c++: -include and header unit translation

 Within a source file, #include is translated to import if a suitable header
 unit is available, but this wasn't working with -include.  This turned out
 to be because we suppressed the translation before the beginning of the
 main file.  After removing that, I had to tweak libcpp file handling to
 accommodate the way it moves from an -include to the main file.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Modules): Mention -include.

gcc/cp/ChangeLog:

	* module.cc (maybe_translate_include): Allow before the main file.

libcpp/ChangeLog:

	* files.cc (_cpp_stack_file): LC_ENTER for -include header unit.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dashinclude-1_b.C: New test.
	* g++.dg/modules/dashinclude-1_a.H: New test.

4 files changed, 31 insertions, 4 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index ed24814..21fc851 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -19976,10 +19976,6 @@ maybe_translate_include (cpp_reader *reader, line_maps *lmaps, location_t loc,
       return nullptr;
     }
 
-  if (!spans.init_p ())
-    /* Before the main file, don't divert.  */
-    return nullptr;
-
   dump.push (NULL);
 
   dump () && dump ("Checking include translation '%s'", path);
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index e5a5d1d..ca2591c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -37764,6 +37764,23 @@ installed.  Specifying the language as one of these variants also
 inhibits output of the object file, as header files have no associated
 object file.
 
+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
+point in the source file, and multiple header units can be used
+together.  In particular, the @option{-include} strategy works: with
+the bits/stdc++.h header used for libstdc++ precompiled headers you
+can
+
+@smallexample
+g++ -fmodules-ts -x c++-system-header -c bits/stdc++.h
+g++ -fmodules-ts -include bits/stdc++.h mycode.C
+@end smallexample
+
+and any standard library #includes in mycode.C will be skipped,
+because the import brought in the whole library.  This can be a simple
+way to use modules to speed up compilation without any code changes.
+
 The @option{-fmodule-only} option disables generation of the
 associated object file for compiling a module interface.  Only the CMI
 is generated.  This option is implied when using the
diff --git a/gcc/testsuite/g++.dg/modules/dashinclude-1_a.H b/gcc/testsuite/g++.dg/modules/dashinclude-1_a.H
new file mode 100644
index 0000000..c1b40a5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/dashinclude-1_a.H
@@ -0,0 +1,5 @@
+// { dg-module-do run }
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+inline int f() { return 0; }
diff --git a/gcc/testsuite/g++.dg/modules/dashinclude-1_b.C b/gcc/testsuite/g++.dg/modules/dashinclude-1_b.C
new file mode 100644
index 0000000..6e6a334
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/dashinclude-1_b.C
@@ -0,0 +1,9 @@
+// Test that include translation works with command-line -include.
+// { dg-additional-options "-fmodules-ts -fdump-lang-module -include $srcdir/g++.dg/modules/dashinclude-1_a.H" }
+
+int main ()
+{
+  return f();
+}
+
+// { dg-final { scan-lang-dump {Translating include to import} module } }