preprocessor: Set input_location to the most recently seen token

When preprocessing with -E and -save-temps, input_location points always to the
first character of the current file. This was previously irrelevant because
nothing was called during the token streaming process that would inspect
input_location. But since r13-1544, "#pragma GCC diagnostic" is supported in
preprocess-only mode, and that pragma relies on input_location to decide if a
given source code location is subject to a diagnostic or not. Most diagnostics
work fine anyway, because they are handled as soon as they are seen and so
everything is still seen in the expected order even though all the diagnostic
pragmas are treated as if they applied at the start of the file. One example
that doesn't work correctly is the new testcase, since here the warning is not
triggered until the end of the file and so it is necessary to track the location
properly.

Fixed by setting input_location to point to each token as it is being
streamed, similar to how C++ mode sets it.

gcc/c-family/ChangeLog:

	* c-ppoutput.cc (token_streamer::stream): Update input_location
	prior to streaming each token.

gcc/testsuite/ChangeLog:

	* c-c++-common/pragma-diag-14.c: New test.
	* c-c++-common/pragma-diag-15.c: New test.

3 files changed, 26 insertions, 0 deletions

diff --git a/gcc/c-family/c-ppoutput.cc b/gcc/c-family/c-ppoutput.cc
index cd38c96..98081cc 100644
--- a/gcc/c-family/c-ppoutput.cc
+++ b/gcc/c-family/c-ppoutput.cc
@@ -210,6 +210,10 @@ void
 token_streamer::stream (cpp_reader *pfile, const cpp_token *token,
 			location_t loc)
 {
+  /* Keep input_location up to date, since it is needed for processing early
+     pragmas such as #pragma GCC diagnostic.  */
+  input_location = loc;
+
   if (token->type == CPP_PADDING)
     {
       avoid_paste = true;
diff --git a/gcc/testsuite/c-c++-common/pragma-diag-14.c b/gcc/testsuite/c-c++-common/pragma-diag-14.c
new file mode 100644
index 0000000..618e7e1
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pragma-diag-14.c
@@ -0,0 +1,9 @@
+/* { dg-do preprocess } */
+/* { dg-additional-options "-Wunused-macros" } */
+
+/* In the past, the pragma has erroneously disabled the warning because the
+   location was not tracked properly with -E or -save-temps; check that it works
+   now.  */
+
+#define X /* { dg-warning "-:-Wunused-macros" } */
+#pragma GCC diagnostic ignored "-Wunused-macros"
diff --git a/gcc/testsuite/c-c++-common/pragma-diag-15.c b/gcc/testsuite/c-c++-common/pragma-diag-15.c
new file mode 100644
index 0000000..d8076b4
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/pragma-diag-15.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-Wunused-macros" } */
+
+/* In the past, the pragma has erroneously disabled the warning because the
+   location was not tracked properly with -E or -save-temps; check that it works
+   now.
+
+   This test currently fails for C++ but it's not because of the pragma, it's
+   because the location of the macro definition is incorrectly set.  This is a
+   separate issue, will resolve it in a later patch.  */
+
+#define X /* { dg-warning "-:-Wunused-macros" {} { xfail c++ } } */
+#pragma GCC diagnostic ignored "-Wunused-macros"