diagnostics: escape non-ASCII source bytes for certain diagnostics

This patch adds support to GCC's diagnostic subsystem for escaping certain
bytes and Unicode characters when quoting source code.

Specifically, this patch adds a new flag rich_location::m_escape_on_output
which is a hint from a diagnostic that non-ASCII bytes in the pertinent
lines of the user's source code should be escaped when printed.

The patch sets this for the following diagnostics:
- when complaining about stray bytes in the program (when these
are non-printable)
- when complaining about "null character(s) ignored");
- for -Wnormalized= (and generate source ranges for such warnings)

The escaping is controlled by a new option:
  -fdiagnostics-escape-format=[unicode|bytes]

For example, consider a diagnostic involing a source line containing the
string "before" followed by the Unicode character U+03C0 ("GREEK SMALL
LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF
(a stray UTF-8 trailing byte), followed by the string "after", where the
diagnostic highlights the U+03C0 character.

By default, this line will be printed verbatim to the user when
reporting a diagnostic at it, as:

 beforeπXafter
       ^

(using X for the stray byte to avoid putting invalid UTF-8 in this
commit message)

If the diagnostic sets the "escape" flag, it will be printed as:

 before<U+03C0><BF>after
       ^~~~~~~~

with -fdiagnostics-escape-format=unicode (the default), or as:

  before<CF><80><BF>after
        ^~~~~~~~

if the user supplies -fdiagnostics-escape-format=bytes.

This only affects how the source is printed; it does not affect
how column numbers that are printed (as per -fdiagnostics-column-unit=
and -fdiagnostics-column-origin=).

gcc/c-family/ChangeLog:
	* c-lex.c (c_lex_with_flags): When complaining about non-printable
	CPP_OTHER tokens, set the "escape on output" flag.

gcc/ChangeLog:
	* common.opt (fdiagnostics-escape-format=): New.
	(diagnostics_escape_format): New enum.
	(DIAGNOSTICS_ESCAPE_FORMAT_UNICODE): New enum value.
	(DIAGNOSTICS_ESCAPE_FORMAT_BYTES): Likewise.
	* diagnostic-format-json.cc (json_end_diagnostic): Add
	"escape-source" attribute.
	* diagnostic-show-locus.c
	(exploc_with_display_col::exploc_with_display_col): Replace
	"tabstop" param with a cpp_char_column_policy and add an "aspect"
	param.  Use these to compute m_display_col accordingly.
	(struct char_display_policy): New struct.
	(layout::m_policy): New field.
	(layout::m_escape_on_output): New field.
	(def_policy): New function.
	(make_range): Update for changes to exploc_with_display_col ctor.
	(default_print_decoded_ch): New.
	(width_per_escaped_byte): New.
	(escape_as_bytes_width): New.
	(escape_as_bytes_print): New.
	(escape_as_unicode_width): New.
	(escape_as_unicode_print): New.
	(make_policy): New.
	(layout::layout): Initialize new fields.  Update m_exploc ctor
	call for above change to ctor.
	(layout::maybe_add_location_range): Update for changes to
	exploc_with_display_col ctor.
	(layout::calculate_x_offset_display): Update for change to
	cpp_display_width.
	(layout::print_source_line): Pass policy
	to cpp_display_width_computation. Capture cpp_decoded_char when
	calling process_next_codepoint.  Move printing of source code to
	m_policy.m_print_cb.
	(line_label::line_label): Pass in policy rather than context.
	(layout::print_any_labels): Update for change to line_label ctor.
	(get_affected_range): Pass in policy rather than context, updating
	calls to location_compute_display_column accordingly.
	(get_printed_columns): Likewise, also for cpp_display_width.
	(correction::correction): Pass in policy rather than tabstop.
	(correction::compute_display_cols): Pass m_policy rather than
	m_tabstop to cpp_display_width.
	(correction::m_tabstop): Replace with...
	(correction::m_policy): ...this.
	(line_corrections::line_corrections): Pass in policy rather than
	context.
	(line_corrections::m_context): Replace with...
	(line_corrections::m_policy): ...this.
	(line_corrections::add_hint): Update to use m_policy rather than
	m_context.
	(line_corrections::add_hint): Likewise.
	(layout::print_trailing_fixits): Likewise.
	(selftest::test_display_widths): New.
	(selftest::test_layout_x_offset_display_utf8): Update to use
	policy rather than tabstop.
	(selftest::test_one_liner_labels_utf8): Add test of escaping
	source lines.
	(selftest::test_diagnostic_show_locus_one_liner_utf8): Update to
	use policy rather than tabstop.
	(selftest::test_overlapped_fixit_printing): Likewise.
	(selftest::test_overlapped_fixit_printing_utf8): Likewise.
	(selftest::test_overlapped_fixit_printing_2): Likewise.
	(selftest::test_tab_expansion): Likewise.
	(selftest::test_escaping_bytes_1): New.
	(selftest::test_escaping_bytes_2): New.
	(selftest::diagnostic_show_locus_c_tests): Call the new tests.
	* diagnostic.c (diagnostic_initialize): Initialize
	context->escape_format.
	(convert_column_unit): Update to use default character width policy.
	(selftest::test_diagnostic_get_location_text): Likewise.
	* diagnostic.h (enum diagnostics_escape_format): New enum.
	(diagnostic_context::escape_format): New field.
	* doc/invoke.texi (-fdiagnostics-escape-format=): New option.
	(-fdiagnostics-format=): Add "escape-source" attribute to examples
	of JSON output, and document it.
	* input.c (location_compute_display_column): Pass in "policy"
	rather than "tabstop", passing to
	cpp_byte_column_to_display_column.
	(selftest::test_cpp_utf8): Update to use cpp_char_column_policy.
	* input.h (class cpp_char_column_policy): New forward decl.
	(location_compute_display_column): Pass in "policy" rather than
	"tabstop".
	* opts.c (common_handle_option): Handle
	OPT_fdiagnostics_escape_format_.
	* selftest.c (temp_source_file::temp_source_file): New ctor
	overload taking a size_t.
	* selftest.h (temp_source_file::temp_source_file): Likewise.

gcc/testsuite/ChangeLog:
	* c-c++-common/diagnostic-format-json-1.c: Add regexp to consume
	"escape-source" attribute.
	* c-c++-common/diagnostic-format-json-2.c: Likewise.
	* c-c++-common/diagnostic-format-json-3.c: Likewise.
	* c-c++-common/diagnostic-format-json-4.c: Likewise, twice.
	* c-c++-common/diagnostic-format-json-5.c: Likewise.
	* gcc.dg/cpp/warn-normalized-4-bytes.c: New test.
	* gcc.dg/cpp/warn-normalized-4-unicode.c: New test.
	* gcc.dg/encoding-issues-bytes.c: New test.
	* gcc.dg/encoding-issues-unicode.c: New test.
	* gfortran.dg/diagnostic-format-json-1.F90: Add regexp to consume
	"escape-source" attribute.
	* gfortran.dg/diagnostic-format-json-2.F90: Likewise.
	* gfortran.dg/diagnostic-format-json-3.F90: Likewise.

libcpp/ChangeLog:
	* charset.c (convert_escape): Use encoding_rich_location when
	complaining about nonprintable unknown escape sequences.
	(cpp_display_width_computation::::cpp_display_width_computation):
	Pass in policy rather than tabstop.
	(cpp_display_width_computation::process_next_codepoint): Add "out"
	param and populate *out if non-NULL.
	(cpp_display_width_computation::advance_display_cols): Pass NULL
	to process_next_codepoint.
	(cpp_byte_column_to_display_column): Pass in policy rather than
	tabstop.  Pass NULL to process_next_codepoint.
	(cpp_display_column_to_byte_column): Pass in policy rather than
	tabstop.
	* errors.c (cpp_diagnostic_get_current_location): New function,
	splitting out the logic from...
	(cpp_diagnostic): ...here.
	(cpp_warning_at): New function.
	(cpp_pedwarning_at): New function.
	* include/cpplib.h (cpp_warning_at): New decl for rich_location.
	(cpp_pedwarning_at): Likewise.
	(struct cpp_decoded_char): New.
	(struct cpp_char_column_policy): New.
	(cpp_display_width_computation::cpp_display_width_computation):
	Replace "tabstop" param with "policy".
	(cpp_display_width_computation::process_next_codepoint): Add "out"
	param.
	(cpp_display_width_computation::m_tabstop): Replace with...
	(cpp_display_width_computation::m_policy): ...this.
	(cpp_byte_column_to_display_column): Replace "tabstop" param with
	"policy".
	(cpp_display_width): Likewise.
	(cpp_display_column_to_byte_column): Likewise.
	* include/line-map.h (rich_location::escape_on_output_p): New.
	(rich_location::set_escape_on_output): New.
	(rich_location::m_escape_on_output): New.
	* internal.h (cpp_diagnostic_get_current_location): New decl.
	(class encoding_rich_location): New.
	* lex.c (skip_whitespace): Use encoding_rich_location when
	complaining about null characters.
	(warn_about_normalization): Generate a source range when
	complaining about improperly normalized tokens, rather than just a
	point, and use encoding_rich_location so that the source code
	is escaped on printing.
	* line-map.c (rich_location::rich_location): Initialize
	m_escape_on_output.

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

2 files changed, 81 insertions, 8 deletions

diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index 56b07ac..176f8c5 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -1268,6 +1268,14 @@ extern bool cpp_warning_syshdr (cpp_reader *, enum cpp_warning_reason reason,
 				const char *msgid, ...)
   ATTRIBUTE_PRINTF_3;
 
+/* As their counterparts above, but use RICHLOC.  */
+extern bool cpp_warning_at (cpp_reader *, enum cpp_warning_reason,
+			    rich_location *richloc, const char *msgid, ...)
+  ATTRIBUTE_PRINTF_4;
+extern bool cpp_pedwarning_at (cpp_reader *, enum cpp_warning_reason,
+			       rich_location *richloc, const char *msgid, ...)
+  ATTRIBUTE_PRINTF_4;
+
 /* Output a diagnostic with "MSGID: " preceding the
    error string of errno.  No location is printed.  */
 extern bool cpp_errno (cpp_reader *, enum cpp_diagnostic_level,
@@ -1442,43 +1450,95 @@ extern const char * cpp_get_userdef_suffix
 
 /* In charset.c */
 
+/* The result of attempting to decode a run of UTF-8 bytes.  */
+
+struct cpp_decoded_char
+{
+  const char *m_start_byte;
+  const char *m_next_byte;
+
+  bool m_valid_ch;
+  cppchar_t m_ch;
+};
+
+/* Information for mapping between code points and display columns.
+
+   This is a tabstop value, along with a callback for getting the
+   widths of characters.  Normally this callback is cpp_wcwidth, but we
+   support other schemes for escaping non-ASCII unicode as a series of
+   ASCII chars when printing the user's source code in diagnostic-show-locus.c
+
+   For example, consider:
+   - the Unicode character U+03C0 "GREEK SMALL LETTER PI" (UTF-8: 0xCF 0x80)
+   - the Unicode character U+1F642 "SLIGHTLY SMILING FACE"
+     (UTF-8: 0xF0 0x9F 0x99 0x82)
+   - the byte 0xBF (a stray trailing byte of a UTF-8 character)
+   Normally U+03C0 would occupy one display column, U+1F642
+   would occupy two display columns, and the stray byte would be
+   printed verbatim as one display column.
+
+   However when escaping them as unicode code points as "<U+03C0>"
+   and "<U+1F642>" they occupy 8 and 9 display columns respectively,
+   and when escaping them as bytes as "<CF><80>" and "<F0><9F><99><82>"
+   they occupy 8 and 16 display columns respectively.  In both cases
+   the stray byte is escaped to <BF> as 4 display columns.  */
+
+struct cpp_char_column_policy
+{
+  cpp_char_column_policy (int tabstop,
+			  int (*width_cb) (cppchar_t c))
+  : m_tabstop (tabstop),
+    m_undecoded_byte_width (1),
+    m_width_cb (width_cb)
+  {}
+
+  int m_tabstop;
+  /* Width in display columns of a stray byte that isn't decodable
+     as UTF-8.  */
+  int m_undecoded_byte_width;
+  int (*m_width_cb) (cppchar_t c);
+};
+
 /* A class to manage the state while converting a UTF-8 sequence to cppchar_t
    and computing the display width one character at a time.  */
 class cpp_display_width_computation {
  public:
   cpp_display_width_computation (const char *data, int data_length,
-				 int tabstop);
+				 const cpp_char_column_policy &policy);
   const char *next_byte () const { return m_next; }
   int bytes_processed () const { return m_next - m_begin; }
   int bytes_left () const { return m_bytes_left; }
   bool done () const { return !bytes_left (); }
   int display_cols_processed () const { return m_display_cols; }
 
-  int process_next_codepoint ();
+  int process_next_codepoint (cpp_decoded_char *out);
   int advance_display_cols (int n);
 
  private:
   const char *const m_begin;
   const char *m_next;
   size_t m_bytes_left;
-  const int m_tabstop;
+  const cpp_char_column_policy &m_policy;
   int m_display_cols;
 };
 
 /* Convenience functions that are simple use cases for class
    cpp_display_width_computation.  Tab characters will be expanded to spaces
-   as determined by TABSTOP.  */
+   as determined by POLICY.m_tabstop, and non-printable-ASCII characters
+   will be escaped as per POLICY.  */
 
 int cpp_byte_column_to_display_column (const char *data, int data_length,
-				       int column, int tabstop);
+				       int column,
+				       const cpp_char_column_policy &policy);
 inline int cpp_display_width (const char *data, int data_length,
-			      int tabstop)
+			      const cpp_char_column_policy &policy)
 {
   return cpp_byte_column_to_display_column (data, data_length, data_length,
-					    tabstop);
+					    policy);
 }
 int cpp_display_column_to_byte_column (const char *data, int data_length,
-				       int display_col, int tabstop);
+				       int display_col,
+				       const cpp_char_column_policy &policy);
 int cpp_wcwidth (cppchar_t c);
 
 bool cpp_input_conversion_is_trivial (const char *input_charset);
diff --git a/libcpp/include/line-map.h b/libcpp/include/line-map.h
index 464494b..8b5e2f8 100644
--- a/libcpp/include/line-map.h
+++ b/libcpp/include/line-map.h
@@ -1787,6 +1787,18 @@ class rich_location
   const diagnostic_path *get_path () const { return m_path; }
   void set_path (const diagnostic_path *path) { m_path = path; }
 
+  /* A flag for hinting that the diagnostic involves character encoding
+     issues, and thus that it will be helpful to the user if we show some
+     representation of how the characters in the pertinent source lines
+     are encoded.
+     The default is false (i.e. do not escape).
+     When set to true, non-ASCII bytes in the pertinent source lines will
+     be escaped in a manner controlled by the user-supplied option
+     -fdiagnostics-escape-format=, so that the user can better understand
+     what's going on with the encoding in their source file.  */
+  bool escape_on_output_p () const { return m_escape_on_output; }
+  void set_escape_on_output (bool flag) { m_escape_on_output = flag; }
+
 private:
   bool reject_impossible_fixit (location_t where);
   void stop_supporting_fixits ();
@@ -1813,6 +1825,7 @@ protected:
   bool m_fixits_cannot_be_auto_applied;
 
   const diagnostic_path *m_path;
+  bool m_escape_on_output;
 };
 
 /* A struct for the result of range_label::get_text: a NUL-terminated buffer