copy context correctly

src/libexpr/eval.cc

@@ -957,6 +957,15 @@ void Value::mkString(std::string_view s, const NixStringContext & context)
     copyContextToValue(*this, context);
 }
 
+void Value::mkEmptyString(const Value & other)
+{
+    mkString("");
+
+    NixStringContext context;
+    copyContext(other, context);
+    copyContextToValue(*this, context);
+}
+
 void Value::mkStringMove(const char * s, const NixStringContext & context)
 {
     mkString(s);

src/libexpr/primops.cc

@@ -3724,10 +3724,10 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
 
     // Special-case on empty substring to avoid O(n) strlen
     // This allows for the use of empty substrings to efficently capture string context
-    if (len <= 0) {
+    if (len == 0) {
         state.forceValue(*args[2], pos);
         if (args[2]->type() == nString) {
-            v.mkString("", args[2]->string.context);
+            v.mkEmptyString(*args[2]);
             return;
         }
     }

src/libexpr/value.hh

@@ -293,6 +293,8 @@ public:
 
     void mkStringMove(const char * s, const NixStringContext & context);
 
+    void mkEmptyString(const Value & other);
+
     inline void mkString(const Symbol & s)
     {
         mkString(((const std::string &) s).c_str());