diagnostics: Honor #pragma GCC diagnostic in the preprocessor [PR53431]

As discussed on PR c++/53431, currently, "#pragma GCC diagnostic" does
not always take effect for diagnostics generated by libcpp. The reason
is that libcpp itself does not interpret this pragma and only sends it on
to the frontend, hence the pragma is only honored if the frontend
arranges for it. The C frontend does process the pragma immediately
(more or less) after seeing the token, so things work fine there. The PR
points out that it doesn't work for C++, because the C++ frontend
doesn't handle anything until it has read all the tokens from
libcpp. The underlying problem is not C++-specific, though, and for
instance, gcc -E has the same issue.

This commit fixes the PR by adding the concept of an early pragma handler that
can be registered by frontends, which gives them a chance to process
diagnostic pragmas from libcpp before it is too late for them to take
effect. The C++ and preprocess-only frontends are modified to use early
pragmas and correct the behavior.

gcc/c-family/ChangeLog:

	PR preprocessor/53920
	PR c++/53431
	* c-common.cc (c_option_is_from_cpp_diagnostics): New function.
	* c-common.h (c_option_is_from_cpp_diagnostics): Declare.
	(c_pp_stream_token): Declare.
	* c-ppoutput.cc (init_pp_output): Refactor logic about skipping
	pragmas to...
	(should_output_pragmas): ...here. New function.
	(token_streamer::stream): Support handling early pragmas.
	(do_line_change): Likewise.
	(c_pp_stream_token): New function.
	* c-pragma.cc (struct pragma_diagnostic_data): New helper class.
	(pragma_diagnostic_lex_normal): New function. Moved logic for
	interpreting GCC diagnostic pragmas here.
	(pragma_diagnostic_lex_pp): New function for parsing diagnostic pragmas
	directly from libcpp.
	(handle_pragma_diagnostic): Refactor into helper function...
	(handle_pragma_diagnostic_impl): ...here.  New function.
	(handle_pragma_diagnostic_early): New function.
	(handle_pragma_diagnostic_early_pp): New function.
	(struct pragma_ns_name): Renamed to...
	(struct pragma_pp_data): ...this.  Add new "early_handler" member.
	(c_register_pragma_1): Support early pragmas in the preprocessor.
	(c_register_pragma_with_early_handler): New function.
	(c_register_pragma): Support the new early handlers in struct
	internal_pragma_handler.
	(c_register_pragma_with_data): Likewise.
	(c_register_pragma_with_expansion): Likewise.
	(c_register_pragma_with_expansion_and_data): Likewise.
	(c_invoke_early_pragma_handler): New function.
	(c_pp_invoke_early_pragma_handler): New function.
	(init_pragma): Add early pragma support for diagnostic pragmas.
	* c-pragma.h (struct internal_pragma_handler): Add new early handler
	members.
	(c_register_pragma_with_early_handler): Declare.
	(c_invoke_early_pragma_handler): Declare.
	(c_pp_invoke_early_pragma_handler): Declare.

gcc/cp/ChangeLog:

	PR c++/53431
	* parser.cc (cp_parser_pragma_kind): Move earlier in the file.
	(cp_lexer_handle_early_pragma): New function.
	(cp_lexer_new_main): Support parsing and handling early pragmas.
	(c_parse_file): Adapt to changes in cp_lexer_new_main.

gcc/testsuite/ChangeLog:

	PR preprocessor/53920
	PR c++/53431
	* c-c++-common/pragma-diag-11.c: New test.
	* c-c++-common/pragma-diag-12.c: New test.
	* c-c++-common/pragma-diag-13.c: New test.

1 files changed, 12 insertions, 1 deletions

diff --git a/gcc/c-family/c-pragma.h b/gcc/c-family/c-pragma.h
index d5d4fe3..de806a6 100644
--- a/gcc/c-family/c-pragma.h
+++ b/gcc/c-family/c-pragma.h
@@ -219,7 +219,7 @@ union gen_pragma_handler {
 };
 /* Internally used to keep the data of the handler.  */
 struct internal_pragma_handler {
-  union gen_pragma_handler handler;
+  union gen_pragma_handler handler, early_handler;
   /* Permits to know if handler is a pragma_handler_1arg (extra_data is false)
      or a pragma_handler_2arg (extra_data is true).  */
   bool extra_data;
@@ -242,6 +242,17 @@ extern void c_register_pragma_with_expansion_and_data (const char *space,
                                                        void *data);
 extern void c_invoke_pragma_handler (unsigned int);
 
+/* Early pragma handlers run in addition to the normal ones.  They can be used
+   by frontends such as C++ that may want to process some pragmas during lexing
+   before they start processing them.  */
+extern void
+c_register_pragma_with_early_handler (const char *space, const char *name,
+				      pragma_handler_1arg handler,
+				      pragma_handler_1arg early_handler);
+extern void c_invoke_early_pragma_handler (unsigned int);
+extern void c_pp_invoke_early_pragma_handler (unsigned int);
+
+
 extern void maybe_apply_pragma_weak (tree);
 extern void maybe_apply_pending_pragma_weaks (void);
 extern tree maybe_apply_renaming_pragma (tree, tree);