Since r6-4582-g8a64515099e645 (which added class rich_location), ranges
of quoted source code have been colorized using the following rules:
- the primary range used the same color of the kind of the diagnostic
i.e. "error" vs "warning" etc (defaulting to bold red and bold magenta
respectively)
- secondary ranges alternate between "range1" and "range2" (defaulting
to green and blue respectively)

This works for cases with large numbers of highlighted ranges, but is
suboptimal for common cases.

The following patch adds a pair of color names: "highlight-a" and
"highlight-b", and uses them whenever it makes sense to highlight and
contrast two different things in the source code (e.g. a type mismatch).
These are used by diagnostic-show-locus.cc for highlighting quoted
source.  In addition the patch adds colorization to fragments within the
corresponding diagnostic messages themselves, using consistent
colorization between the message and the quoted source code for the two
different things being contrasted.

For example, consider:

demo.c: In function ‘test_bad_format_string_args’:
../../src/demo.c:25:18: warning: format ‘%i’ expects argument of
  type ‘int’, but argument 2 has type ‘const char *’ [-Wformat=]
   25 |   printf("hello %i", msg);
      |                 ~^   ~~~
      |                  |   |
      |                  int const char *
      |                 %s

Previously, the types within the message in quotes would be in bold but
not colorized, and the labelled ranges of quoted source code would use
bold magenta for the "int" and non-bold green for the "const char *".

With this patch:
- the "%i" and "int" in the message and the "int" in the quoted source
  are all colored bold green
- the "const char *" in the message and in the quoted source are both
  colored bold blue
so that the consistent use of contrasting color draws the reader's eyes
to the relationships between the diagnostic message and the source.

I've tried this with gnome-terminal with many themes, including a
variety of light versus dark backgrounds, solarized versus non-solarized
themes, etc, and it was readable in all.

My initial version of the patch used the existing %r and %R facilities
within pretty-print.cc for the messages, but this turned out to be very
uncomfortable, leading to error-prone format strings such as:

  error_at (richloc,
            "invalid operands to binary %s (have %<%r%T%R%> and %<%r%T%R%>)",
            opname,
            "highlight-a", type0,
            "highlight-b", type1);

To avoid requiring monstrosities such as the above, the patch adds a new
"%e" format code to pretty-print.cc, which expects a pp_element *, where
pp_element is a new abstract base class (actually a pp_markup::element),
along with various useful subclasses.  This lets the above be written
as:

  pp_markup::element_quoted_type element_0 (type0, highlight_colors::lhs);
  pp_markup::element_quoted_type element_1 (type1, highlight_colors::rhs);
  error_at (richloc,
            "invalid operands to binary %s (have %e and %e)",
            opname, &element_0, &element_1);

which I feel is maintainable and clear to translators; the use of %e and
pp_element * captures the type-unsafe part of the variadic call, and the
subclasses allow for type-safety (so e.g. an element_quoted_type expects
a type and a highlighting color).  This approach allows for some nice
simplifications within c-format.cc.

The patch also extends -Wformat to "teach" it about the new %e and
pp_element *.  Doing so requires c-format.cc to be able to determine
if a T * is a pp_element * (i.e. if T is a subclass).  To do so I added
a new comp_types callback for comparing types, where the C++ frontend
supplies a suitable implementation (and %e will always be wrong for C).

I've manually tested this on many diagnostics with both C and C++ and it
seems a subtle but significant improvement in readability.

I've added a new option -fno-diagnostics-show-highlight-colors in case
people prefer the old behavior.

gcc/c-family/ChangeLog:
	* c-common.cc: Include "tree-pretty-print-markup.h".
	(binary_op_error): Use pp_markup::element_quoted_type and %e.
	(check_function_arguments): Add "comp_types" param and pass it to
	check_function_format.
	* c-common.h (check_function_arguments): Add "comp_types" param.
	(check_function_format): Likewise.
	* c-format.cc: Include "tree-pretty-print-markup.h".
	(local_pp_element_ptr_node): New.
	(PP_FORMAT_CHAR_TABLE): Add entry for %e.
	(struct format_check_context): Add "m_comp_types" field.
	(check_function_format): Add "comp_types" param and pass it to
	check_format_info.
	(check_format_info): Likewise, passing it to format_ctx's ctor.
	(check_format_arg): Extract m_comp_types from format_ctx and
	pass it to check_format_info_main.
	(check_format_info_main): Add "comp_types" param and pass it to
	arg_parser's ctor.
	(class argument_parser): Add "m_comp_types" field.
	(argument_parser::check_argument_type): Pass m_comp_types to
	check_format_types.
	(handle_subclass_of_pp_element_p): New.
	(check_format_types): Add "comp_types" param, and use it to
	call handle_subclass_of_pp_element_p.
	(class element_format_substring): New.
	(class element_expected_type_with_indirection): New.
	(format_type_warning): Use element_expected_type_with_indirection
	to unify the if (wanted_type_name) branches, reducing from four
	emit_warning calls to two.  Simplify these further using %e.
	Doing so also gives suitable colorization of the text within the
	diagnostics.
	(init_dynamic_diag_info): Initialize local_pp_element_ptr_node.
	(selftest::test_type_mismatch_range_labels): Add nullptr for new
	param of gcc_rich_location label overload.
	* c-format.h (T_PP_ELEMENT_PTR): New.
	* c-type-mismatch.cc: Include "diagnostic-highlight-colors.h".
	(binary_op_rich_location::binary_op_rich_location): Use
	highlight_colors::lhs and highlight_colors::rhs for the ranges.
	* c-type-mismatch.h (class binary_op_rich_location): Add comment
	about highlight_colors.

gcc/c/ChangeLog:
	* c-objc-common.cc: Include "tree-pretty-print-markup.h".
	(print_type): Add optional "highlight_color" param and use it
	to show highlight colors in "aka" text.
	(pp_markup::element_quoted_type::print_type): New.
	* c-typeck.cc: Include "tree-pretty-print-markup.h".
	(comp_parm_types): New.
	(build_function_call_vec): Pass it to check_function_arguments.
	(inform_for_arg): Use %e and highlight colors to contrast actual
	versus expected.
	(convert_for_assignment): Use highlight_colors::actual for the
	rhs_label.
	(build_binary_op): Use highlight_colors::lhs and highlight_colors::rhs
	for the ranges.

gcc/ChangeLog:
	* common.opt (fdiagnostics-show-highlight-colors): New option.
	* common.opt.urls: Regenerate.
	* coretypes.h (pp_markup::element): New forward decl.
	(pp_element): New typedef.
	* diagnostic-color.cc (gcc_color_defaults): Add "highlight-a"
	and "highlight-b".
	* diagnostic-format-json.cc (diagnostic_output_format_init_json):
	Disable highlight colors.
	* diagnostic-format-sarif.cc (diagnostic_output_format_init_sarif):
	Likewise.
	* diagnostic-highlight-colors.h: New file.
	* diagnostic-path.cc (struct event_range): Pass nullptr for
	highlight color of m_rich_loc.
	* diagnostic-show-locus.cc (colorizer::set_range): Handle ranges
	with m_highlight_color.
	(colorizer::STATE_NAMED_COLOR): New.
	(colorizer::m_richloc): New field.
	(colorizer::colorizer): Add richloc param for initializing
	m_richloc.
	(colorizer::set_named_color): New.
	(colorizer::begin_state): Add case STATE_NAMED_COLOR.
	(layout::layout): Pass richloc to m_colorizer's ctor.
	(selftest::test_one_liner_labels): Pass nullptr for new param of
	gcc_rich_location ctor for labels.
	(selftest::test_one_liner_labels_utf8): Likewise.
	* diagnostic.h (diagnostic_context::set_show_highlight_colors):
	New.
	* doc/invoke.texi: Add option -fdiagnostics-show-highlight-colors
	and highlight-a and highlight-b color caps.
	* doc/ux.texi
	(Use color consistently when highlighting mismatches): New
	subsection.
	* gcc-rich-location.cc (gcc_rich_location::add_expr): Add
	"highlight_color" param.
	(gcc_rich_location::maybe_add_expr): Likewise.
	* gcc-rich-location.h (gcc_rich_location::gcc_rich_location):
	Split out into a pair of ctors, where if a range_label is supplied
	the caller must also supply a highlight color.
	(gcc_rich_location::add_expr): Add "highlight_color" param.
	(gcc_rich_location::maybe_add_expr): Likewise.
	* gcc.cc (driver_handle_option): Handle
	OPT_fdiagnostics_show_highlight_colors.
	* lto-wrapper.cc (merge_and_complain): Likewise.
	(append_compiler_options): Likewise.
	(append_diag_options): Likewise.
	(run_gcc): Likewise.
	* opts-common.cc (decode_cmdline_options_to_array): Add comment
	about -fno-diagnostics-show-highlight-colors.
	* opts-global.cc (init_options_once): Preserve
	pp_show_highlight_colors in case the global_dc's printer is
	recreated.
	* opts.cc (common_handle_option): Handle
	OPT_fdiagnostics_show_highlight_colors.
	(gen_command_line_string): Likewise.
	* pretty-print-markup.h: New file.
	* pretty-print.cc: Include "pretty-print-markup.h" and
	"diagnostic-highlight-colors.h".
	(pretty_printer::format): Handle %e.
	(pretty_printer::pretty_printer): Handle new field
	m_show_highlight_colors.
	(pp_string_n): New.
	(pp_markup::context::begin_quote): New.
	(pp_markup::context::end_quote): New.
	(pp_markup::context::begin_color): New.
	(pp_markup::context::end_color): New.
	(highlight_colors::expected): New.
	(highlight_colors::actual): New.
	(highlight_colors::lhs): New.
	(highlight_colors::rhs): New.
	(class selftest::test_element): New.
	(selftest::test_pp_format): Add tests of %e.
	(selftest::test_urlification): Likewise.
	* pretty-print.h (pp_markup::context): New forward decl.
	(class chunk_info): Add friend class pp_markup::context.
	(class pretty_printer): Add friend pp_show_highlight_colors.
	(pretty_printer::m_show_highlight_colors): New field.
	(pp_show_highlight_colors): New inline function.
	(pp_string_n): New decl.
	* substring-locations.cc: Include "diagnostic-highlight-colors.h".
	(format_string_diagnostic_t::highlight_color_format_string): New.
	(format_string_diagnostic_t::highlight_color_param): New.
	(format_string_diagnostic_t::emit_warning_n_va): Use highlight
	colors.
	* substring-locations.h
	(format_string_diagnostic_t::highlight_color_format_string): New.
	(format_string_diagnostic_t::highlight_color_param): New.
	* toplev.cc (general_init): Initialize global_dc's
	show_highlight_colors.
	* tree-pretty-print-markup.h: New file.

gcc/cp/ChangeLog:
	* call.cc: Include "tree-pretty-print-markup.h".
	(implicit_conversion_error): Use highlight_colors::percent_h for
	the labelled range.
	(op_error_string): Split out into...
	(concat_op_error_string): ...this.
	(binop_error_string): New.
	(op_error): Use %e, binop_error_string, highlight_colors::lhs,
	and highlight_colors::rhs.
	(maybe_inform_about_fndecl_for_bogus_argument_init): Add
	"highlight_color" param; use it for the richloc.
	(convert_like_internal): Use highlight_colors::percent_h for the
	labelled_range, and highlight_colors::percent_i for the call to
	maybe_inform_about_fndecl_for_bogus_argument_init.
	(build_over_call): Pass cp_comp_parm_types for new "comp_types"
	param of check_function_arguments.
	(complain_about_bad_argument): Use highlight_colors::percent_h for
	the labelled_range, and highlight_colors::percent_i for the call
	to maybe_inform_about_fndecl_for_bogus_argument_init.
	* cp-tree.h (maybe_inform_about_fndecl_for_bogus_argument_init):
	Add optional highlight_color param.
	(cp_comp_parm_types): New decl.
	(highlight_colors::const percent_h): New decl.
	(highlight_colors::const percent_i): New decl.
	* error.cc: Include "tree-pretty-print-markup.h".
	(highlight_colors::const percent_h): New defn.
	(highlight_colors::const percent_i): New defn.
	(type_to_string): Add param "highlight_color" and use it.
	(print_nonequal_arg): Likewise.
	(print_template_differences): Add params "highlight_color_a" and
	"highlight_color_b".
	(type_to_string_with_compare): Add params "this_highlight_color"
	and "peer_highlight_color".
	(print_template_tree_comparison): Add params "highlight_color_a"
	and "highlight_color_b".
	(cxx_format_postprocessor::handle):
	Use highlight_colors::percent_h and highlight_colors::percent_i.
	(pp_markup::element_quoted_type::print_type): New.
	(range_label_for_type_mismatch::get_text): Pass nullptr for new
	params of type_to_string_with_compare.
	* typeck.cc (cp_comp_parm_types): New.
	(cp_build_function_call_vec): Pass it to check_function_arguments.
	(convert_for_assignment): Use highlight_colors::percent_h for the
	labelled_range.

gcc/testsuite/ChangeLog:
	* g++.dg/diagnostic/bad-binary-ops-highlight-colors.C: New test.
	* g++.dg/diagnostic/bad-binary-ops-no-highlight-colors.C: New test.
	* g++.dg/plugin/plugin.exp (plugin_test_list): Add
	show-template-tree-color-no-highlight-colors.C to
	show_template_tree_color_plugin.c.
	* g++.dg/plugin/show-template-tree-color-labels.C: Update expected
	output to reflect use of highlight-a and highlight-b to contrast
	mismatches.
	* g++.dg/plugin/show-template-tree-color-no-elide-type.C:
	Likewise.
	* g++.dg/plugin/show-template-tree-color-no-highlight-colors.C:
	New test.
	* g++.dg/plugin/show-template-tree-color.C: Update expected output
	to reflect use of highlight-a and highlight-b to contrast
	mismatches.
	* g++.dg/warn/Wformat-gcc_diag-1.C: New test.
	* g++.dg/warn/Wformat-gcc_diag-2.C: New test.
	* g++.dg/warn/Wformat-gcc_diag-3.C: New test.
	* gcc.dg/bad-binary-ops-highlight-colors.c: New test.
	* gcc.dg/format/colors.c: New test.
	* gcc.dg/plugin/diagnostic_plugin_show_trees.c (show_tree): Pass
	nullptr for new param of gcc_rich_location::add_expr.

libcpp/ChangeLog:
	* include/rich-location.h (location_range::m_highlight_color): New
	field.
	(rich_location::rich_location): Add optional label_highlight_color
	param.
	(rich_location::set_highlight_color): New decl.
	(rich_location::add_range): Add optional label_highlight_color
	param.
	(rich_location::set_range): Likewise.
	* line-map.cc (rich_location::rich_location): Add
	"label_highlight_color" param and pass it to add_range.
	(rich_location::set_highlight_color): New.
	(rich_location::add_range): Add "label_highlight_color" param.
	(rich_location::set_range): Add "highlight_color" param.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>