cont. cleanup: remove superfluous std::string copies

src/libexpr/primops.cc

@@ -40,6 +40,13 @@ namespace nix {
  * Miscellaneous
  *************************************************************/
 
+static inline Value * mkString(EvalState & state, const std::csub_match & match)
+{
+    Value * v = state.allocValue();
+    v->mkString({match.first, match.second});
+    return v;
+}
+
 StringMap EvalState::realiseContext(const NixStringContext & context, StorePathSet * maybePathsOut, bool isIFD)
 {
     std::vector<DerivedPath::Built> drvs;
@@ -4268,7 +4275,7 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
             if (!match[i + 1].matched)
                 v2 = &state.vNull;
             else
-                (v2 = state.allocValue())->mkString(match[i + 1].str());
+                v2 = mkString(state, match[i + 1]);
         v.mkList(list);
 
     } catch (std::regex_error & e) {
@@ -4352,7 +4359,7 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
             auto match = *i;
 
             // Add a string for non-matched characters.
-            (list[idx++] = state.allocValue())->mkString(match.prefix().str());
+            list[idx++] = mkString(state, match.prefix());
 
             // Add a list for matched substrings.
             const size_t slen = match.size() - 1;
@@ -4363,14 +4370,14 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
                 if (!match[si + 1].matched)
                     v2 = &state.vNull;
                 else
-                    (v2 = state.allocValue())->mkString(match[si + 1].str());
+                    v2 = mkString(state, match[si + 1]);
             }
 
             (list[idx++] = state.allocValue())->mkList(list2);
 
             // Add a string for non-matched suffix characters.
             if (idx == 2 * len)
-                (list[idx++] = state.allocValue())->mkString(match.suffix().str());
+                list[idx++] = mkString(state, match.suffix());
         }
 
         assert(idx == 2 * len + 1);

src/libstore/daemon.cc

@@ -90,11 +90,11 @@ struct TunnelLogger : public Logger
     {
         if (ei.level > verbosity) return;
 
-        std::stringstream oss;
+        std::ostringstream oss;
         showErrorInfo(oss, ei, false);
 
         StringSink buf;
-        buf << STDERR_NEXT << oss.str();
+        buf << STDERR_NEXT << oss.view();
         enqueueMsg(buf.s);
     }
 

src/libstore/outputs-spec.cc

@@ -30,16 +30,15 @@ std::optional<OutputsSpec> OutputsSpec::parseOpt(std::string_view s)
 {
     static std::regex regex(std::string { outputSpecRegexStr });
 
-    std::smatch match;
-    std::string s2 { s }; // until some improves std::regex
-    if (!std::regex_match(s2, match, regex))
+    std::cmatch match;
+    if (!std::regex_match(s.cbegin(), s.cend(), match, regex))
         return std::nullopt;
 
     if (match[1].matched)
         return { OutputsSpec::All {} };
 
     if (match[2].matched)
-        return OutputsSpec::Names { tokenizeString<StringSet>(match[2].str(), ",") };
+        return OutputsSpec::Names { tokenizeString<StringSet>({match[2].first, match[2].second}, ",") };
 
     assert(false);
 }

src/libutil/args.cc

@@ -293,7 +293,7 @@ void RootArgs::parseCmdline(const Strings & _cmdline, bool allowShebang)
                     // We match one space after `nix` so that we preserve indentation.
                     // No space is necessary for an empty line. An empty line has basically no effect.
                     if (std::regex_match(line, match, std::regex("^#!\\s*nix(:? |$)(.*)$")))
-                        shebangContent += match[2].str() + "\n";
+                        shebangContent += std::string_view{match[2].first, match[2].second} + "\n";
                 }
                 for (const auto & word : parseShebangContent(shebangContent)) {
                     cmdline.push_back(word);

src/nix-build/nix-build.cc

@@ -36,7 +36,7 @@ extern char * * environ __attribute__((weak));
 /* Recreate the effect of the perl shellwords function, breaking up a
  * string into arguments like a shell word, including escapes
  */
-static std::vector<std::string> shellwords(const std::string & s)
+static std::vector<std::string> shellwords(std::string_view s)
 {
     std::regex whitespace("^\\s+");
     auto begin = s.cbegin();
@@ -51,7 +51,7 @@ static std::vector<std::string> shellwords(const std::string & s)
     auto it = begin;
     for (; it != s.cend(); ++it) {
         if (st == sBegin) {
-            std::smatch match;
+            std::cmatch match;
             if (regex_search(it, s.cend(), match, whitespace)) {
                 cur.append(begin, it);
                 res.push_back(cur);
@@ -173,7 +173,7 @@ static void main_nix_build(int argc, char * * argv)
                     line = chomp(line);
                     std::smatch match;
                     if (std::regex_match(line, match, std::regex("^#!\\s*nix-shell\\s+(.*)$")))
-                        for (const auto & word : shellwords(match[1].str()))
+                        for (const auto & word : shellwords({match[1].first, match[1].second}))
                             args.push_back(word);
                 }
             }

src/nix-env/user-env.cc

@@ -111,9 +111,7 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
     auto manifestFile = ({
         std::ostringstream str;
         printAmbiguous(manifest, state.symbols, str, nullptr, std::numeric_limits<int>::max());
-        // TODO with C++20 we can use str.view() instead and avoid copy.
-        std::string str2 = str.str();
-        StringSource source { str2 };
+        StringSource source { str.view() };
         state.store->addToStoreFromDump(
             source, "env-manifest.nix", FileSerialisationMethod::Flat, ContentAddressMethod::Raw::Text, HashAlgorithm::SHA256, references);
     });