[M3] add codegen::flake_nix

tests/codegen_flake.cpp

@@ -0,0 +1,138 @@
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.codegen;
+import cargoxx.manifest;
+import cargoxx.layout;
+import cargoxx.lockfile;
+import cargoxx.linkdb;
+import std;
+
+using cargoxx::codegen::flake_nix;
+using cargoxx::codegen::GenerateInputs;
+using cargoxx::layout::DiscoveredLayout;
+using cargoxx::linkdb::Recipe;
+using cargoxx::lockfile::Lockfile;
+using cargoxx::lockfile::LockfilePackage;
+using cargoxx::manifest::Edition;
+using cargoxx::manifest::Manifest;
+using cargoxx::manifest::Package;
+
+namespace {
+
+auto pkg(std::string name) -> Package {
+    return Package{
+        .name = std::move(name),
+        .version = "0.1.0",
+        .edition = Edition::Cpp23,
+        .authors = {},
+        .license = std::nullopt,
+    };
+}
+
+auto recipe(std::string attr) -> Recipe {
+    return Recipe{
+        .nixpkgs_attr = std::move(attr),
+        .find_package = "",
+        .targets = {},
+        .source = "curated",
+    };
+}
+
+auto root_pkg(std::string name, std::string version,
+              std::optional<std::string> rev = std::nullopt) -> LockfilePackage {
+    return LockfilePackage{
+        .name = std::move(name),
+        .version = std::move(version),
+        .dependencies = {},
+        .nixpkgs_attr = std::nullopt,
+        .nixpkgs_rev = std::move(rev),
+        .linkdb_source = std::nullopt,
+    };
+}
+
+} // namespace
+
+TEST_CASE("flake_nix renders the package description and rev",
+          "[codegen][flake]") {
+    Manifest m{pkg("hello"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("hello", "0.1.0", "abc123def456")}};
+    GenerateInputs in{m, layout, lock, {}, "/tmp/hello"};
+
+    auto out = flake_nix(in);
+    REQUIRE(out.find("description = \"hello\";") != std::string::npos);
+    REQUIRE(out.find("nixpkgs/abc123def456") != std::string::npos);
+}
+
+TEST_CASE("flake_nix uses 'nixos-unstable' when no rev is pinned",
+          "[codegen][flake]") {
+    Manifest m{pkg("hello"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    GenerateInputs in{m, layout, lock, {}, "/tmp/hello"};
+
+    auto out = flake_nix(in);
+    REQUIRE(out.find("nixpkgs/nixos-unstable") != std::string::npos);
+}
+
+TEST_CASE("flake_nix emits an empty buildInputs list when there are no deps",
+          "[codegen][flake]") {
+    Manifest m{pkg("hello"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    GenerateInputs in{m, layout, lock, {}, "/tmp/hello"};
+
+    auto out = flake_nix(in);
+    REQUIRE(out.find("buildInputs = [\n          ];") != std::string::npos);
+}
+
+TEST_CASE("flake_nix emits one pkgs.<attr> line per dep",
+          "[codegen][flake]") {
+    Manifest m{pkg("app"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("app", "0.1.0", "rev42")}};
+    std::vector<Recipe> recipes = {recipe("fmt_10"), recipe("spdlog")};
+    GenerateInputs in{m, layout, lock, recipes, "/tmp/app"};
+
+    auto out = flake_nix(in);
+    REQUIRE(out.find("pkgs.fmt_10") != std::string::npos);
+    REQUIRE(out.find("pkgs.spdlog") != std::string::npos);
+}
+
+TEST_CASE("flake_nix dedupes duplicate nixpkgs_attrs", "[codegen][flake]") {
+    Manifest m{pkg("app"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("app", "0.1.0", "rev42")}};
+    // boost appears twice — same nixpkgs_attr from two component-bearing entries
+    std::vector<Recipe> recipes = {recipe("boost"), recipe("boost")};
+    GenerateInputs in{m, layout, lock, recipes, "/tmp/app"};
+
+    auto out = flake_nix(in);
+    auto first = out.find("pkgs.boost");
+    REQUIRE(first != std::string::npos);
+    REQUIRE(out.find("pkgs.boost", first + 1) == std::string::npos);
+}
+
+TEST_CASE("flake_nix produces deterministic output", "[codegen][flake]") {
+    Manifest m{pkg("app"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("app", "0.1.0", "rev42")}};
+    std::vector<Recipe> recipes = {recipe("fmt_10"), recipe("spdlog"),
+                                   recipe("nlohmann_json")};
+    GenerateInputs in{m, layout, lock, recipes, "/tmp/app"};
+
+    auto a = flake_nix(in);
+    auto b = flake_nix(in);
+    REQUIRE(a == b);
+}
+
+TEST_CASE("flake_nix output ends with a newline", "[codegen][flake]") {
+    Manifest m{pkg("hello"), {}, {}};
+    DiscoveredLayout layout{};
+    Lockfile lock{1, {root_pkg("hello", "0.1.0")}};
+    GenerateInputs in{m, layout, lock, {}, "/tmp/hello"};
+
+    auto out = flake_nix(in);
+    REQUIRE_FALSE(out.empty());
+    REQUIRE(out.back() == '\n');
+}