preprocessor: Better line info for <builtin> & <command-line>

With C++ module header units it becomes important to distinguish
between macros defined in forced headers (& commandline & builtins)
from those defined in the header file being processed.  We weren't
making that easy because we treated the builtins and command-line
locations somewhat file-like, with incrementing line numbers, and
showing them as included from line 1 of the main file.  This patch does
3 things:

0) extend the idiom that 'line 0' of a file means 'the file as a whole'

1) builtins and command-line macros are shown as-if included from line zero.

2) when emitting preprocessed output we keep resetting the line number
so that re-reading that preprocessed output will get the same set of
locations for the command line etc.

For instance the new c-c++-common/cpp/line-2.c test, now emits

In file included from <command-line>:
./line-2.h:4:2: error: #error wrong
    4 | #error wrong
      |  ^~~~~
line-2.c:3:11: error: macro "bill" passed 1 arguments, but takes just 0
    3 | int bill(1);
      |           ^
In file included from <command-line>:
./line-2.h:3: note: macro "bill" defined here
    3 | #define bill() 2
      |

Before it told you about including from <command-line>:31.

the preprocessed output looks like:
...

(There's a new optimization in do_line_marker to stop each of these
line markers causing a new line map.  We can simply rewind the
location, and keep using the same line map.)

	libcpp/
	* directives.c (do_linemarker): Optimize rewinding to line zero.
	* files.c (_cpp_stack_file): Start on line zero when about to inject
	headers.
	(cpp_push_include, cpp_push_default_include): Use highest_line as
	the location.
	* include/cpplib.h (cpp_read_main_file): Add injecting parm.
	* init.c (cpp_read_main_file): Likewise, inform _cpp_stack_file.
	* internal.h (enum include_type): Add IT_MAIN_INJECT.
	gcc/c-family/
	* c-opts.c (c_common_post_options): Add 'injecting' arg to
	cpp_read_main_file.
	(c_finish_options): Add linemap_line_start calls for builtin and cmd
	maps.  Force token position to line_table's highest line.
	* c-ppoutput.c (print_line_1): Refactor, print line zero.
	(cb_define): Always increment source line.
	gcc/testsuite/
	* c-c++-common/cpp/line-2.c: New.
	* c-c++-common/cpp/line-2.h: New.
	* c-c++-common/cpp/line-3.c: New.
	* c-c++-common/cpp/line-4.c: New.
	* c-c++-common/cpp/line-4.h: New.

1 files changed, 30 insertions, 14 deletions

diff --git a/libcpp/directives.c b/libcpp/directives.c
index bbfdfcd..f597187 100644
--- a/libcpp/directives.c
+++ b/libcpp/directives.c
@@ -940,7 +940,7 @@ strtolinenum (const uchar *str, size_t len, linenum_type *nump, bool *wrapped)
 
 /* Interpret #line command.
    Note that the filename string (if any) is a true string constant
-   (escapes are interpreted), unlike in #line.  */
+   (escapes are interpreted).  */
 static void
 do_line (cpp_reader *pfile)
 {
@@ -1115,27 +1115,43 @@ do_linemarker (cpp_reader *pfile)
   line_table->seen_line_directive = true;
 }
 
-/* Arrange the file_change callback.  pfile->line has changed to
-   FILE_LINE of TO_FILE, for reason REASON.  SYSP is 1 for a system
-   header, 2 for a system header that needs to be extern "C" protected,
-   and zero otherwise.  */
+/* Arrange the file_change callback.  Changing to TO_FILE:TO_LINE for
+   REASON.  SYSP is 1 for a system header, 2 for a system header that
+   needs to be extern "C" protected, and zero otherwise.  */
 void
 _cpp_do_file_change (cpp_reader *pfile, enum lc_reason reason,
-		     const char *to_file, linenum_type file_line,
+		     const char *to_file, linenum_type to_line,
 		     unsigned int sysp)
 {
   linemap_assert (reason != LC_ENTER_MACRO);
-  const struct line_map *map = linemap_add (pfile->line_table, reason, sysp,
-					    to_file, file_line);
+
   const line_map_ordinary *ord_map = NULL;
-  if (map != NULL)
-    {
-      ord_map = linemap_check_ordinary (map);
-      linemap_line_start (pfile->line_table,
-			  ORDINARY_MAP_STARTING_LINE_NUMBER (ord_map),
-			  127);
+  if (!to_line && reason == LC_RENAME_VERBATIM)
+    {
+      /* A linemarker moving to line zero.  If we're on the second
+         line of the current map, and it also starts at zero, just
+         rewind -- we're probably reading the builtins of a
+         preprocessed source.  */
+      line_map_ordinary *last = LINEMAPS_LAST_ORDINARY_MAP (pfile->line_table);
+      if (!ORDINARY_MAP_STARTING_LINE_NUMBER (last)
+	  && SOURCE_LINE (last, pfile->line_table->highest_line) == 2)
+	{
+	  ord_map = last;
+	  pfile->line_table->highest_location
+	    = pfile->line_table->highest_line = MAP_START_LOCATION (last);
+	}
     }
 
+  if (!ord_map)
+    if (const line_map *map = linemap_add (pfile->line_table, reason, sysp,
+					   to_file, to_line))
+      {
+	ord_map = linemap_check_ordinary (map);
+	linemap_line_start (pfile->line_table,
+			    ORDINARY_MAP_STARTING_LINE_NUMBER (ord_map),
+			    127);
+      }
+
   if (pfile->cb.file_change)
     pfile->cb.file_change (pfile, ord_map);
 }