Fix clang-tidy performance-inefficient-string-concatenation warnings

.clang-tidy

@@ -7,6 +7,7 @@ Checks: [-*,
          performance-inefficient-vector-operation,
          performance-move-const-argument,
          performance-unnecessary-copy-initialization,
+         performance-inefficient-string-concatenation,
 
          # Skipping these temporarily because they are very noisy
          -bugprone-narrowing-conversions,

src/Var.cc

@@ -467,7 +467,7 @@ static std::optional<FuncType::Prototype> func_type_check(const FuncType* decl,
                 auto msg = ad->DeprecationMessage();
 
                 if ( ! msg.empty() )
-                    msg = ": " + msg;
+                    msg = std::string{": "}.append(msg);
 
                 reporter->Deprecation(util::fmt("use of deprecated parameter '%s'%s (%s)", rval->args->FieldName(i),
                                                 msg.data(), obj_desc_short(impl).c_str()),

src/analyzer/protocol/irc/IRC.cc

@@ -357,8 +357,10 @@ void IRC_Analyzer::DeliverStream(int length, const u_char* line, bool orig) {
                 parts.erase(parts.begin(), parts.begin() + 4);
 
                 string real_name = parts[0];
-                for ( size_t i = 1; i < parts.size(); ++i )
+                for ( size_t i = 1; i < parts.size(); ++i ) {
-                    real_name = real_name + " " + parts[i];
+                    real_name += " ";
+                    real_name += parts[i];
+                }
 
                 if ( real_name[0] == ':' )
                     real_name = real_name.substr(1);

src/logging/Manager.cc

@@ -768,7 +768,7 @@ bool Manager::TraverseRecord(Stream* stream, Filter* filter, RecordType* rt, Tab
 
         // Add the ext prefix if this is an ext field.
         if ( j < num_ext_fields )
-            new_path = filter->ext_prefix + new_path;
+            new_path = string{filter->ext_prefix}.append(new_path);
 
         if ( t->InternalType() == TYPE_INTERNAL_OTHER ) {
             if ( t->Tag() == TYPE_RECORD ) {

src/script_opt/CPP/Driver.cc

@@ -341,9 +341,12 @@ void CPPCompile::RegisterCompiledBody(const string& f) {
     auto be = body_events.find(f);
     if ( be != body_events.end() )
         for ( const auto& e : be->second ) {
-            if ( events.size() > 0 )
+            if ( ! events.empty() )
                 events += ", ";
-            events = events + "\"" + e + "\"";
+
+            events += "\"";
+            events += e;
+            events += "\"";
         }
 
     events = string("{") + events + "}";

src/script_opt/CPP/Exprs.cc

@@ -28,7 +28,7 @@ string CPPCompile::GenListExpr(const Expr* e, GenType gt, bool nested) {
 
         if ( nested && e_i->Tag() == EXPR_LIST )
             // These are table or set indices.
-            gen_i = string("index_val__CPP({") + gen_i + "})";
+            gen_i = util::fmt("index_val__CPP({%s})", gen_i.c_str());
 
         gen += gen_i;
 
@@ -1196,7 +1196,7 @@ string CPPCompile::GenLambdaClone(const LambdaExpr* l, bool all_deep) {
         if ( captures && ! IsNativeType(id_t) ) {
             for ( const auto& c : *captures )
                 if ( id == c.Id() && (c.IsDeepCopy() || all_deep) )
-                    arg = string("cast_intrusive<") + TypeName(id_t) + ">(" + arg + "->Clone())";
+                    arg = util::fmt("cast_intrusive<%s>(%s->Clone())", TypeName(id_t), arg.c_str());
         }
 
         cl_args += ", " + arg;

src/script_opt/CPP/Inits.cc

@@ -299,7 +299,7 @@ void CPPCompile::GenStandaloneActivation() {
             hashes += Fmt(h);
         }
 
-        hashes = "{" + hashes + "}";
+        hashes = std::string{"{"}.append(hashes).append("}");
 
         auto f = fb.first;
         const auto& fn = f->GetName();