enable two new clang-tidy checks (#3988)

* enable two new clang-tidy checks

* Use better loop var for char

.clang-tidy

@@ -10,13 +10,15 @@ cppcoreguidelines-pro-type-static-cast-downcast,
 cppcoreguidelines-slicing,
 google-explicit-constructor,
 llvm-namespace-comment,
+misc-definitions-in-headers,
 misc-misplaced-const,
 misc-non-copyable-objects,
 misc-static-assert,
 misc-throw-by-value-catch-by-reference,
 misc-uniqueptr-reset-release,
 misc-unused-parameters,
 modernize-avoid-bind,
+modernize-loop-convert,
 modernize-make-shared,
 modernize-redundant-void-arg,
 modernize-replace-auto-ptr,
@@ -63,6 +65,8 @@ readability-uniqueptr-delete-release,
 CheckOptions:
 - key:             performance-for-range-copy.WarnOnAllAutoCopies
   value:           true
+- key:             performance-inefficient-string-concatenation.StrictMode
+  value:           true
 - key:             performance-unnecessary-value-param.AllowedTypes
   value:           'exception_ptr$;'
 - key:             readability-implicit-bool-conversion.AllowPointerConditions

tests/test_constants_and_functions.cpp

@@ -31,8 +31,8 @@ py::bytes return_bytes() {
 std::string print_bytes(const py::bytes &bytes) {
     std::string ret = "bytes[";
     const auto value = static_cast<std::string>(bytes);
-    for (size_t i = 0; i < value.length(); ++i) {
-        ret += std::to_string(static_cast<int>(value[i])) + " ";
+    for (char c : value) {
+        ret += std::to_string(static_cast<int>(c)) + ' ';
     }
     ret.back() = ']';
     return ret;