preprocessor: main file searching

This adds the capability to locate the main file on the user or system
include paths.  That's extremely useful to users building header
units.  Searching has to be requiested (plain header-unit compilation
will not search).  Also, to make include_next work as expected when
building a header unit, we add a mechanism to retrofit a non-searched
source file as one on the include path.

	libcpp/
	* include/cpplib.h (enum cpp_main_search): New.
	(struct cpp_options): Add main_search field.
	(cpp_main_loc): Declare.
	(cpp_retrofit_as_include): Declare.
	* internal.h (struct cpp_reader): Add main_loc field.
	(_cpp_in_main_source_file): Not main if main is a header.
	* init.c (cpp_read_main_file): Use main_search option to locate
	main file.  Set main_loc
	* files.c (cpp_retrofit_as_include): New.

1 files changed, 31 insertions, 0 deletions

diff --git a/libcpp/files.c b/libcpp/files.c
index ba52d2b..301b237 100644
--- a/libcpp/files.c
+++ b/libcpp/files.c
@@ -1131,6 +1131,37 @@ cpp_find_header_unit (cpp_reader *pfile, const char *name, bool angle,
   return file->path;
 }
 
+/* Retrofit the just-entered main file asif it was an include.  This
+   will permit correct include_next use, and mark it as a system
+   header if that's where it resides.  We use filesystem-appropriate
+   prefix matching of the include path to locate the main file.  */
+void
+cpp_retrofit_as_include (cpp_reader *pfile)
+{
+  /* We should be the outermost.  */
+  gcc_assert (!pfile->buffer->prev);
+
+  if (const char *name = pfile->main_file->name)
+    {
+      /* Locate name on the include dir path, using a prefix match.  */
+      size_t name_len = strlen (name);
+      for (cpp_dir *dir = pfile->quote_include; dir; dir = dir->next)
+	if (dir->len < name_len
+	    && IS_DIR_SEPARATOR (name[dir->len])
+	    && !filename_ncmp (name, dir->name, dir->len))
+	  {
+	    pfile->main_file->dir = dir;
+	    if (dir->sysp)
+	      cpp_make_system_header (pfile, 1, 0);
+	    break;
+	  }
+    }
+
+  /* Initialize controlling macro state.  */
+  pfile->mi_valid = true;
+  pfile->mi_cmacro = 0;
+}
+
 /* Could not open FILE.  The complication is dependency output.  */
 static void
 open_file_failed (cpp_reader *pfile, _cpp_file *file, int angle_brackets,