Merge pull request #8398 from polykernel/perf/lazy-eval-replacements-replacestrings

primops: lazy evaluation of replaceStrings replacements

doc/manual/src/release-notes/rl-next.md

@@ -4,3 +4,4 @@
   The number of parallel downloads (also known as substitutions) has been separated from the [`--max-jobs` setting](../command-ref/conf-file.md#conf-max-jobs).
   The new setting is called [`max-substitution-jobs`](../command-ref/conf-file.md#conf-max-substitution-jobs).
   The number of parallel downloads is now set to 16 by default (previously, the default was 1 due to the coupling to build jobs).
+- The function `builtins.replaceStrings` is now lazy in the value of its second argument `to`, that is a replacee in `to` is only evaluated when its corresponding pattern in `from` is matched in the string `s`.

src/libexpr/primops.cc

@@ -3908,13 +3908,8 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     for (auto elem : args[0]->listItems())
         from.emplace_back(state.forceString(*elem, pos, "while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
-    std::vector<std::pair<std::string, NixStringContext>> to;
-    to.reserve(args[1]->listSize());
-    for (auto elem : args[1]->listItems()) {
-        NixStringContext ctx;
-        auto s = state.forceString(*elem, ctx, pos, "while evaluating one of the replacement strings passed to builtins.replaceStrings");
-        to.emplace_back(s, std::move(ctx));
-    }
+    std::unordered_map<size_t, std::string> cache;
+    auto to = args[1]->listItems();
 
     NixStringContext context;
     auto s = state.forceString(*args[2], context, pos, "while evaluating the third argument passed to builtins.replaceStrings");
@@ -3925,10 +3920,19 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
         bool found = false;
         auto i = from.begin();
         auto j = to.begin();
-        for (; i != from.end(); ++i, ++j)
+        size_t j_index = 0;
+        for (; i != from.end(); ++i, ++j, ++j_index)
             if (s.compare(p, i->size(), *i) == 0) {
                 found = true;
-                res += j->first;
+                auto v = cache.find(j_index);
+                if (v == cache.end()) {
+                    NixStringContext ctx;
+                    auto ts = state.forceString(**j, ctx, pos, "while evaluating one of the replacement strings passed to builtins.replaceStrings");
+                    v = (cache.emplace(j_index, ts)).first;
+                    for (auto& path : ctx)
+                        context.insert(path);
+                }
+                res += v->second;
                 if (i->empty()) {
                     if (p < s.size())
                         res += s[p];
@@ -3936,9 +3940,6 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
                 } else {
                     p += i->size();
                 }
-                for (auto& path : j->second)
-                    context.insert(path);
-                j->second.clear();
                 break;
             }
         if (!found) {

src/libexpr/tests/error_traces.cc

@@ -171,7 +171,7 @@ namespace nix {
                       hintfmt("value is %s while a string was expected", "an integer"),
                       hintfmt("while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
-        ASSERT_TRACE2("replaceStrings [ \"old\" ] [ true ] {}",
+        ASSERT_TRACE2("replaceStrings [ \"oo\" ] [ true ] \"foo\"",
                       TypeError,
                       hintfmt("value is %s while a string was expected", "a Boolean"),
                       hintfmt("while evaluating one of the replacement strings passed to builtins.replaceStrings"));

tests/lang/eval-okay-replacestrings.exp

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

tests/lang/eval-okay-replacestrings.nix

@@ -8,4 +8,5 @@ with builtins;
   (replaceStrings [""] ["X"] "abc")
   (replaceStrings [""] ["X"] "")
   (replaceStrings ["-"] ["_"] "a-b")
+  (replaceStrings ["oo" "XX"] ["u" (throw "unreachable")] "foobar")
 ]