c-family: add -fsearch-include-path

The C++ modules code has a -fmodule-header (or -x c++-{user,system}-header)
option to specify looking up headers to compile to header units on the usual
include paths.  I'd like to have the same functionality for full C++20
modules such as module std, which I proposed to live on the include path at
bits/std.cc.  But this behavior doesn't seem necessarily connected to
modules, so I'm proposing a general C/C++ option to specify the behavior of
looking in the include path for the input files specified on the command
line.

Other ideas for the name of the option are very welcome.

The libcpp change is to allow -fsearch-include-path{,=user} to find files in
the current working directory, like -include.  This can be handy for a quick
compile of both std.cc and a file that imports it, e.g.

g++ -std=c++20 -fmodules -fsearch-include-path bits/std.cc importer.cc

gcc/ChangeLog:

	* doc/cppopts.texi: Document -fsearch-include-path.
	* doc/invoke.texi: Mention it for modules.

gcc/c-family/ChangeLog:

	* c.opt: Add -fsearch-include-path.
	* c-opts.cc (c_common_post_options): Handle it.

gcc/cp/ChangeLog:

	* module.cc (module_preprocess_options): Don't override it.

libcpp/ChangeLog:

	* internal.h (search_path_head): Declare.
	* files.cc (search_path_head): No longer static.
	* init.cc (cpp_read_main_file): Use it.

3 files changed, 8 insertions, 11 deletions

diff --git a/libcpp/files.cc b/libcpp/files.cc
index c63f582..840dffc 100644
--- a/libcpp/files.cc
+++ b/libcpp/files.cc
@@ -189,9 +189,6 @@ static bool read_file_guts (cpp_reader *pfile, _cpp_file *file,
 			    location_t loc, const char *input_charset);
 static bool read_file (cpp_reader *pfile, _cpp_file *file,
 		       location_t loc);
-static struct cpp_dir *search_path_head (cpp_reader *, const char *fname,
-					 int angle_brackets, enum include_type,
-					 bool suppress_diagnostic = false);
 static const char *dir_name_of_file (_cpp_file *file);
 static void open_file_failed (cpp_reader *pfile, _cpp_file *file, int,
 			      location_t);
@@ -1081,7 +1078,7 @@ _cpp_mark_file_once_only (cpp_reader *pfile, _cpp_file *file)
 /* Return the directory from which searching for FNAME should start,
    considering the directive TYPE and ANGLE_BRACKETS.  If there is
    nothing left in the path, returns NULL.  */
-static struct cpp_dir *
+struct cpp_dir *
 search_path_head (cpp_reader *pfile, const char *fname, int angle_brackets,
 		  enum include_type type, bool suppress_diagnostic)
 {
diff --git a/libcpp/init.cc b/libcpp/init.cc
index 355e501..3ab120a 100644
--- a/libcpp/init.cc
+++ b/libcpp/init.cc
@@ -744,14 +744,12 @@ cpp_read_main_file (cpp_reader *pfile, const char *fname, bool injecting)
     /* Set the default target (if there is none already).  */
     deps_add_default_target (deps, fname);
 
+  auto main_search = CPP_OPTION (pfile, main_search);
+  bool angle = main_search == CMS_system;
+  cpp_dir *start_dir = (main_search < CMS_user ? &pfile->no_search_path
+			: search_path_head (pfile, fname, angle, IT_CMDLINE));
   pfile->main_file
-    = _cpp_find_file (pfile, fname,
-		      CPP_OPTION (pfile, preprocessed) ? &pfile->no_search_path
-		      : CPP_OPTION (pfile, main_search) == CMS_user
-		      ? pfile->quote_include
-		      : CPP_OPTION (pfile, main_search) == CMS_system
-		      ? pfile->bracket_include : &pfile->no_search_path,
-		      /*angle=*/0, _cpp_FFK_NORMAL, 0);
+    = _cpp_find_file (pfile, fname, start_dir, angle, _cpp_FFK_NORMAL, 0);
 
   if (_cpp_find_failed (pfile->main_file))
     return NULL;
diff --git a/libcpp/internal.h b/libcpp/internal.h
index e65198e..d91acd6 100644
--- a/libcpp/internal.h
+++ b/libcpp/internal.h
@@ -766,6 +766,8 @@ extern _cpp_file *_cpp_find_file (cpp_reader *, const char *, cpp_dir *,
 				  int angle, _cpp_find_file_kind, location_t);
 extern bool _cpp_find_failed (_cpp_file *);
 extern void _cpp_mark_file_once_only (cpp_reader *, struct _cpp_file *);
+extern cpp_dir *search_path_head (cpp_reader *, const char *, int,
+				  include_type, bool = false);
 extern const char *_cpp_find_header_unit (cpp_reader *, const char *file,
 					  bool angle_p,  location_t);
 extern int _cpp_stack_embed (cpp_reader *, const char *, bool,