(perf): Add missing move in sp matrix caster and microopt char concats (#3823)

* Add missing move in sp matrix caster and microopt char concat

* Remove useless move

* Add a couple more std::move

* Missed one char

* Improve error_string

* Ensure no temp reallocs in errorString concat

* Remove useless move

include/pybind11/detail/type_caster_base.h

@@ -225,8 +225,8 @@ PYBIND11_NOINLINE detail::type_info *get_type_info(const std::type_index &tp,
     if (throw_if_missing) {
         std::string tname = tp.name();
         detail::clean_type_id(tname);
-        pybind11_fail("pybind11::detail::get_type_info: unable to find type info for \"" + tname
-                      + "\"");
+        pybind11_fail("pybind11::detail::get_type_info: unable to find type info for \""
+                      + std::move(tname) + '"');
     }
     return nullptr;
 }
@@ -512,9 +512,13 @@ PYBIND11_NOINLINE std::string error_string() {
             Py_INCREF(f_code);
 #    endif
             int lineno = PyFrame_GetLineNumber(frame);
-            errorString += "  " + handle(f_code->co_filename).cast<std::string>() + "("
-                           + std::to_string(lineno)
-                           + "): " + handle(f_code->co_name).cast<std::string>() + "\n";
+            errorString += "  ";
+            errorString += handle(f_code->co_filename).cast<std::string>();
+            errorString += '(';
+            errorString += std::to_string(lineno);
+            errorString += "): ";
+            errorString += handle(f_code->co_name).cast<std::string>();
+            errorString += '\n';
             Py_DECREF(f_code);
 #    if PY_VERSION_HEX >= 0x030900B1
             auto *b_frame = PyFrame_GetBack(frame);

include/pybind11/eigen.h

@@ -668,7 +668,7 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
                                      Type::Flags &(Eigen::RowMajor | Eigen::ColMajor),
                                      StorageIndex>(shape[0].cast<Index>(),
                                                    shape[1].cast<Index>(),
-                                                   nnz,
+                                                   std::move(nnz),
                                                    outerIndices.mutable_data(),
                                                    innerIndices.mutable_data(),
                                                    values.mutable_data());
@@ -686,7 +686,8 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
         array outerIndices((rowMajor ? src.rows() : src.cols()) + 1, src.outerIndexPtr());
         array innerIndices(src.nonZeros(), src.innerIndexPtr());
 
-        return matrix_type(std::make_tuple(data, innerIndices, outerIndices),
+        return matrix_type(std::make_tuple(
+                               std::move(data), std::move(innerIndices), std::move(outerIndices)),
                            std::make_pair(src.rows(), src.cols()))
             .release();
     }

include/pybind11/numpy.h

@@ -940,7 +940,7 @@ protected:
 
     void fail_dim_check(ssize_t dim, const std::string &msg) const {
         throw index_error(msg + ": " + std::to_string(dim) + " (ndim = " + std::to_string(ndim())
-                          + ")");
+                          + ')');
     }
 
     template <typename... Ix>
@@ -1144,11 +1144,11 @@ struct format_descriptor<T, detail::enable_if_t<detail::is_pod_struct<T>::value>
 
 template <size_t N>
 struct format_descriptor<char[N]> {
-    static std::string format() { return std::to_string(N) + "s"; }
+    static std::string format() { return std::to_string(N) + 's'; }
 };
 template <size_t N>
 struct format_descriptor<std::array<char, N>> {
-    static std::string format() { return std::to_string(N) + "s"; }
+    static std::string format() { return std::to_string(N) + 's'; }
 };
 
 template <typename T>

include/pybind11/pybind11.h

@@ -561,14 +561,14 @@ protected:
         for (auto *it = chain_start; it != nullptr; it = it->next) {
             if (options::show_function_signatures()) {
                 if (index > 0) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
                 if (chain) {
                     signatures += std::to_string(++index) + ". ";
                 }
                 signatures += rec->name;
                 signatures += it->signature;
-                signatures += "\n";
+                signatures += '\n';
             }
             if (it->doc && it->doc[0] != '\0' && options::show_user_defined_docstrings()) {
                 // If we're appending another docstring, and aren't printing function signatures,
@@ -577,15 +577,15 @@ protected:
                     if (first_user_def) {
                         first_user_def = false;
                     } else {
-                        signatures += "\n";
+                        signatures += '\n';
                     }
                 }
                 if (options::show_function_signatures()) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
                 signatures += it->doc;
                 if (options::show_function_signatures()) {
-                    signatures += "\n";
+                    signatures += '\n';
                 }
             }
         }
@@ -1055,7 +1055,7 @@ protected:
                     msg += it2->signature;
                 }
 
-                msg += "\n";
+                msg += '\n';
             }
             msg += "\nInvoked with: ";
             auto args_ = reinterpret_borrow<tuple>(args_in);