Specialize StrCat for constant string literals

- For single-char literals (N==2): use char append to avoid strlen scan
- For longer literals (N>2): combine string+space into single append via
  _MakeStringWithSpace helper using index_sequence
- Add #include <array> and #include <utility> required by new templates

Agent-Logs-Url: https://github.com/Cpp2Rust/cpp2rust/sessions/3ee80ab3-b415-4172-90d7-3a80ee9aa747

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

Author: Copilot

cpp2rust/converter/converter.h

@@ -7,12 +7,14 @@
 #include <clang/AST/RecursiveASTVisitor.h>
 #include <clang/Sema/Sema.h>
 
+#include <array>
 #include <functional>
 #include <optional>
 #include <stack>
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
+#include <utility>
 #include <vector>
 
 #include "converter/lex.h"
@@ -315,7 +317,40 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
   template <typename... Ts>
   inline void _StrCat(const char *func, int line, const Ts &...vals) {
     llvm::errs() << '[' << func << ':' << line << "] ";
-    ((llvm::errs() << vals << '\n', *rs_code_ += vals, *rs_code_ += ' '), ...);
+    ((llvm::errs() << vals << '\n', AppendToCode(*rs_code_, vals)), ...);
+  }
+
+  // Builds a std::array of N chars from the first N-1 chars of s followed by a
+  // space. Used by AppendToCode to combine a string literal and its trailing
+  // space into a single contiguous buffer for one append call.
+  template <std::size_t N, std::size_t... I>
+  static constexpr std::array<char, N>
+  _MakeStringWithSpace(const char (&s)[N], std::index_sequence<I...>) {
+    return {s[I]..., ' '};
+  }
+
+  // General fallback: append val then a space (two operations).
+  template <typename T>
+  static void AppendToCode(std::string &rs_code, const T &val) {
+    rs_code += val;
+    rs_code += ' ';
+  }
+
+  // Specialization for string literals (const char[N]):
+  //  - N == 2 (single char, e.g. "("): use character append rather than string
+  //    append to avoid a strlen scan.
+  //  - N > 2 (longer string): combine the string and trailing space into one
+  //    contiguous buffer and append it in a single call.
+  template <std::size_t N>
+  static void AppendToCode(std::string &rs_code, const char (&val)[N]) {
+    if constexpr (N == 2) {
+      rs_code += val[0];
+      rs_code += ' ';
+    } else {
+      const auto combined =
+          _MakeStringWithSpace(val, std::make_index_sequence<N - 1>{});
+      rs_code.append(combined.data(), N);
+    }
   }
 
   class Buffer {