current)");
     let pure = std::env::var("SHEN_X_SHA256").ok().as_deref() == Some("pure");
     if pure {
-        assert!(value.is_nil(), "pure SHA mode must advertise no host feature: {value:?}");
+        assert!(
+            value.is_nil(),
+            "pure SHA mode must advertise no host feature: {value:?}"
+        );
     } else {
         let feature = value
             .head()
@@ -108,8 +111,13 @@ fn shen_batteries_features_query_reports_sha_host() {
         let sym = interp.intern("shen.x.features.current");
         let direct = interp.get_aot_direct(sym).expect("AOT feature query");
         let direct_value = direct(&mut interp, &[]).expect("feature query succeeds");
-        assert_eq!(direct_value.head().and_then(|v| v.as_sym()).map(|s| interp.resolve(s).to_string()),
-                   Some("shen.x/sha256-host".to_string()));
+        assert_eq!(
+            direct_value
+                .head()
+                .and_then(|v| v.as_sym())
+                .map(|s| interp.resolve(s).to_string()),
+            Some("shen.x/sha256-host".to_string())
+        );
     }
 }
 

flake.nix

@@ -10,16 +10,23 @@
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = nixpkgs.legacyPackages.${system};
+        tools = [
+          pkgs.rustc
+          pkgs.cargo
+          pkgs.rustfmt
+          pkgs.clippy
+          pkgs.rust-analyzer
+          pkgs.pkg-config
+        ];
       in {
+        packages.toolchain = pkgs.buildEnv {
+          name = "shen-rust-toolchain";
+          paths = tools;
+        };
+        packages.default = self.packages.${system}.toolchain;
+
         devShells.default = pkgs.mkShell {
-          buildInputs = [
-            pkgs.rustc
-            pkgs.cargo
-            pkgs.rustfmt
-            pkgs.clippy
-            pkgs.rust-analyzer
-            pkgs.pkg-config
-          ];
+          packages = tools;
 
           shellHook = ''
             echo "shen-rust dev shell"