gdb: add operator+= and operator+ overload for std::string

This commit adds operator+= and operator+ overloads for adding
gdb::unique_xmalloc_ptr<char> to a std::string.  I could only find 3
places in GDB where this was useful right now, and these all make use
of operator+=.

I've also added a self test for gdb::unique_xmalloc_ptr<char>, which
makes use of both operator+= and operator+, so they are both getting
used/tested.

There should be no user visible changes after this commit, except when
running 'maint selftest', where the new self test is visible.

gdb/Makefile.in

@@ -478,6 +478,7 @@ SELFTESTS_SRCS = \
 	unittests/tracepoint-selftests.c \
 	unittests/tui-selftests.c \
 	unittests/ui-file-selftests.c \
+	unittests/unique_xmalloc_ptr_char.c \
 	unittests/unpack-selftests.c \
 	unittests/utils-selftests.c \
 	unittests/vec-utils-selftests.c \

gdb/symfile.c

@@ -1438,7 +1438,7 @@ find_separate_debug_file (const char *dir,
   for (const gdb::unique_xmalloc_ptr<char> &debugdir : debugdir_vec)
     {
       debugfile = target_prefix ? "target:" : "";
-      debugfile += debugdir.get ();
+      debugfile += debugdir;
       debugfile += "/";
       debugfile += drive;
       debugfile += dir_notarget;
@@ -1460,7 +1460,7 @@ find_separate_debug_file (const char *dir,
 	  /* If the file is in the sysroot, try using its base path in
 	     the global debugfile directory.  */
 	  debugfile = target_prefix ? "target:" : "";
-	  debugfile += debugdir.get ();
+	  debugfile += debugdir;
 	  debugfile += "/";
 	  debugfile += base_path;
 	  debugfile += "/";
@@ -1473,7 +1473,7 @@ find_separate_debug_file (const char *dir,
 	     the sysroot's global debugfile directory.  */
 	  debugfile = target_prefix ? "target:" : "";
 	  debugfile += gdb_sysroot;
-	  debugfile += debugdir.get ();
+	  debugfile += debugdir;
 	  debugfile += "/";
 	  debugfile += base_path;
 	  debugfile += "/";

gdb/unittests/unique_xmalloc_ptr_char.c

@@ -0,0 +1,57 @@
+/* Self tests for gdb::unique_xmalloc_ptr<char>.
+
+   Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "defs.h"
+#include "gdbsupport/selftest.h"
+#include "selftest-arch.h"
+#include "gdbsupport/gdb_unique_ptr.h"
+
+namespace selftests {
+namespace unpack {
+
+static void
+unique_xmalloc_ptr_char ()
+{
+  gdb::unique_xmalloc_ptr<char> a = make_unique_xstrdup ("abc");
+  gdb::unique_xmalloc_ptr<char> b = make_unique_xstrndup ("defghi", 3);
+
+  SELF_CHECK (strcmp (a.get (), "abc") == 0);
+  SELF_CHECK (strcmp (b.get (), "def") == 0);
+
+  std::string str = "xxx";
+
+  /* Check the operator+= overload.  */
+  str += a;
+  SELF_CHECK (str == "xxxabc");
+
+  /* Check the operator+ overload.  */
+  str = str + b;
+  SELF_CHECK (str == "xxxabcdef");
+}
+
+}
+}
+
+void _initialize_unique_xmalloc_ptr_char ();
+void
+_initialize_unique_xmalloc_ptr_char ()
+{
+  selftests::register_test ("unique_xmalloc_ptr_char",
+			    selftests::unpack::unique_xmalloc_ptr_char);
+}

gdbsupport/gdb_unique_ptr.h

@@ -21,6 +21,7 @@
 #define COMMON_GDB_UNIQUE_PTR_H
 
 #include <memory>
+#include <string>
 #include "gdbsupport/gdb-xfree.h"
 
 namespace gdb
@@ -74,4 +75,22 @@ make_unique_xstrndup (const char *str, size_t n)
   return gdb::unique_xmalloc_ptr<char> (xstrndup (str, n));
 }
 
+/* An overload of operator+= fo adding gdb::unique_xmalloc_ptr<char> to a
+   std::string.  */
+
+static inline std::string &
+operator+= (std::string &lhs, const gdb::unique_xmalloc_ptr<char> &rhs)
+{
+  return lhs += rhs.get ();
+}
+
+/* An overload of operator+ for adding gdb::unique_xmalloc_ptr<char> to a
+   std::string.  */
+
+static inline std::string
+operator+ (const std::string &lhs, const gdb::unique_xmalloc_ptr<char> &rhs)
+{
+  return lhs + rhs.get ();
+}
+
 #endif /* COMMON_GDB_UNIQUE_PTR_H */