libcpp: Implement clang -Wheader-guard warning [PR96842]

The following patch implements the clang -Wheader-guard warning, which warns
if a valid multiple inclusion header guard's #ifndef/#if !defined directive
is immediately (no other non-line directives nor other (non-comment)
tokens in between) followed by #define directive for some different macro,
which in get_suggestion rules is close enough to the actual header guard
macro (i.e. likely misspelling), the #define is object-like with empty
definition (I've followed what clang implements) and the macro isn't defined
later on (at least not on the final #endif at the end of a header).

In this case it emits a warning, so that
  #ifndef STDIO_H
  #define STDOI_H
  ...
  #endif
or similar misspellings can be caught.

clang enables this warning by default, but I've put it into -Wall instead
as it still seems to be a style warning, nothing more severe; if a header
doesn't survive multiple inclusion because of the misspelling, users will
get different diagnostics.

2024-10-02  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/96842
libcpp/
	* include/cpplib.h (struct cpp_options): Add warn_header_guard member.
	(enum cpp_warning_reason): Add CPP_W_HEADER_GUARD enumerator.
	* internal.h (struct cpp_reader): Add mi_def_cmacro, mi_loc and
	mi_def_loc members.
	(_cpp_defined_macro_p): Constify type pointed by argument type.
	Formatting fix.
	* init.cc (cpp_create_reader): Clear
	CPP_OPTION (pfile, warn_header_guard).
	* directives.cc (struct if_stack): Add def_loc and mi_def_cmacro
	members.
	(DIRECTIVE_TABLE): Add IF_COND flag to define.
	(do_define): Set ifs->mi_def_cmacro on a define immediately following
	#ifndef directive for the guard.  Clear pfile->mi_valid.  Formatting
	fix.
	(do_endif): Copy over pfile->mi_def_cmacro and pfile->mi_def_loc
	if ifs->mi_def_cmacro is set and pfile->mi_cmacro isn't a defined
	macro.
	(push_conditional): Clear mi_def_cmacro and mi_def_loc members.
	* files.cc (_cpp_pop_file_buffer): Emit -Wheader-guard diagnostics.
gcc/
	* doc/invoke.texi (Wheader-guard): Document.
gcc/c-family/
	* c.opt (Wheader-guard): New option.
	* c.opt.urls: Regenerated.
	* c-ppoutput.cc (init_pp_output): Initialize also cb->get_suggestion.
gcc/testsuite/
	* c-c++-common/cpp/Wheader-guard-1.c: New test.
	* c-c++-common/cpp/Wheader-guard-1-1.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-2.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-3.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-4.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-5.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-6.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-7.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-8.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-9.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-10.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-11.h: New test.
	* c-c++-common/cpp/Wheader-guard-1-12.h: New test.
	* c-c++-common/cpp/Wheader-guard-2.c: New test.
	* c-c++-common/cpp/Wheader-guard-2.h: New test.
	* c-c++-common/cpp/Wheader-guard-3.c: New test.
	* c-c++-common/cpp/Wheader-guard-3.h: New test.

1 files changed, 5 insertions, 2 deletions

diff --git a/libcpp/internal.h b/libcpp/internal.h
index fa64a69..b69a037 100644
--- a/libcpp/internal.h
+++ b/libcpp/internal.h
@@ -495,9 +495,11 @@ struct cpp_reader
      been used.  */
   bool seen_once_only;
 
-  /* Multiple include optimization.  */
+  /* Multiple include optimization and -Wheader-guard warning.  */
   const cpp_hashnode *mi_cmacro;
   const cpp_hashnode *mi_ind_cmacro;
+  const cpp_hashnode *mi_def_cmacro;
+  location_t mi_loc, mi_def_loc;
   bool mi_valid;
 
   /* Lexing.  */
@@ -696,7 +698,8 @@ _cpp_in_main_source_file (cpp_reader *pfile)
 }
 
 /* True if NODE is a macro for the purposes of ifdef, defined etc.  */
-inline bool _cpp_defined_macro_p (cpp_hashnode *node)
+inline bool
+_cpp_defined_macro_p (const cpp_hashnode *node)
 {
   /* Do not treat conditional macros as being defined.  This is due to
      the powerpc port using conditional macros for 'vector', 'bool',