[M7] lockfile: pin top-level nixpkgs_rev + flake_utils_rev

src/codegen/flake.cpp

@@ -99,20 +99,26 @@ auto pinned_inputs_dedup(const std::vector<DepBinding>& bindings)
     return out;
 }
 
-auto emit_inputs_block(const std::vector<const DepBinding*>& pinned)
-    -> std::string {
-    // Always emit the shared toolchain `nixpkgs` and `flake-utils`
-    // inputs. Per-pinned-dep inputs land between them so the output
-    // diff stays stable across reruns.
+auto emit_inputs_block(const std::vector<const DepBinding*>& pinned,
+                        const lockfile::Lockfile& lock) -> std::string {
+    auto nixpkgs_url =
+        lock.nixpkgs_rev_pin && !lock.nixpkgs_rev_pin->empty()
+            ? std::format("github:NixOS/nixpkgs/{}", *lock.nixpkgs_rev_pin)
+            : std::string{"github:NixOS/nixpkgs/nixos-unstable"};
+    auto flake_utils_url =
+        lock.flake_utils_rev_pin && !lock.flake_utils_rev_pin->empty()
+            ? std::format("github:numtide/flake-utils/{}",
+                          *lock.flake_utils_rev_pin)
+            : std::string{"github:numtide/flake-utils"};
     std::string out =
         "  inputs = {\n"
-        "    nixpkgs.url = \"github:NixOS/nixpkgs/nixos-unstable\";\n";
+        + std::format("    nixpkgs.url = \"{}\";\n", nixpkgs_url);
     for (const auto* b : pinned) {
         out += std::format("    {}.url = \"github:NixOS/nixpkgs/{}\";\n",
                            b->sanitized, *b->rev);
     }
-    out += "    flake-utils.url = \"github:numtide/flake-utils\";\n"
-           "  };\n";
+    out += std::format("    flake-utils.url = \"{}\";\n", flake_utils_url);
+    out += "  };\n";
     return out;
 }
 
@@ -165,7 +171,7 @@ auto flake_nix(const GenerateInputs& in) -> std::string {
     out += "{\n";
     out += std::format("  description = \"{}\";\n\n", in.manifest.package.name);
 
-    out += emit_inputs_block(pinned);
+    out += emit_inputs_block(pinned, in.lock);
 
     const bool any_pkg_config =
         std::ranges::any_of(in.recipes,