[committed] diagnostics: fix bad interaction between line spans and line numbers

[David Malcolm <dmalcolm at redhat dot com>]

Without this patch, the "line span" markers and the line numbering
interacted badly, leading to stray copies of the line-span markers
appearing as prefixes on the first source line in a span:

missing-header-fixit-3.c: In function 'test':
missing-header-fixit-3.c:9:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
9 |   printf ("%i of %i\n", i, j);
  |   ^~~~~~
missing-header-fixit-3.c:9:3: warning: incompatible implicit declaration of built-in function 'printf'
missing-header-fixit-3.c:9:3: note: include '<stdio.h>' or provide a declaration of 'printf'
missing-header-fixit-3.c:1:1:
  |+#include <stdio.h>
missing-header-fixit-3.c:1:1:1 | /* Example of a fix-it hint that adds a #include directive,
missing-header-fixit-3.c:9:3:
missing-header-fixit-3.c:9:3:9 |   printf ("%i of %i\n", i, j);
  |   ^~~~~~

With this patch, we now correctly print:

missing-header-fixit-3.c: In function 'test':
missing-header-fixit-3.c:9:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
9 |   printf ("%i of %i\n", i, j);
  |   ^~~~~~
missing-header-fixit-3.c:9:3: warning: incompatible implicit declaration of built-in function 'printf'
missing-header-fixit-3.c:9:3: note: include '<stdio.h>' or provide a declaration of 'printf'
missing-header-fixit-3.c:1:1:
+ |+#include <stdio.h>
1 | /* Example of a fix-it hint that adds a #include directive,
missing-header-fixit-3.c:9:3:
9 |   printf ("%i of %i\n", i, j);
  |   ^~~~~~

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.

Committed to trunk as r263606.

gcc/ChangeLog:
	* diagnostic.c (default_diagnostic_start_span_fn): Call pp_string
	to emit the span, rather than setting it as the prefix.

gcc/testsuite/ChangeLog:
	* gcc.dg/missing-header-fixit-3.c: New test.
---
 gcc/diagnostic.c                              |  6 +++---
 gcc/testsuite/gcc.dg/missing-header-fixit-3.c | 27 +++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/missing-header-fixit-3.c

diff --git a/gcc/diagnostic.c b/gcc/diagnostic.c
index 59477ce..7e8bcf5 100644
--- a/gcc/diagnostic.c
+++ b/gcc/diagnostic.c
@@ -629,9 +629,9 @@ void
 default_diagnostic_start_span_fn (diagnostic_context *context,
 				  expanded_location exploc)
 {
-  pp_set_prefix (context->printer,
-		 diagnostic_get_location_text (context, exploc));
-  pp_string (context->printer, "");
+  char *text = diagnostic_get_location_text (context, exploc);
+  pp_string (context->printer, text);
+  free (text);
   pp_newline (context->printer);
 }
 
diff --git a/gcc/testsuite/gcc.dg/missing-header-fixit-3.c b/gcc/testsuite/gcc.dg/missing-header-fixit-3.c
new file mode 100644
index 0000000..8f2fb5b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/missing-header-fixit-3.c
@@ -0,0 +1,27 @@
+/* Example of a fix-it hint that adds a #include directive,
+   adding them to the top of the file, given that there is no
+   pre-existing #include.  */
+
+/* { dg-options "-fdiagnostics-show-caret -fdiagnostics-show-line-numbers" } */
+
+void test (int i, int j)
+{
+  printf ("%i of %i\n", i, j); /* { dg-warning "implicit declaration" } */
+  /* { dg-message "include '<stdio.h>' or provide a declaration of 'printf'" "" { target *-*-* } .-1 } */
+#if 0
+/* { dg-begin-multiline-output "" }
+9 |   printf ("%i of %i\n", i, j);
+  |   ^~~~~~
+   { dg-end-multiline-output "" } */
+/* { dg-regexp ".*missing-header-fixit-3.c:1:1:" } */
+/* { dg-begin-multiline-output "" }
++ |+#include <stdio.h>
+1 | /* Example of a fix-it hint that adds a #include directive,
+   { dg-end-multiline-output "" } */
+/* { dg-regexp ".*missing-header-fixit-3.c:9:3:" } */
+/* { dg-begin-multiline-output "" }
+9 |   printf ("%i of %i\n", i, j);
+  |   ^~~~~~
+   { dg-end-multiline-output "" } */
+#endif
+}
-- 
1.8.5.3