diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5629865f0..93916d465 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -193,27 +193,6 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
 
 static constexpr size_t BASE_ENV_SIZE = 128;
 
-struct EvalState::SrcToStore
-{
-    boost::concurrent_flat_map<SourcePath, StorePath> inner;
-};
-
-struct EvalState::ImportResolutionCache
-{
-    boost::concurrent_flat_map<SourcePath, SourcePath> inner;
-};
-
-struct EvalState::FileEvalCache
-{
-    boost::concurrent_flat_map<
-        SourcePath,
-        Value *,
-        std::hash<SourcePath>,
-        std::equal_to<SourcePath>,
-        traceable_allocator<std::pair<const SourcePath, Value *>>>
-        inner;
-};
-
 EvalState::EvalState(
     const LookupPath & lookupPathFromArguments,
     ref<Store> store,
@@ -286,9 +265,9 @@ EvalState::EvalState(
     , debugRepl(nullptr)
     , debugStop(false)
     , trylevel(0)
-    , srcToStore(make_ref<SrcToStore>())
-    , importResolutionCache(make_ref<ImportResolutionCache>())
-    , fileEvalCache(make_ref<FileEvalCache>())
+    , srcToStore(make_ref<decltype(srcToStore)::element_type>())
+    , importResolutionCache(make_ref<decltype(importResolutionCache)::element_type>())
+    , fileEvalCache(make_ref<decltype(fileEvalCache)::element_type>())
     , regexCache(makeRegexCache())
 #if NIX_USE_BOEHMGC
     , valueAllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
@@ -1100,14 +1079,14 @@ struct ExprParseFile : Expr
 
 void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
 {
-    auto resolvedPath = getConcurrent(importResolutionCache->inner, path);
+    auto resolvedPath = getConcurrent(*importResolutionCache, path);
 
     if (!resolvedPath) {
         resolvedPath = resolveExprPath(path);
-        importResolutionCache->inner.emplace(path, *resolvedPath);
+        importResolutionCache->emplace(path, *resolvedPath);
     }
 
-    if (auto v2 = getConcurrent(fileEvalCache->inner, *resolvedPath)) {
+    if (auto v2 = getConcurrent(*fileEvalCache, *resolvedPath)) {
         forceValue(**v2, noPos);
         v = **v2;
         return;
@@ -1116,7 +1095,7 @@ void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
     Value * vExpr;
     ExprParseFile expr{*resolvedPath, mustBeTrivial};
 
-    fileEvalCache->inner.try_emplace_and_cvisit(
+    fileEvalCache->try_emplace_and_cvisit(
         *resolvedPath,
         nullptr,
         [&](auto & i) {
@@ -1133,8 +1112,8 @@ void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
 
 void EvalState::resetFileCache()
 {
-    fileEvalCache->inner.clear();
-    fileEvalCache->inner.rehash(0);
+    importResolutionCache->clear();
+    fileEvalCache->clear();
     inputCache->clear();
 }
 
@@ -2419,7 +2398,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
     if (nix::isDerivation(path.path.abs()))
         error<EvalError>("file names are not allowed to end in '%1%'", drvExtension).debugThrow();
 
-    auto dstPathCached = getConcurrent(srcToStore->inner, path);
+    auto dstPathCached = getConcurrent(*srcToStore, path);
 
     auto dstPath = dstPathCached ? *dstPathCached : [&]() {
         auto dstPath = fetchToStore(
@@ -2432,7 +2411,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
             nullptr,
             repair);
         allowPath(dstPath);
-        srcToStore->inner.try_emplace(path, dstPath);
+        srcToStore->try_emplace(path, dstPath);
         printMsg(lvlChatty, "copied source '%1%' -> '%2%'", path, store->printStorePath(dstPath));
         return dstPath;
     }();
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index 57f0f3f9d..a5e3a2bc7 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -21,6 +21,8 @@
 #include "nix/expr/config.hh"
 
 #include <boost/unordered/unordered_flat_map.hpp>
+#include <boost/unordered/concurrent_flat_map_fwd.hpp>
+
 #include <map>
 #include <optional>
 #include <functional>
@@ -411,21 +413,24 @@ private:
 
     /* Cache for calls to addToStore(); maps source paths to the store
        paths. */
-    struct SrcToStore;
-    ref<SrcToStore> srcToStore;
+    ref<boost::concurrent_flat_map<SourcePath, StorePath>> srcToStore;
 
     /**
      * A cache that maps paths to "resolved" paths for importing Nix
      * expressions, i.e. `/foo` to `/foo/default.nix`.
      */
-    struct ImportResolutionCache;
-    ref<ImportResolutionCache> importResolutionCache;
+    ref<boost::concurrent_flat_map<SourcePath, SourcePath>> importResolutionCache;
 
     /**
      * A cache from resolved paths to values.
      */
-    struct FileEvalCache;
-    ref<FileEvalCache> fileEvalCache;
+    ref<boost::concurrent_flat_map<
+        SourcePath,
+        Value *,
+        std::hash<SourcePath>,
+        std::equal_to<SourcePath>,
+        traceable_allocator<std::pair<const SourcePath, Value *>>>>
+        fileEvalCache;
 
     /**
      * Associate source positions of certain AST nodes with their preceding doc comment, if they have one.