[cfe-dev] Clarifying the roles and requirements on fixit hints in diagnostics

[Nick Lewycky]

On 10 April 2011 01:37, Chandler Carruth <chandlerc at google.com> wrote:

> I know I have been confused on more than one occasion about the precise
> requirements for issuing a fixit hint. After several IRC conversations
> (thanks to those that participated) I think I've got a decent understanding,
> and I'd like to add some sections to the Clang documentation which spell
> this out for future reference. A brief summary of my understanding lest I
> have gotten confused again:
>
> 1) Fixit hints attached to an error diagnostic *must* recover the
> parse/semantic analysis/etc as if the fix had been applied. This won't ever
> miscompile code as the compilation cannot succeed after issuing an error
> unless Clang has been asked to automatically apply these fixes, in which
> case the compile will succeed, and accurately reflect the (newly updated)
> code.
>
> 2) Fixit hints on notes *must not* have any impact on the compilation. The
> also are not automatically applied to the code by the -fixit flag.
>
> 3) There can be only one hint attached to a diagnostic, and thus if the
> hint is attached to the error or warning diagnostic it must be an extremely
> confident fix with no other viable candidates. When there are multiple
> viable candidate fixes, they should be presented as multiple fixit hint
> bearing notes.
>
>
> The one area this doesn't cover are fixit hints attached to warnings. These
> are trickier. Previously, it has been suggested that they should follow the
> same rules as those attached to errors, but that has some serious problems.
> Suppose this is the approach is used for -Wmismatched-tags:
> ---- x.cc ----
> class X;
> struct X {
>   X() {}
> };
> X x;
> ----
> This code is well formed, but the warning will suggest replacing 'struct'
> with 'class'. Doing so makes the code ill-formed. If we recover as-if the
> fixit hint were applied, we would error on this code when the warning is
> turned on, which seems rather surprising. ;] If we don't recover as if the
> fixit hint were applied, and run Clang with -fixit, we accept the code, but
> then alter the code to a text sequence which if we recompile is rejected.
> Again, rather surprising. Currently, Clang's warnings which have fixit hints
> attached have a mixture of these policies, but more often follow the latter
> policy of no recovery. For some, this is a moot point -- the code parses the
> same either way and the hint merely removes redundant text or adds
> clarifying text. The question is what *should* the rest of the warnings with
> fixit hints do?
>
> One option would be to require that warnings with direct fixit hints *can*
> recover as if the hint were applied, but provide a hook so that the recovery
> is only performed when that warning is promoted to an error or when running
> with -fixit in effect.
>
> A second option is to require that recovery not be performed for warning
> fixit hints and that -fixit not apply them automatically. Essentially treat
> them the same as note fixit hints.
>
> I prefer the second option as it is simpler, and I think provides a better
> user experience. There are several warnings with a note attached to them
> purely to provide a fixit hint without recover or automatic application. The
> output would be simpler and more clear if these could be directly attached
> to the warnings.
>
> Thoughts? Once this is a bit more clear, I'll start fixing documentation.
>

I like the second option with a refinement: the warning fixit must be one
that removes this warning and does not change the semantics or validity of
the program (it may change the AST). If it may change the program then it
needs to be a note. Then, -fixit should apply warning

That way, we never do recovery that follows the fixits except on errors, the
fixits like -Wparenthesis will still fire to add the parens in a way that
doesn't actually change anything, -Wmismatched-tags would use a note fixit
to suggest its change.

Actually -Wmismatched-tags should also add public: / private: so that we
could upgrade its fixit from a note to a warning and let clang -fixit fix
things.

Making sure all the warnings choose the right level gives us a place for
bugs to live, but I think it's worth it for the ability to see "clang
-fixit" fix the warnings it knows how to fix.

Nick
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