diagnostics: add line numbers to source (PR other/84889)

diagnostics: add line numbers to source (PR other/84889)

This patch adds a left margin to the lines of source (and annotations)
printed by diagnostic_show_locus, so that e.g. rather than:

test.c: In function 'test':
test.c:12:15: error: 'struct foo' has no member named 'm_bar'; did you mean 'bar'?
   return ptr->m_bar;
               ^~~~~
               bar

we print:

test.c: In function 'test':
test.c:12:15: error: 'struct foo' has no member named 'm_bar'; did you mean 'bar'?
12 |   return ptr->m_bar;
   |               ^~~~~
   |               bar

Similarly, for a multiline case (in C++ this time), this:

bad-binary-ops.C: In function 'int test_2()':
bad-binary-ops.C:26:4: error: no match for 'operator+' (operand types are 's' and 't')
   return (some_function ()
           ~~~~~~~~~~~~~~~~
    + some_other_function ());
    ^~~~~~~~~~~~~~~~~~~~~~~~

becomes:

bad-binary-ops.C: In function 'int test_2()':
bad-binary-ops.C:26:4: error: no match for 'operator+' (operand types are 's' and 't')
25 |   return (some_function ()
   |           ~~~~~~~~~~~~~~~~
26 |    + some_other_function ());
   |    ^~~~~~~~~~~~~~~~~~~~~~~~

I believe this slightly improves the readability of the output, in that it:
- distinguishes between the user's source code vs the annotation lines
  that we're adding (the underlinings and fix-it hints here)
- shows the line numbers in another place (potentially helpful for
  multiline diagnostics, where the user can see the line numbers directly,
  rather than have to figure them out relative to the caret: in the 2nd
  example, note how the diagnostic is reported at line 26, but the first
  line printed is actually line 25)

I'm not sure that this is the precise format we want to go with [1], but
I think it's an improvement over the status quo, and we're in stage 1
of gcc 9, so there's plenty of time to shake out issues.

I've turned it on by default; it can be disabled via
-fno-diagnostics-show-line-numbers (it's also turned off in the testsuite, to
avoid breaking numerous existing test cases).

[1] Some possible variants:
  - maybe just "LL|" rather than "LL | "
  - maybe ':' rather than '|'
  - maybe we should have some leading indentation, to better split up
    the diagnostics visually via the left-hand column
  - etc

gcc/ChangeLog:
	PR other/84889
	* common.opt (fdiagnostics-show-line-numbers): New option.
	* diagnostic-show-locus.c (class layout): Add fields
	"m_show_line_numbers_p" and "m_linenum_width";
	(num_digits): New function.
	(test_num_digits): New function.
	(layout::layout): Initialize new fields.  Update m_x_offset
	logic to handle any left margin.
	(layout::print_source_line): Print line number when requested.
	(layout::start_annotation_line): New member function.
	(layout::print_annotation_line): Call it.
	(layout::print_leading_fixits): Likewise.
	(layout::print_trailing_fixits): Likewise.  Update calls to
	move_to_column for new parameter.
	(layout::get_x_bound_for_row): Add "add_left_margin" param and use
	it to potentially call start_annotation_line.
	(layout::show_ruler): Call start_annotation_line.
	(selftest::test_line_numbers_multiline_range): New selftest.
	(selftest::diagnostic_show_locus_c_tests): Call test_num_digits
	and selftest::test_line_numbers_multiline_range.
	* diagnostic.c (diagnostic_initialize): Initialize
	show_line_numbers_p.
	* diagnostic.h (struct diagnostic_context): Add field
	"show_line_numbers_p".
	* doc/invoke.texi (Diagnostic Message Formatting Options): Add
	-fno-diagnostics-show-line-numbers.
	* dwarf2out.c (gen_producer_string): Add
	OPT_fdiagnostics_show_line_numbers to the ignored options.
	* lto-wrapper.c (merge_and_complain): Likewise to the "pick
	one setting" options.
	(append_compiler_options): Likewise to the dropped options.
	(append_diag_options): Likewise to the passed-on options.
	* opts.c (common_handle_option): Handle the new option.
	* toplev.c (general_init): Set up global_dc->show_line_numbers_p.

gcc/testsuite/ChangeLog:
	PR other/84889
	* gcc.dg/plugin/diagnostic-test-show-locus-bw-line-numbers.c: New
	test.
	* gcc.dg/plugin/diagnostic-test-show-locus-color-line-numbers.c:
	New test.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add the new tests.
	* lib/prune.exp: Add -fno-diagnostics-show-line-numbers to
	TEST_ALWAYS_FLAGS.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263450 138bc75d-0d04-0410-961f-82ee72b054a4