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.

gcc/c-family/c-opts.cc

@@ -770,6 +770,19 @@ c_common_handle_option (size_t scode, const char *arg, HOST_WIDE_INT value,
       cpp_opts->traditional = 1;
       break;
 
+    case OPT_fsearch_include_path:
+      cpp_opts->main_search = CMS_user;
+      break;
+
+    case OPT_fsearch_include_path_:
+      if (!strcmp (arg, "user"))
+	cpp_opts->main_search = CMS_user;
+      else if (!strcmp (arg, "system"))
+	cpp_opts->main_search = CMS_system;
+      else
+	error ("invalid argument %qs to %<-fsearch-include-path%>", arg);
+      break;
+
     case OPT_v:
       verbose = true;
       break;

gcc/c-family/c.opt

@@ -2280,6 +2280,13 @@ frtti
 C++ ObjC++ Optimization Var(flag_rtti) Init(1)
 Generate run time type descriptor information.
 
+fsearch-include-path
+C ObjC C++ ObjC++
+Look for the main source file on the include path.
+
+fsearch-include-path=
+C++ ObjC++ Joined RejectNegative Undocumented
+
 fshort-enums
 C ObjC C++ ObjC++ LTO Optimization Var(flag_short_enums)
 Use the narrowest integer type possible for enumeration types.

gcc/cp/module.cc

@@ -21224,7 +21224,8 @@ module_preprocess_options (cpp_reader *reader)
 	}
       auto *opt = cpp_get_options (reader);
       opt->module_directives = true;
-      opt->main_search = cpp_main_search (flag_header_unit);
+      if (opt->main_search == CMS_none)
+	opt->main_search = cpp_main_search (flag_header_unit);
     }
 }
 

gcc/doc/cppopts.texi

@@ -270,6 +270,21 @@ When preprocessing, do not shorten system header paths with canonicalization.
 @item -fmax-include-depth=@var{depth}
 Set the maximum depth of the nested #include. The default is 200. 
 
+@opindex fsearch-include-path
+@item -fsearch-include-path@r{[}=@var{kind}@r{]}
+Look for input files on the #include path, not just the current
+directory.  This is particularly useful with C++20 modules, for which
+both header units and module interface units need to be compiled
+directly:
+
+@smallexample
+g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc
+@end smallexample
+
+@var{kind} defaults to @samp{user}, which looks on the @code{#include
+"@dots{}"} search path; you can also explicitly specify @samp{system}
+for the @code{#include <@dots{}>} search path.
+
 @opindex ftabstop
 @item -ftabstop=@var{width}
 Set the distance between tab stops.  This helps the preprocessor report

gcc/doc/invoke.texi

@@ -38001,6 +38001,11 @@ installed.  Specifying the language as one of these variants also
 inhibits output of the object file, as header files have no associated
 object file.
 
+Alternately, or for a module interface unit in an installed location,
+you can use @option{-fsearch-include-path} to specify that the main
+source file should be found on the include path rather than the
+current directory.
+
 Header units can be used in much the same way as precompiled headers
 (@pxref{Precompiled Headers}), but with fewer restrictions: an
 #include that is translated to a header unit import can appear at any

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)
 {

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;

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,