diagnostics: Add new option -fdiagnostics-plain-output

Adds the new option -fdiagnostics-plain-output, which is an alias for
several others:

    -fno-diagnostics-show-caret
    -fno-diagnostics-show-line-numbers
    -fdiagnostics-color=never
    -fdiagnostics-urls=never

The idea is that in the future, if the default behavior of diagnostics is
changed to add some fancy feature or other, then the
-fdiagnostics-plain-output option will also be changed accordingly so that
the old behavior is preserved in the presence of this option. This allows
us to use -fdiagnostics-plain-output in in the testsuite, such that the
testsuite (specifically the setting of TEST_ALWAYS_FLAGS in prune.exp)
does not need to be touched whenever diagnostics get a new look. This also
removes the need to add workarounds to compat.exp for every new option
that may be needed in a newer version of the compiler, but is not
supported in older versions.

gcc/ChangeLog:

	* common.opt: Add new option -fdiagnostics-plain-output.
	* doc/invoke.texi: Document it.
	* opts-common.c (decode_cmdline_options_to_array): Implement it.
	(decode_cmdline_option): Add missing const qualifier to argv.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp: Use the new option
	-fdiagnostics-plain-output.

gcc/testsuite/ChangeLog:

	* lib/prune.exp: Change TEST_ALWAYS_FLAGS to use -fdiagnostics-plain-output.
	* lib/c-compat.exp: Adapt to the prune.exp change.

2 files changed, 28 insertions, 6 deletions

diff --git a/gcc/testsuite/lib/c-compat.exp b/gcc/testsuite/lib/c-compat.exp
index 9493c214aea..2b673179115 100644
--- a/gcc/testsuite/lib/c-compat.exp
+++ b/gcc/testsuite/lib/c-compat.exp
@@ -36,24 +36,34 @@ load_lib target-libpath.exp
 proc compat-use-alt-compiler { } {
     global GCC_UNDER_TEST ALT_CC_UNDER_TEST
     global compat_same_alt compat_alt_caret compat_alt_color compat_no_line_no
-    global compat_alt_urls
+    global compat_alt_urls compat_alt_plain_output
     global TEST_ALWAYS_FLAGS
 
     # We don't need to do this if the alternate compiler is actually
     # the same as the compiler under test.
     if { $compat_same_alt == 0 } then {
 	set GCC_UNDER_TEST $ALT_CC_UNDER_TEST
+
+	# These flags are no longer added to TEST_ALWAYS_FLAGS by prune.exp
+	# because they are subsumed by -fdiagnostics-plain-output. Add them back
+	# for compatibility testing with older compilers that do not understand
+	# -fdiagnostics-plain-output.
+	set TEST_ALWAYS_FLAGS "-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never $TEST_ALWAYS_FLAGS"
+
 	if { $compat_alt_caret == 0 } then {
-	    regsub -- "-fno-diagnostics-show-caret" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
+	    regsub -all -- "-fno-diagnostics-show-caret" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
 	}
 	if { $compat_alt_color == 0 } then {
-	    regsub -- "-fdiagnostics-color=never" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
+	    regsub -all -- "-fdiagnostics-color=never" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
 	}
 	if { $compat_alt_urls == 0 } then {
-	    regsub -- "-fdiagnostics-urls=never" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
+	    regsub -all -- "-fdiagnostics-urls=never" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
 	}
 	if { $compat_no_line_no == 0 } then {
-	    regsub -- "-fno-diagnostics-show-line-numbers" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
+	    regsub -all -- "-fno-diagnostics-show-line-numbers" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
+	}
+	if { $compat_alt_plain_output == 0 } then {
+	    regsub -all -- "-fdiagnostics-plain-output" $TEST_ALWAYS_FLAGS "" TEST_ALWAYS_FLAGS
 	}
 	restore_gcc_exec_prefix_env_var
     }
@@ -85,12 +95,14 @@ proc compat_setup_dfp { } {
     global compat_alt_caret
     global compat_alt_color
     global compat_alt_urls
+    global compat_alt_plain_output
     global compat_no_line_no
     global TEST_ALWAYS_FLAGS compat_save_TEST_ALWAYS_FLAGS
 
     set compat_alt_caret 0
     set compat_alt_color 0
     set compat_alt_urls 0
+    set compat_alt_plain_output 0
     set compat_no_line_no 0
     set compat_save_TEST_ALWAYS_FLAGS $TEST_ALWAYS_FLAGS
 
@@ -119,6 +131,10 @@ proc compat_setup_dfp { } {
 		int dummy; } "-fno-diagnostics-show-line-numbers"] != 0 } {
 	    set compat_no_line_no 1
 	}
+	if { [check_no_compiler_messages_nocache compat_alt_has_plain_output object {
+		int dummy; } "-fdiagnostics-plain-output"] != 0 } {
+	    set compat_alt_plain_output 1
+	}
 	
 	compat-use-tst-compiler
     }
diff --git a/gcc/testsuite/lib/prune.exp b/gcc/testsuite/lib/prune.exp
index 58a739684a5..190367c44e0 100644
--- a/gcc/testsuite/lib/prune.exp
+++ b/gcc/testsuite/lib/prune.exp
@@ -18,10 +18,16 @@
 
 load_lib multiline.exp
 
+# Add options to TEST_ALWAYS_FLAGS so that diagnostics have the expected output
+# format.  Note: You should not normally need to add more options here.  If you
+# have made a change to the default diagnostic output format and are wanting to
+# undo that in the testsuite here, then please update the handling of
+# -fdiagnostics-plain-output in opts-common.c instead.
+
 if ![info exists TEST_ALWAYS_FLAGS] {
     set TEST_ALWAYS_FLAGS ""
 }
-set TEST_ALWAYS_FLAGS "-fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never  -fdiagnostics-urls=never $TEST_ALWAYS_FLAGS"
+set TEST_ALWAYS_FLAGS "-fdiagnostics-plain-output $TEST_ALWAYS_FLAGS"
 
 proc prune_gcc_output { text } {
     global srcdir