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27.1 Guidelines for Diagnostics

27.1.1 Talk in terms of the user’s code

Diagnostics should be worded in terms of the user’s source code, and the source language, rather than GCC’s own implementation details.

27.1.2 Diagnostics are actionable

A good diagnostic is actionable: it should assist the user in taking action.

Consider what an end user will want to do when encountering a diagnostic.

Given an error, an end user will think: “How do I fix this?”

Given a warning, an end user will think:

A good diagnostic provides pertinent information to allow the user to easily answer the above questions.

27.1.3 The user’s attention is important

A perfect compiler would issue a warning on every aspect of the user’s source code that ought to be fixed, and issue no other warnings. Naturally, this ideal is impossible to achieve.

Warnings should have a good signal-to-noise ratio: we should have few false positives (falsely issuing a warning when no warning is warranted) and few false negatives (failing to issue a warning when one is justified).

Note that a false positive can mean, in practice, a warning that the user doesn’t agree with. Ideally a diagnostic should contain enough information to allow the user to make an informed choice about whether they should care (and how to fix it), but a balance must be drawn against overloading the user with irrelevant data.

27.1.4 Precision of Wording

Provide the user with details that allow them to identify what the problem is. For example, the vaguely-worded message:

demo.c:1:1: warning: 'noinline' attribute ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      | ^~~

doesn’t tell the user why the attribute was ignored, or what kind of entity the compiler thought the attribute was being applied to (the source location for the diagnostic is also poor; see discussion of input_location). A better message would be:

demo.c:1:24: warning: attribute 'noinline' on variable 'foo' was
   ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      |     ~~~ ~~~~~~~~~~~~~~~^~~~~~~~~
demo.c:1:24: note: attribute 'noinline' is only applicable to functions

which spells out the missing information (and fixes the location information, as discussed below).

The above example uses a note to avoid a combinatorial explosion of possible messages.

27.1.5 Try the diagnostic on real-world code

It’s worth testing a new warning on many instances of real-world code, written by different people, and seeing what it complains about, and what it doesn’t complain about.

This may suggest heuristics that silence common false positives.

It may also suggest ways to improve the precision of the message.

27.1.6 Make mismatches clear

Many diagnostics relate to a mismatch between two different places in the user’s source code. Examples include:

In each case, the diagnostic should indicate both pertinent locations (so that the user can easily see the problem and how to fix it).

The standard way to do this is with a note (via inform). For example:

  auto_diagnostic_group d;
  if (warning_at (loc, OPT_Wduplicated_cond,
                  "duplicated %<if%> condition"))
    inform (EXPR_LOCATION (t), "previously used here");

which leads to:

demo.c: In function 'test':
demo.c:5:17: warning: duplicated 'if' condition [-Wduplicated-cond]
    5 |   else if (flag > 3)
      |            ~~~~~^~~
demo.c:3:12: note: previously used here
    3 |   if (flag > 3)
      |       ~~~~~^~~

The inform call should be guarded by the return value from the warning_at call so that the note isn’t emitted when the warning is suppressed.

For cases involving punctuation where the locations might be near each other, they can be conditionally consolidated via gcc_rich_location::add_location_if_nearby:

    auto_diagnostic_group d;
    gcc_rich_location richloc (primary_loc);
    bool added secondary = richloc.add_location_if_nearby (secondary_loc);
    error_at (&richloc, "main message");
    if (!added secondary)
      inform (secondary_loc, "message for secondary");

This will emit either one diagnostic with two locations:

  demo.c:42:10: error: main message
    ~   ^

or two diagnostics:

  demo.c:42:4: error: main message
  demo.c:40:2: note: message for secondary

27.1.7 Location Information

GCC’s location_t type can support both ordinary locations, and locations relating to a macro expansion.

As of GCC 6, ordinary locations changed from supporting just a point in the user’s source code to supporting three points: the caret location, plus a start and a finish:

      a = foo && bar;
          |   |    |
          |   |    finish
          |   caret

Tokens coming out of libcpp have locations of the form caret == start, such as for foo here:

      a = foo && bar;
          | |
          | finish
          caret == start

Compound expressions should be reported using the location of the expression as a whole, rather than just of one token within it.

For example, in -Wformat, rather than underlining just the first token of a bad argument:

   printf("hello %i %s", (long)0, "world");
                 ~^      ~

the whole of the expression should be underlined, so that the user can easily identify what is being referred to:

   printf("hello %i %s", (long)0, "world");
                 ~^      ~~~~~~~

Avoid using the input_location global, and the diagnostic functions that implicitly use it—use error_at and warning_at rather than error and warning, and provide the most appropriate location_t value available at that phase of the compilation. It’s possible to supply secondary location_t values via rich_location.

For example, in the example of imprecise wording above, generating the diagnostic using warning:

  // BAD: implicitly uses input_location
  warning (OPT_Wattributes, "%qE attribute ignored", name);

leads to:

// BAD: uses input_location
demo.c:1:1: warning: 'noinline' attribute ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      | ^~~

which thus happened to use the location of the int token, rather than that of the attribute. Using warning_at with the location of the attribute, providing the location of the declaration in question as a secondary location, and adding a note:

  auto_diagnostic_group d;
  gcc_rich_location richloc (attrib_loc);
  richloc.add_range (decl_loc);
  if (warning_at (OPT_Wattributes, &richloc,
                  "attribute %qE on variable %qE was ignored", name))
    inform (attrib_loc, "attribute %qE is only applicable to functions");

would lead to:

// OK: use location of attribute, with a secondary location
demo.c:1:24: warning: attribute 'noinline' on variable 'foo' was
   ignored [-Wattributes]
    1 | int foo __attribute__((noinline));
      |     ~~~ ~~~~~~~~~~~~~~~^~~~~~~~~
demo.c:1:24: note: attribute 'noinline' is only applicable to functions

27.1.8 Coding Conventions

See the diagnostics section of the GCC coding conventions.

In the C++ front end, when comparing two types in a message, use ‘%H’ and ‘%I’ rather than ‘%T’, as this allows the diagnostics subsystem to highlight differences between template-based types. For example, rather than using ‘%qT’:

  // BAD: a pair of %qT used in C++ front end for type comparison
  error_at (loc, "could not convert %qE from %qT to %qT", expr,
            TREE_TYPE (expr), type);

which could lead to:

error: could not convert 'map<int, double>()' from 'map<int,double>'
   to 'map<int,int>'

using ‘%H’ and ‘%I’ (via ‘%qH’ and ‘%qI’):

  // OK: compare types in C++ front end via %qH and %qI
  error_at (loc, "could not convert %qE from %qH to %qI", expr,
            TREE_TYPE (expr), type);

allows the above output to be simplified to:

error: could not convert 'map<int, double>()' from 'map<[...],double>'
   to 'map<[...],int>'

where the double and int are colorized to highlight them.

27.1.9 Group logically-related diagnostics

Use auto_diagnostic_group when issuing multiple related diagnostics (seen in various examples on this page). This informs the diagnostic subsystem that all diagnostics issued within the lifetime of the auto_diagnostic_group are related. For example, -fdiagnostics-format=json will treat the first diagnostic emitted within the group as a top-level diagnostic, and all subsequent diagnostics within the group as its children.

27.1.10 Quoting

Text should be quoted by either using the ‘q’ modifier in a directive such as ‘%qE’, or by enclosing the quoted text in a pair of ‘%<’ and ‘%>’ directives, and never by using explicit quote characters. The directives handle the appropriate quote characters for each language and apply the correct color or highlighting.

The following elements should be quoted in GCC diagnostics:

Other elements such as numbers that do not refer to numeric constants that appear in the source code should not be quoted. For example, in the message:

argument %d of %qE must be a pointer type

since the argument number does not refer to a numerical constant in the source code it should not be quoted.

27.1.11 Spelling and Terminology

See the terminology and markup section of the GCC coding conventions.

27.1.12 Fix-it hints

GCC’s diagnostic subsystem can emit fix-it hints: small suggested edits to the user’s source code.

They are printed by default underneath the code in question. They can also be viewed via -fdiagnostics-generate-patch and -fdiagnostics-parseable-fixits. With the latter, an IDE ought to be able to offer to automatically apply the suggested fix.

Fix-it hints contain code fragments, and thus they should not be marked for translation.

Fix-it hints can be added to a diagnostic by using a rich_location rather than a location_t - the fix-it hints are added to the rich_location using one of the various add_fixit member functions of rich_location. They are documented with rich_location in libcpp/line-map.h. It’s easiest to use the gcc_rich_location subclass of rich_location found in gcc-rich-location.h, as this implicitly supplies the line_table variable.

For example:

   if (const char *suggestion = hint.suggestion ())
       gcc_rich_location richloc (location);
       richloc.add_fixit_replace (suggestion);
       error_at (&richloc,
                 "%qE does not name a type; did you mean %qs?",
                 id, suggestion);

which can lead to:

spellcheck-typenames.C:73:1: error: 'singed' does not name a type; did
   you mean 'signed'?
   73 | singed char ch;
      | ^~~~~~
      | signed

Non-trivial edits can be built up by adding multiple fix-it hints to one rich_location. It’s best to express the edits in terms of the locations of individual tokens. Various handy functions for adding fix-it hints for idiomatic C and C++ can be seen in gcc-rich-location.h. Fix-it hints should work

When implementing a fix-it hint, please verify that the suggested edit leads to fixed, compilable code. (Unfortunately, this currently must be done by hand using -fdiagnostics-generate-patch. It would be good to have an automated way of verifying that fix-it hints actually fix the code).

For example, a “gotcha” here is to forget to add a space when adding a missing reserved word. Consider a C++ fix-it hint that adds typename in front of a template declaration. A naive way to implement this might be:

gcc_rich_location richloc (loc);
// BAD: insertion is missing a trailing space
richloc.add_fixit_insert_before ("typename");
error_at (&richloc, "need %<typename%> before %<%T::%E%> because "
                     "%qT is a dependent scope",
                     parser->scope, id, parser->scope);

When applied to the code, this might lead to:

T::type x;

being “corrected” to:

typenameT::type x;

In this case, the correct thing to do is to add a trailing space after typename:

gcc_rich_location richloc (loc);
// OK: note that here we have a trailing space
richloc.add_fixit_insert_before ("typename ");
error_at (&richloc, "need %<typename%> before %<%T::%E%> because "
                     "%qT is a dependent scope",
                     parser->scope, id, parser->scope);

leading to this corrected code:

typename T::type x; Express deletion in terms of deletion, not replacement

It’s best to express deletion suggestions in terms of deletion fix-it hints, rather than replacement fix-it hints. For example, consider this:

    auto_diagnostic_group d;
    gcc_rich_location richloc (location_of (retval));
    tree name = DECL_NAME (arg);
    richloc.add_fixit_replace (IDENTIFIER_POINTER (name));
    warning_at (&richloc, OPT_Wredundant_move,
                "redundant move in return statement");

which is intended to e.g. replace a std::move with the underlying value:

   return std::move (retval);

where the change has been expressed as replacement, replacing with the name of the declaration. This works for simple cases, but consider this case:

# define CONFIGURY_GLOBAL global_a
# define CONFIGURY_GLOBAL global_b

int fn ()
  return std::move (CONFIGURY_GLOBAL /* some comment */);

The above implementation erroneously strips out the macro and the comment in the fix-it hint:

   return std::move (CONFIGURY_GLOBAL /* some comment */);

and thus this resulting code:

   return global_a;

It’s better to do deletions in terms of deletions; deleting the std::move ( and the trailing close-paren, leading to this:

   return std::move (CONFIGURY_GLOBAL /* some comment */);
          CONFIGURY_GLOBAL /* some comment */

and thus this result:

   return CONFIGURY_GLOBAL /* some comment */;

Unfortunately, the pertinent location_t values are not always available. Multiple suggestions

In the rare cases where you need to suggest more than one mutually exclusive solution to a problem, this can be done by emitting multiple notes and calling rich_location::fixits_cannot_be_auto_applied on each note’s rich_location. If this is called, then the fix-it hints in the rich_location will be printed, but will not be added to generated patches.

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