[M5+] verify_link retries with libc++ stdenv override on link failure

src/codegen/flake.cpp

@@ -18,6 +18,7 @@ struct DepBinding {
     std::string sanitized;     // "nixpkgs_fmt_10_2_1" — input attr,
                                //   let-binding stem, lambda param
     std::optional<std::string> rev; // pinned commit (null → unpinned)
+    bool libcxx_override = false;   // wrap in .override { stdenv = libcxx }
 };
 
 // Replaces every char outside [a-zA-Z0-9_] with '_'. The result is safe
@@ -76,6 +77,7 @@ auto build_bindings(const GenerateInputs& in) -> std::vector<DepBinding> {
             .nixpkgs_attr = std::move(attr),
             .sanitized = sanitize_input_attr(dep.name, dep.version_spec),
             .rev = std::move(ref.rev),
+            .libcxx_override = rec.requires_libcxx_override,
         };
         out.push_back(std::move(b));
     }
@@ -138,20 +140,36 @@ auto emit_let_bindings(const std::vector<const DepBinding*>& pinned)
     return out;
 }
 
+auto base_expr(const DepBinding& b) -> std::string {
+    return b.rev && !b.rev->empty()
+               ? std::format("pkgs_{}.{}", b.sanitized, b.nixpkgs_attr)
+               : std::format("pkgs.{}", b.nixpkgs_attr);
+}
+
 auto emit_build_input_line(const DepBinding& b) -> std::string {
-    if (b.rev && !b.rev->empty()) {
-        return std::format("            pkgs_{}.{}\n", b.sanitized, b.nixpkgs_attr);
+    auto expr = base_expr(b);
+    if (b.libcxx_override) {
+        // Stdenv swap (libc++) plus a `doCheck = false` overrideAttrs
+        // to skip the package's test suite — without it, the rebuilt
+        // dep would re-run its full check phase under the new stdenv,
+        // adding minutes to hours of evaluation/build time.
+        expr = std::format(
+            "(({}.override {{ stdenv = llvmPkgs.libcxxStdenv; }})"
+            ".overrideAttrs (old: {{ doCheck = false; }}))",
+            expr);
     }
-    return std::format("            pkgs.{}\n", b.nixpkgs_attr);
+    return std::format("            {}\n", expr);
 }
 
 auto emit_build_inputs(const std::vector<DepBinding>& bindings) -> std::string {
     std::set<std::string> seen;
     std::string out;
     for (const auto& b : bindings) {
-        auto key = b.rev && !b.rev->empty()
-                       ? std::format("pkgs_{}.{}", b.sanitized, b.nixpkgs_attr)
-                       : std::format("pkgs.{}", b.nixpkgs_attr);
+        // Dedupe by base expression — two deps that resolve to the same
+        // (set, attr) but differ only in override are considered distinct
+        // because the dedup key includes the override flag.
+        auto key = std::format("{}{}", base_expr(b),
+                               b.libcxx_override ? "@libcxx" : "");
         if (seen.insert(key).second) {
             out += emit_build_input_line(b);
         }