libnixt: add HackCache.h

libnixt/include/nixt/HackCache.h

@@ -0,0 +1,50 @@
+/// Access ParseCache in \p nix::EvalState
+
+#pragma once
+
+#include <nix/eval.hh>
+
+namespace nixt {
+
+#if HAVE_BOEHMGC
+using FileParseCache = std::map<
+    nix::SourcePath, nix::Expr *, std::less<nix::SourcePath>,
+    traceable_allocator<std::pair<const nix::SourcePath, nix::Expr *>>>;
+#else
+using FileParseCache = std::map<nix::SourcePath, nix::Expr *>;
+#endif
+
+namespace detail {
+
+template <typename Tag> struct R {
+  using type = typename Tag::type;
+  static type P;
+};
+
+template <typename Tag> typename R<Tag>::type R<Tag>::P;
+
+template <typename Tag, typename Tag::type p> struct RB : R<Tag> {
+  struct F {
+    F() { R<Tag>::P = p; }
+  };
+  static F FO;
+};
+
+template <typename Tag, typename Tag::type p>
+typename RB<Tag, p>::F RB<Tag, p>::FO;
+
+// Impl
+
+struct ParseCacheF {
+  using type = FileParseCache nix::EvalState::*;
+};
+
+template struct RB<ParseCacheF, &nix::EvalState::fileParseCache>;
+
+} // namespace detail
+
+inline FileParseCache &getFileParseCache(nix::EvalState &S) {
+  return S.*detail::R<detail::ParseCacheF>::P;
+}
+
+} // namespace nixt