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

src/linkdb/linkdb.cppm

@@ -14,6 +14,12 @@ struct Recipe {
     std::string find_package; // post-substitution
     std::vector<std::string> targets; // post-substitution
     std::string source; // 'curated' | 'manual' | etc.
+    // Set when the dep needs `.override { stdenv = llvmPkgs.libcxxStdenv; }`
+    // to link against the user's libc++-built project (e.g. nixpkgs ships
+    // it built against libstdc++ — abseil-cpp, llvm). verify_link toggles
+    // this on after a libstdc++/libc++ link failure. Codegen wraps the
+    // buildInputs entry in the override expression when true.
+    bool requires_libcxx_override = false;
 
     bool operator==(const Recipe&) const = default;
 };
@@ -37,6 +43,7 @@ struct OverlayRow {
     std::vector<std::string> targets;
     std::string source;
     std::int64_t verified_at = 0;
+    bool requires_libcxx_override = false;
 };
 
 // RAII wrapper for an open sqlite3 connection used by the overlay database.
@@ -90,6 +97,13 @@ auto overlay_delete_recipe(OverlayState& state, const std::string& package,
                            const std::string& source)
     -> cargoxx::util::Result<void>;
 
+// Toggle the libcxx-override flag for an existing row. Used by
+// `verify_link` after a libstdc++/libc++ link mismatch.
+auto overlay_set_libcxx_override(OverlayState& state, const std::string& package,
+                                 const std::string& version_range,
+                                 const std::string& source, bool value)
+    -> cargoxx::util::Result<void>;
+
 auto overlay_query(OverlayState& state, const std::string& package)
     -> cargoxx::util::Result<std::vector<OverlayRow>>;
 
@@ -125,6 +139,11 @@ class Database {
                            const std::string& version_range,
                            const std::string& source) -> util::Result<void>;
 
+    auto set_libcxx_override(const std::string& package,
+                             const std::string& version_range,
+                             const std::string& source, bool value)
+        -> util::Result<void>;
+
   private:
     Database() = default;
     std::map<std::string, std::vector<detail::CuratedRecipe>> curated_;