GNU Compiler Collection (GCC) Internals: Guidelines for Diagnostics

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### 28.1 Guidelines for Diagnostics

#### 28.1.1 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.

#### 28.1.2 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:

- “Is this a real problem?”
- “Do I care?”
- if they decide it’s genuine: “How do I fix this?”

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

#### 28.1.3 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.

#### 28.1.4 Sometimes the user didn’t write the code

GCC is typically used in two different ways:

- Semi-interactive usage: GCC is used as a development tool when the user is writing code, as the “compile” part of the “edit-compile-debug” cycle. The user is actively hacking on the code themself (perhaps a project they wrote, or someone else’s), where they just made a change to the code and want to see what happens, and to be warned about mistakes.
- Batch rebuilds: where the user is recompiling one or more existing packages, and GCC is a detail that’s being invoked by various build scripts. Examples include a user trying to bring up an operating system consisting of hundreds of packages on a new CPU architecture, where the packages were written by many different people, or simply rebuilding packages after a dependency changed, where the user is hoping “nothing breaks”, since they are unfamiliar with the code.

Keep both of these styles of usage in mind when implementing diagnostics.

#### 28.1.5 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.

#### 28.1.6 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.

#### 28.1.7 Make mismatches clear

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

- a type mismatch, where the type at a usage site does not match the type at a declaration
- the argument count at a call site does not match the parameter count at the declaration
- something is erroneously duplicated (e.g. an error, due to breaking a uniqueness requirement, or a warning, if it’s suggestive of a bug)
- an “opened” syntactic construct (such as an open-parenthesis) is not closed

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
        (foo)
        ~   ^

or two diagnostics:

      demo.c:42:4: error: main message
        foo)
           ^
      demo.c:40:2: note: message for secondary
        (
        ^

#### 28.1.8 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
              start

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");
                     ~^      ~
                     %li

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");
                     ~^      ~~~~~~~
                     %li

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

#### 28.1.9 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.

#### 28.1.10 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.

#### 28.1.11 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:

- Language keywords.
- Tokens.
- Boolean, numerical, character, and string constants that appear in the source code.
- Identifiers, including function, macro, type, and variable names.

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.

#### 28.1.12 Spelling and Terminology

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

#### 28.1.13 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`.

#### 28.1.13.1 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;

#### 28.1.13.2 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);
              ~~~~~~~~~~^~~~~~~~
              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:

    #ifdef SOME_CONFIG_FLAG
    # define CONFIGURY_GLOBAL global_a
    #else
    # define CONFIGURY_GLOBAL global_b
    #endif

    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 */);
              ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              global_a

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.

#### 28.1.13.3 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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