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diff --git a/gcc/diagnostics/selftest-source-printing.h b/gcc/diagnostics/selftest-source-printing.h
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+/* Support for selftests involving diagnostic_show_locus.
+   Copyright (C) 1999-2025 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC 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, or (at your option) any later
+version.
+
+GCC 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 GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_DIAGNOSTICS_SELFTEST_SOURCE_PRINTING_H
+#define GCC_DIAGNOSTICS_SELFTEST_SOURCE_PRINTING_H
+
+#include "selftest.h"
+#include "diagnostics/file-cache.h"
+
+/* The selftest code should entirely disappear in a production
+   configuration, hence we guard all of it with #if CHECKING_P.  */
+
+#if CHECKING_P
+
+namespace diagnostics {
+namespace selftest {
+
+/* RAII class for use in selftests involving diagnostic_show_locus.
+
+   Manages creating and cleaning up the following:
+   - writing out a temporary .c file containing CONTENT
+   - temporarily override the global "line_table" (using CASE_) and
+     push a line_map starting at the first line of the temporary file
+   - provide a file_cache.  */
+
+struct source_printing_fixture
+{
+  source_printing_fixture (const ::selftest::line_table_case &case_,
+			   const char *content);
+
+  const char *get_filename () const
+  {
+    return m_tmp_source_file.get_filename ();
+  }
+
+  const char *m_content;
+  ::selftest::temp_source_file m_tmp_source_file;
+  ::selftest::line_table_test m_ltt;
+  file_cache m_fc;
+};
+
+/* Fixture for one-liner tests exercising multibyte awareness.  For
+   simplicity we stick to using two multibyte characters in the test, U+1F602
+   == "\xf0\x9f\x98\x82", which uses 4 bytes and 2 display columns, and U+03C0
+   == "\xcf\x80", which uses 2 bytes and 1 display column.
+
+   This works with the following 1-line source file:
+
+     .0000000001111111111222222   display
+     .1234567890123456789012345   columns
+     "SS_foo = P_bar.SS_fieldP;\n"
+     .0000000111111111222222223   byte
+     .1356789012456789134567891   columns
+
+   Here SS represents the two display columns for the U+1F602 emoji and
+   P represents the one display column for the U+03C0 pi symbol.  */
+
+struct source_printing_fixture_one_liner_utf8
+  : public source_printing_fixture
+{
+  source_printing_fixture_one_liner_utf8 (const ::selftest::line_table_case &case_);
+};
+
+} // namespace diagnostics::selftest
+} // namespace diagnostics
+
+#endif /* #if CHECKING_P */
+
+#endif /* GCC_DIAGNOSTICS_SELFTEST_SOURCE_PRINTING_H */