cleanup: remove superfluous std::string copies

src/libcmd/repl.cc

@@ -654,7 +654,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 ss << "No documentation found.\n\n";
             }
 
-            auto markdown = ss.str();
+            auto markdown = ss.view();
             logger->cout(trim(renderMarkdownToTerminal(markdown)));
 
         } else

src/libexpr/eval.cc

@@ -572,16 +572,13 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
         s << docStr;
 
         s << '\0'; // for making a c string below
-        std::string ss = s.str();
 
         return Doc {
             .pos = pos,
             .name = name,
             .arity = 0, // FIXME: figure out how deep by syntax only? It's not semantically useful though...
             .args = {},
-            .doc =
+            .doc = makeImmutableString(s.view()), // NOTE: memory leak when compiled without GC
-                // FIXME: this leaks; make the field std::string?
-                strdup(ss.data()),
         };
     }
     if (isFunctor(v)) {
@@ -1805,11 +1802,9 @@ void ExprIf::eval(EvalState & state, Env & env, Value & v)
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
     if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
-        auto exprStr = ({
+        std::ostringstream out;
-            std::ostringstream out;
+        cond->show(state.symbols, out);
-            cond->show(state.symbols, out);
+        auto exprStr = out.view();
-            out.str();
-        });
 
         if (auto eq = dynamic_cast<ExprOpEq *>(cond)) {
             try {

src/libexpr/get-drvs.cc

@@ -374,11 +374,12 @@ static void getDerivations(EvalState & state, Value & vIn,
            bound to the attribute with the "lower" name should take
            precedence). */
         for (auto & i : v.attrs()->lexicographicOrder(state.symbols)) {
+            std::string_view symbol{state.symbols[i->name]};
             try {
-                debug("evaluating attribute '%1%'", state.symbols[i->name]);
+                debug("evaluating attribute '%1%'", symbol);
-                if (!std::regex_match(std::string(state.symbols[i->name]), attrRegex))
+                if (!std::regex_match(symbol.begin(), symbol.end(), attrRegex))
                     continue;
-                std::string pathPrefix2 = addToPath(pathPrefix, state.symbols[i->name]);
+                std::string pathPrefix2 = addToPath(pathPrefix, symbol);
                 if (combineChannels)
                     getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 else if (getDerivation(state, *i->value, pathPrefix2, drvs, done, ignoreAssertionFailures)) {
@@ -392,7 +393,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                     }
                 }
             } catch (Error & e) {
-                e.addTrace(state.positions[i->pos], "while evaluating the attribute '%s'", state.symbols[i->name]);
+                e.addTrace(state.positions[i->pos], "while evaluating the attribute '%s'", symbol);
                 throw;
             }
         }

src/libexpr/primops.cc

@@ -2129,7 +2129,7 @@ static void prim_toXML(EvalState & state, const PosIdx pos, Value * * args, Valu
     std::ostringstream out;
     NixStringContext context;
     printValueAsXML(state, true, false, *args[0], out, context, pos);
-    v.mkString(out.str(), context);
+    v.mkString(out.view(), context);
 }
 
 static RegisterPrimOp primop_toXML({
@@ -2237,7 +2237,7 @@ static void prim_toJSON(EvalState & state, const PosIdx pos, Value * * args, Val
     std::ostringstream out;
     NixStringContext context;
     printValueAsJSON(state, true, *args[0], pos, out, context);
-    v.mkString(out.str(), context);
+    v.mkString(out.view(), context);
 }
 
 static RegisterPrimOp primop_toJSON({

src/libexpr/primops/fromTOML.cc

@@ -66,7 +66,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
                         attrs.alloc("_type").mkString("timestamp");
                         std::ostringstream s;
                         s << t;
-                        attrs.alloc("value").mkString(s.str());
+                        attrs.alloc("value").mkString(s.view());
                         v.mkAttrs(attrs);
                     } else {
                         throw std::runtime_error("Dates and times are not supported");

src/libexpr/print.cc

@@ -460,7 +460,7 @@ private:
 
                 std::ostringstream s;
                 s << state.positions[v.payload.lambda.fun->pos];
-                output << " @ " << filterANSIEscapes(s.str());
+                output << " @ " << filterANSIEscapes(s.view());
             }
         } else if (v.isPrimOp()) {
             if (v.primOp())

src/nix-build/nix-build.cc

@@ -260,9 +260,9 @@ static void main_nix_build(int argc, char * * argv)
                 // read the shebang to understand which packages to read from. Since
                 // this is handled via nix-shell -p, we wrap our ruby script execution
                 // in ruby -e 'load' which ignores the shebangs.
-                envCommand = fmt("exec %1% %2% -e 'load(ARGV.shift)' -- %3% %4%", execArgs, interpreter, shellEscape(script), joined.str());
+                envCommand = fmt("exec %1% %2% -e 'load(ARGV.shift)' -- %3% %4%", execArgs, interpreter, shellEscape(script), joined.view());
             } else {
-                envCommand = fmt("exec %1% %2% %3% %4%", execArgs, interpreter, shellEscape(script), joined.str());
+                envCommand = fmt("exec %1% %2% %3% %4%", execArgs, interpreter, shellEscape(script), joined.view());
             }
         }
 

src/nix/config-check.cc

@@ -26,17 +26,17 @@ std::string formatProtocol(unsigned int proto)
     return "unknown";
 }
 
-bool checkPass(const std::string & msg) {
+bool checkPass(std::string_view msg) {
     notice(ANSI_GREEN "[PASS] " ANSI_NORMAL + msg);
     return true;
 }
 
-bool checkFail(const std::string & msg) {
+bool checkFail(std::string_view msg) {
     notice(ANSI_RED "[FAIL] " ANSI_NORMAL + msg);
     return false;
 }
 
-void checkInfo(const std::string & msg) {
+void checkInfo(std::string_view msg) {
     notice(ANSI_BLUE "[INFO] " ANSI_NORMAL + msg);
 }
 
@@ -91,7 +91,7 @@ struct CmdConfigCheck : StoreCommand
             ss << "Multiple versions of nix found in PATH:\n";
             for (auto & dir : dirs)
                 ss << "  " << dir << "\n";
-            return checkFail(ss.str());
+            return checkFail(ss.view());
         }
 
         return checkPass("PATH contains only one nix version.");
@@ -132,7 +132,7 @@ struct CmdConfigCheck : StoreCommand
             for (auto & dir : dirs)
                 ss << "  " << dir << "\n";
             ss << "\n";
-            return checkFail(ss.str());
+            return checkFail(ss.view());
         }
 
         return checkPass("All profiles are gcroots.");
@@ -151,7 +151,7 @@ struct CmdConfigCheck : StoreCommand
                << "sync with the daemon.\n\n"
                << "Client protocol: " << formatProtocol(clientProto) << "\n"
                << "Store protocol: " << formatProtocol(storeProto) << "\n\n";
-            return checkFail(ss.str());
+            return checkFail(ss.view());
         }
 
         return checkPass("Client protocol matches store protocol.");