diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 7a00f4ddf..a6973f590 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -3067,7 +3067,7 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path)
     return parseExprFromFile(path, staticBaseEnv);
 }
 
-Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromFile(const SourcePath & path, const std::shared_ptr<StaticEnv> & staticEnv)
 {
     auto buffer = path.resolveSymlinks().readFile();
     // readFile hopefully have left some extra space for terminators
@@ -3075,8 +3075,8 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
     return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
 }
 
-Expr *
-EvalState::parseExprFromString(std::string s_, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromString(
+    std::string s_, const SourcePath & basePath, const std::shared_ptr<StaticEnv> & staticEnv)
 {
     // NOTE this method (and parseStdin) must take care to *fully copy* their input
     // into their respective Pos::Origin until the parser stops overwriting its input
@@ -3210,7 +3210,11 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
 }
 
 Expr * EvalState::parse(
-    char * text, size_t length, Pos::Origin origin, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    const SourcePath & basePath,
+    const std::shared_ptr<StaticEnv> & staticEnv)
 {
     DocCommentMap tmpDocComments; // Only used when not origin is not a SourcePath
     DocCommentMap * docComments = &tmpDocComments;
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index 76ce62b87..0c7f9cf09 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -191,7 +191,7 @@ std::ostream & operator<<(std::ostream & os, const ValueType t);
 
 struct RegexCache;
 
-std::shared_ptr<RegexCache> makeRegexCache();
+ref<RegexCache> makeRegexCache();
 
 struct DebugTrace
 {
@@ -372,6 +372,7 @@ public:
 
     const fetchers::Settings & fetchSettings;
     const EvalSettings & settings;
+
     SymbolTable symbols;
     PosTable positions;
 
@@ -418,7 +419,7 @@ public:
 
     RootValue vImportedDrvToDerivation = nullptr;
 
-    ref<fetchers::InputCache> inputCache;
+    const ref<fetchers::InputCache> inputCache;
 
     /**
      * Debugger
@@ -471,18 +472,18 @@ private:
 
     /* Cache for calls to addToStore(); maps source paths to the store
        paths. */
-    ref<boost::concurrent_flat_map<SourcePath, StorePath>> srcToStore;
+    const 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`.
      */
-    ref<boost::concurrent_flat_map<SourcePath, SourcePath>> importResolutionCache;
+    const ref<boost::concurrent_flat_map<SourcePath, SourcePath>> importResolutionCache;
 
     /**
      * A cache from resolved paths to values.
      */
-    ref<boost::concurrent_flat_map<
+    const ref<boost::concurrent_flat_map<
         SourcePath,
         Value *,
         std::hash<SourcePath>,
@@ -504,7 +505,7 @@ private:
     /**
      * Cache used by prim_match().
      */
-    std::shared_ptr<RegexCache> regexCache;
+    const ref<RegexCache> regexCache;
 
 public:
 
@@ -592,12 +593,13 @@ public:
      * Parse a Nix expression from the specified file.
      */
     Expr * parseExprFromFile(const SourcePath & path);
-    Expr * parseExprFromFile(const SourcePath & path, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parseExprFromFile(const SourcePath & path, const std::shared_ptr<StaticEnv> & staticEnv);
 
     /**
      * Parse a Nix expression from the specified string.
      */
-    Expr * parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr *
+    parseExprFromString(std::string s, const SourcePath & basePath, const std::shared_ptr<StaticEnv> & staticEnv);
     Expr * parseExprFromString(std::string s, const SourcePath & basePath);
 
     Expr * parseStdin();
@@ -766,7 +768,7 @@ public:
 
 #if NIX_USE_BOEHMGC
     /** A GC root for the baseEnv reference. */
-    std::shared_ptr<Env *> baseEnvP;
+    const std::shared_ptr<Env *> baseEnvP;
 #endif
 
 public:
@@ -780,7 +782,7 @@ public:
     /**
      * The same, but used during parsing to resolve variables.
      */
-    std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
+    const std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
 
     /**
      * Internal primops not exposed to the user.
@@ -862,7 +864,7 @@ private:
         size_t length,
         Pos::Origin origin,
         const SourcePath & basePath,
-        std::shared_ptr<StaticEnv> & staticEnv);
+        const std::shared_ptr<StaticEnv> & staticEnv);
 
     /**
      * Current Nix call stack depth, used with `max-call-depth` setting to throw stack overflow hopefully before we run
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 5f06bf009..d87825a6d 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -4611,9 +4611,9 @@ struct RegexCache
     }
 };
 
-std::shared_ptr<RegexCache> makeRegexCache()
+ref<RegexCache> makeRegexCache()
 {
-    return std::make_shared<RegexCache>();
+    return make_ref<RegexCache>();
 }
 
 void prim_match(EvalState & state, const PosIdx pos, Value ** args, Value & v)