Revert "libexpr: Fix prim_replaceStrings() to work on an empty source string"

This reverts commit 4ea9707591.

It causes an infinite loop in Nixpkgs evaluation,
e.g. "nix-instantiate -A hello" hung.

PR #1886.

src/libexpr/primops.cc

@@ -1913,26 +1913,21 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     auto s = state.forceString(*args[2], context, pos);
 
     string res;
-    // Loops one past last character to handle the case where 'from' contains an empty string.
-    for (size_t p = 0; p <= s.size(); ) {
+    for (size_t p = 0; p < s.size(); ) {
         bool found = false;
         auto i = from.begin();
         auto j = to.begin();
         for (; i != from.end(); ++i, ++j)
             if (s.compare(p, i->size(), *i) == 0) {
                 found = true;
+                p += i->size();
                 res += j->first;
-                if (i->empty()) {
-                    res += s[p++];
-                } else {
-                    p += i->size();
-                }
                 for (auto& path : j->second)
                     context.insert(path);
                 j->second.clear();
                 break;
             }
-        if (!found && p < s.size()) res += s[p++];
+        if (!found) res += s[p++];
     }
 
     mkString(v, res, context);

tests/lang/eval-okay-replacestrings.exp

@@ -1 +1 @@
-[ "faabar" "fbar" "fubar" "faboor" "fubar" "XaXbXcX" "X" ]
+[ "faabar" "fbar" "fubar" "faboor" "fubar" ]

tests/lang/eval-okay-replacestrings.nix

@@ -5,6 +5,4 @@ with builtins;
   (replaceStrings ["oo"] ["u"] "foobar")
   (replaceStrings ["oo" "a"] ["a" "oo"] "foobar")
   (replaceStrings ["oo" "oo"] ["u" "i"] "foobar")
-  (replaceStrings [""] ["X"] "abc")
-  (replaceStrings [""] ["X"] "")
 ]