Re: [PATCH/RFC] Add "User Experience Guidelines" to gccint.texi

[Martin Sebor <msebor at gmail dot com>]

On 10/12/2018 09:43 AM, David Malcolm wrote:
> Here's a proposed "User Experience Guidelines" section for our
> internals manual
>
> It's a mixture of proposed policy, together with notes on how to
> implement the recommendations.
>
> Thoughts?

To improve consistency among diagnostic messages it's important
to have a set of guidelines in place.  Thank you for taking
the first step!

> gcc/ChangeLog:
> 	* Makefile.in (TEXI_GCCINT_FILES): Add ux.texi.
> 	* doc/gccint.texi: Include ux.texi and use it in top-level menu.
> 	* doc/ux.texi: New file.
> ---
>  gcc/Makefile.in     |   2 +-
>  gcc/doc/gccint.texi |   2 +
>  gcc/doc/ux.texi     | 455 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 458 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/doc/ux.texi
>
> diff --git a/gcc/Makefile.in b/gcc/Makefile.in
> index 70efab7..3f05e95 100644
> --- a/gcc/Makefile.in
> +++ b/gcc/Makefile.in
> @@ -3176,7 +3176,7 @@ TEXI_GCCINT_FILES = gccint.texi gcc-common.texi gcc-vers.texi		\
>  	 gnu.texi gpl_v3.texi fdl.texi contrib.texi languages.texi	\
>  	 sourcebuild.texi gty.texi libgcc.texi cfg.texi tree-ssa.texi	\
>  	 loop.texi generic.texi gimple.texi plugins.texi optinfo.texi   \
> -	 match-and-simplify.texi poly-int.texi
> +	 match-and-simplify.texi ux.texi poly-int.texi
>
>  TEXI_GCCINSTALL_FILES = install.texi install-old.texi fdl.texi		\
>  	 gcc-common.texi gcc-vers.texi
> diff --git a/gcc/doc/gccint.texi b/gcc/doc/gccint.texi
> index 1a1af41..2554b31 100644
> --- a/gcc/doc/gccint.texi
> +++ b/gcc/doc/gccint.texi
> @@ -125,6 +125,7 @@ Additional tutorial information is linked to from
>  * LTO::             Using Link-Time Optimization.
>
>  * Match and Simplify:: How to write expression simplification patterns for GIMPLE and GENERIC
> +* User Experience Guidelines:: Guidelines for implementing diagnostics and options.
>  * Funding::         How to help assure funding for free software.
>  * GNU Project::     The GNU Project and GNU/Linux.
>
> @@ -162,6 +163,7 @@ Additional tutorial information is linked to from
>  @include plugins.texi
>  @include lto.texi
>  @include match-and-simplify.texi
> +@include ux.texi
>
>  @include funding.texi
>  @include gnu.texi
> diff --git a/gcc/doc/ux.texi b/gcc/doc/ux.texi
> new file mode 100644
> index 0000000..87ff599
> --- /dev/null
> +++ b/gcc/doc/ux.texi
> @@ -0,0 +1,455 @@
> +@c Copyright (C) 2018 Free Software Foundation, Inc.
> +@c Free Software Foundation, Inc.
> +@c This is part of the GCC manual.
> +@c For copying conditions, see the file gcc.texi.
> +
> +@node User Experience Guidelines
> +@chapter User Experience Guidelines
> +@cindex User Experience Guidelines
> +@cindex Guidelines, User Experience
> +
> +To borrow a slogan from
> + @uref{https://elm-lang.org/blog/compilers-as-assistants, Elm},
> +
> +@quotation
> +@strong{Compilers should be assistants, not adversaries.}  A compiler should
> +not just detect bugs, it should then help you understand why there is a bug.
> +It should not berate you in a robot voice, it should give you specific hints
> +that help you write better code. Ultimately, a compiler should make
> +programming faster and more fun!
> +@author Evan Czaplicki
> +@end quotation
> +
> +This chapter provides guidelines on how to implement diagnostics and
> +command-line options in ways that we hope achieve the above ideal.
> +
> +@menu
> +* Guidelines for Diagnostics::       How to implement diagnostics.
> +* Guidelines for Options::           Guidelines for command-line options.
> +@end menu
> +
> +
> +@node Guidelines for Diagnostics
> +@cindex Guidelines for Diagnostics
> +@cindex Diagnostics, Guidelines for
> +
> +@section Guidelines for Diagnostics
> +
> +@subsection 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.
> +
> +@subsection Diagnostics are ``actionable''
> +
> +A good diagnostic is @emph{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 want to review the warning and think:
> +
> +@itemize @bullet
> +@item
> +``Is this a ``true'' result?''

Sounds like you mean a true positive here but I'm not 100% sure
so expanding on this a bit it might be helpful -- maybe simply
phrasing it as "is this a real problem?"

> +@item
> +``Do I care?''
> +@item
> +if they decide it's genuine: ``How do I fix this?''
> +@end itemize
> +
> +A good diagnostic provides pertinent information to allow the user to
> +easily answer the above questions.
> +
> +@subsection 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: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 @emph{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.
> +
> +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.
> +
> +@subsection Make mismatches clear
> +
> +Many diagnostics relate to a mismatch between two different places in the
> +user's source code.  Examples include:
> +@itemize @bullet
> +  @item
> +  a type mismatch, where the type at a usage site does not match the type
> +  at a declaration
> +
> +  @item
> +  the argument count at a call site does not match the parameter count
> +  at the declaration
> +
> +  @item
> +  something is erroneously duplicated (e.g. an error, due to breaking a
> +  uniqueness requirement, or a warning, if it's suggestive of a bug)
> +
> +  @item
> +  an ``opened'' syntactic construct (such as an open-parenthesis) is not
> +  closed
> +
> +  @c TODO: more examples?

The area of attributes and attribute checking is rife with examples
of things to improve.  A poster child for a warning with an unclear
rationale is:

  'foobar' attribute ignored.

(There are nearly a hundred of these.)  Most are due to a mismatch
between the kind of a symbol the attribute is expected to apply to
(function, type, variable, etc.) and the symbol it is being used
with.  This kind of a mismatch should also be made clear.  Doing
it right could be a little tedious because mentioning both kinds
of symbols involved could lead to a combinatorial explosion of
messages due to internationalization.  A clever use of notes
would help.  (Improving the design of struct attribute_spec
and handling this in the middle-end would relieve each attribute
handler from having to jump through these hoops.)

> +@end itemize
> +
> +In each case, the diagnostic should indicate @strong{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 @code{inform}).  For
> +example:
> +
> +@smallexample
> +  auto_diagnostic_group d;
> +  if (warning_at (loc, OPT_Wduplicated_cond,
> +                  "duplicated %<if%> condition"))
> +    inform (EXPR_LOCATION (t), "previously used here");
> +@end smallexample

I wonder if this could also be used as an example of a missing
detail: is the relationship necessarily obvious between the warning
and the note?  (Can there be two warnings with a note on the same
line?  If not here, I know there can be in other warnings.)  If not,
is it worth solving in a consistent way?

> +
> +For cases involving punctuation where the locations might be near
> +each other, they can be conditionally consolidated via
> +@code{gcc_rich_location::add_location_if_nearby}:
> +
> +@smallexample
> +    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");
> +@end smallexample
> +
> +This will emit either one diagnostic with two locations:
> +@smallexample
> +  demo.c:42:10: error: main message
> +    (foo)
> +    ~   ^
> +@end smallexample
> +
> +or two diagnostics:
> +
> +@smallexample
> +  demo.c:42:4: error: main message
> +    foo)
> +       ^
> +  demo.c:40:2: note: message for secondary
> +    (
> +    ^
> +@end smallexample
> +
> +@subsection Location Information
> +
> +GCC's @code{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:
> +
> +@smallexample
> +      a = foo && bar;
> +          ~~~~^~~~~~
> +          |   |    |
> +          |   |    finish
> +          |   caret
> +          start
> +@end smallexample
> +
> +Tokens coming out of libcpp have locations of the form ``caret == start'',
> +such as for ``foo'' here:
> +
> +@smallexample
> +      a = foo && bar;
> +          ^~~
> +          | |
> +          | finish
> +          caret == start
> +@end smallexample
> +
> +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 @code{-Wformat}, rather than underlining just the first
> +token of a bad argument:
> +
> +@smallexample
> +   printf("hello %i %s", (long)0, "world);
> +                 ~^      ~
> +                 %li
> +@end smallexample
> +
> +the whole of the expression should be underlined, so that the user can
> +easily identify what is being referred to:
> +
> +@smallexample
> +   printf("hello %i %s", (long)0, "world);
> +                 ~^      ~~~~~~~
> +                 %li
> +@end smallexample
> +
> +@c this was r251239

This is really nice.  My experience from the sprintf warnings is
that it was quite to get to work the way I wanted to (put the caret
and underlining just in the right place).  I'm not sure who else
has worked with this and what their experience was but I wonder
if a separate tutorial showing how to make this work would help
people get more comfortable with it.

> +
> +Avoid using the @code{input_location} global, and the diagnostic functions
> +that implicitly use it - use @code{error_at} and @code{warning_at} rather
> +than @code{error} and @code{warning}.
> +
> +@c TODO labelling of ranges
> +
> +@subsection Coding Conventions
> +
> +See the @uref{https://gcc.gnu.org/codingconventions.html#Diagnostics,
> +diagnostics section} of the GCC coding conventions.
> +
> +In the C++ frontend, when comparing two types in a message, use @code{%H}
> +and @code{%I} rather tha @code{%T}, as this allows the diagnostics
> +subsystem to highlight differences between template-based types.
> +
> +Use @code{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 @code{auto_diagnostic_group} are related.  (Currently it doesn't
> +do anything with this information, but we may implement that in the
> +future).

Same here.  I have never used this and even though I saw the %H
and %I patches go in I probably wouldn't have thought of using
these codes.  Having a little tutorial (or an example showing
the effect of these directives) might help.

> +
> +@subsection Spelling and Terminology
> +
> +See the @uref{https://gcc.gnu.org/codingconventions.html#Spelling
> +Spelling, terminology and markup} section of the GCC coding conventions.
> +
> +@subsection Tense of messages
> +Syntax errors occur in the present tense e.g.
> +
> +@smallexample
> +error_at (loc, "cannot convert %qH to %qI");
> +@end smallexample
> +
> +and thus
> +
> +@smallexample
> +// CORRECT: usage of present tense:
> +error: cannot convert 'int' to 'void *'
> +@end smallexample

This sounds fine.  It would be nice to change the few instances
of the past tense.  I think past tense is appropriate in runtime
messages like "could not open file" so it might be worth mentioning.

> +
> +rather than
> +
> +@smallexample
> +// BAD: usage of past tense:
> +error: could not convert 'int' to 'void *'
> +@end smallexample
> +
> +Predictions about run-time behavior should be described in the future tense e.g.
> +
> +@smallexample
> +warning_at (loc, OPT_some_warning,
> +            "this code will read past the end of the array");
> +@end smallexample

I don't think this will always achieve a good result.  Consider
warnings like -Wuninitialized:

    'x' is used uninitialized in this function

A slightly different example is -Walloca, -Warray-bounds, and
-Wvla: it makes more sense (to me) to say "value is too large"
or "index is out of bounds" than "it will be too large" etc.

It would be fine for warnings like -Wformat-truncation but all
it will do there is add a word without making things any clearer:
"output truncated" vs "output will be truncated."

In general, it's also not always the case that the code will
necessarily be executed.  It could be eliminated or replaced
by a trap (as we discussed at Cauldron), so saying "this will
happen" when we know something else entirely might happen
(and what exactly that is might depend on options) would be
misleading.

> +
> +@subsection Fix-it hints

There's a wealth of information in this section and it reads
like a tutorial as much as, if not more than guidelines.  Would
separating the two make sense?  I.e., have a standalone tutorial
page and just the most important high-level guidelines here?

Besides the topics you already covered, some others that I think
would be useful to cover are those I mentioned at Cauldron: e.g.,
how to choose where to implement a warning (pattern rules vs
flow-based rules), the common gotchas to look out for there,
etc.  I'll be happy to contribute those once the first version
is agreed on and committed.

Martin

> +
> +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 @option{-fdiagnostics-generate-patch} and
> +@option{-fdiagnostics-parseable-fixits}.  With the latter, an IDE
> +ought to be able to offer to automatically apply the suggested fix.
> +
> +Fix-it hints can be added to a diagnostic by using a @code{rich_location}
> +rather than a @code{location_t} - the fix-it hints are added to the
> +@code{rich_location} using one of the various @code{add_fixit} member
> +functions of @code{rich_location}.  They are documented with
> +@code{rich_location} in @file{libcpp/line-map.h}.
> +It's easiest to use the @code{gcc_rich_location} subclass of
> +@code{rich_location} found in @file{gcc-rich-location.h}.
> +
> +For example:
> +
> +@smallexample
> +   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);
> +     @}
> +@end smallexample
> +
> +which can lead to:
> +
> +@smallexample
> +spellcheck-typenames.C:73:1: error: 'singed' does not name a type; did you mean 'signed'?
> +73 | singed char ch;
> +   | ^~~~~~
> +   | signed
> +@end smallexample
> +
> +Non-trivial edits can be built up by adding multiple fix-it hints to one
> +@code{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
> +@file{gcc-rich-location.h}.
> +
> +@subsubsection Fix-it hints should be ``applyable''
> +
> +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 @option{-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
> +@code{typename} in front of a template declaration.  A naive way to
> +implement this might be:
> +
> +@smallexample
> +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);
> +@end smallexample
> +
> +When applied to the code, this might lead to:
> +
> +@smallexample
> +T::type x;
> +@end smallexample
> +
> +being ``corrected'' to:
> +
> +@smallexample
> +typenameT::type x;
> +@end smallexample
> +
> +In this case, the correct thing to do is to add a trailing space after
> +@code{typename}:
> +
> +@smallexample
> +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);
> +@end smallexample
> +
> +leading to this corrected code:
> +
> +@smallexample
> +typename T::type x;
> +@end smallexample
> +
> +@subsubsection 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:
> +
> +@smallexample
> +    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");
> +@end smallexample
> +
> +which is intended to e.g. replace a @code{std::move} with the underlying
> +value:
> +
> +@smallexample
> +   return std::move (retval);
> +          ~~~~~~~~~~^~~~~~~~
> +          retval
> +@end smallexample
> +
> +where the change has been expressed as replacement, replacing
> +with the name of the declaration.
> +
> +This works for simple cases, but consider this case:
> +
> +@smallexample
> +#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 */);
> +@}
> +@end smallexample
> +
> +The above implementation will erroneously strip out the macro and the
> +comment in the fix-it hint:
> +
> +@smallexample
> +   return std::move (CONFIGURY_GLOBAL /* some comment */);
> +          ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +          global_a
> +@end smallexample
> +
> +and thus this resulting code:
> +
> +@smallexample
> +   return global_a;
> +@end smallexample
> +
> +It's better to do deletions in terms of deletions; deleting the
> +@code{std::move (} and the trailing close-paren, leading to
> +this:
> +
> +@smallexample
> +   return std::move (CONFIGURY_GLOBAL /* some comment */);
> +          ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +          CONFIGURY_GLOBAL /* some comment */
> +@end smallexample
> +
> +and thus this result:
> +
> +@smallexample
> +   return CONFIGURY_GLOBAL /* some comment */;
> +@end smallexample
> +
> +Unfortunately, the pertinent @code{location_t} values are not always
> +available.
> +
> +@c the above was https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01474.html
> +
> +@subsubsection 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
> +@code{rich_location::fixits_cannot_be_auto_applied} on each note's
> +@code{rich_location}.  If this is called, then the fix-it hints in
> +the @code{rich_location} will be printed, but will not be added to
> +generated patches.
> +
> +
> +@node Guidelines for Options
> +@cindex Options, Guidelines for
> +@cindex Guidelines for Options
> +
> +@section Guidelines for Options
> +
> +@c TODO