[M3] add codegen::flake_nix

src/codegen/codegen.cppm

@@ -1,7 +1,24 @@
 export module cargoxx.codegen;
 
+import std;
 import cargoxx.util;
 import cargoxx.manifest;
-import cargoxx.lockfile;
 import cargoxx.layout;
 import cargoxx.linkdb;
+import cargoxx.lockfile;
+
+export namespace cargoxx::codegen {
+
+// All inputs the generators need. Held by const reference; the caller owns
+// the underlying objects. Not copyable.
+struct GenerateInputs {
+    const manifest::Manifest& manifest;
+    const layout::DiscoveredLayout& layout;
+    const lockfile::Lockfile& lock;
+    std::vector<linkdb::Recipe> recipes; // one per manifest dep, same order
+    std::filesystem::path project_root;
+};
+
+auto flake_nix(const GenerateInputs& in) -> std::string;
+
+} // namespace cargoxx::codegen

src/codegen/flake.cpp

@@ -0,0 +1,108 @@
+module cargoxx.codegen;
+
+import std;
+import cargoxx.manifest;
+import cargoxx.linkdb;
+import cargoxx.lockfile;
+
+namespace cargoxx::codegen {
+
+namespace {
+
+// SPEC.md §7 plus a `buildInputs` slot for resolved dep attrs (TECH_SPEC §10).
+// `${...}` in the env.NIX_CFLAGS_COMPILE block is literal Nix, not a marker —
+// our markers use the @@MARKER@@ form.
+constexpr std::string_view FLAKE_TEMPLATE = R"({
+  description = "@@DESCRIPTION@@";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/@@NIXPKGS_REV@@";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        llvmPkgs = pkgs.llvmPackages;
+      in {
+        devShell = llvmPkgs.libcxxStdenv.mkDerivation {
+          name = "shell";
+          version = "1.0";
+          nativeBuildInputs = [
+            pkgs.ninja
+            pkgs.cmake
+            pkgs.clang-tools
+          ];
+          buildInputs = [
+@@DEP_LINES@@          ];
+          env.NIX_CFLAGS_COMPILE = toString [
+            "-stdlib=libc++"
+            "-Wno-unused-command-line-argument"
+            "-B${pkgs.lib.getLib pkgs.libcxx}/lib"
+            "-isystem ${pkgs.lib.getDev pkgs.libcxx}/include/c++/v1"
+          ];
+          hardeningDisable = [
+            "all"
+          ];
+        };
+      });
+}
+)";
+
+auto substitute(std::string_view tmpl, std::string_view marker, std::string_view value)
+    -> std::string {
+    std::string out;
+    out.reserve(tmpl.size());
+    std::size_t pos = 0;
+    while (pos < tmpl.size()) {
+        auto next = tmpl.find(marker, pos);
+        if (next == std::string_view::npos) {
+            out.append(tmpl.substr(pos));
+            break;
+        }
+        out.append(tmpl.substr(pos, next - pos));
+        out.append(value);
+        pos = next + marker.size();
+    }
+    return out;
+}
+
+auto stable_dedup(const std::vector<std::string>& xs) -> std::vector<std::string> {
+    std::vector<std::string> out;
+    std::set<std::string> seen;
+    for (const auto& x : xs) {
+        if (seen.insert(x).second) {
+            out.push_back(x);
+        }
+    }
+    return out;
+}
+
+} // namespace
+
+auto flake_nix(const GenerateInputs& in) -> std::string {
+    auto rev = in.lock.nixpkgs_rev().value_or("nixos-unstable");
+
+    std::vector<std::string> attrs;
+    attrs.reserve(in.recipes.size());
+    for (const auto& r : in.recipes) {
+        attrs.push_back(r.nixpkgs_attr);
+    }
+    auto deduped = stable_dedup(attrs);
+
+    std::string dep_lines;
+    for (const auto& a : deduped) {
+        dep_lines += "            pkgs.";
+        dep_lines += a;
+        dep_lines += '\n';
+    }
+
+    auto out = std::string{FLAKE_TEMPLATE};
+    out = substitute(out, "@@DESCRIPTION@@", in.manifest.package.name);
+    out = substitute(out, "@@NIXPKGS_REV@@", rev);
+    out = substitute(out, "@@DEP_LINES@@", dep_lines);
+    return out;
+}
+
+} // namespace cargoxx::codegen