recordDecl() AST matcher

[Aaron Ballman via cfe-commits]

On Tue, Sep 8, 2015 at 9:18 AM, Manuel Klimek <klimek at google.com> wrote:
> On Tue, Sep 8, 2015 at 2:23 PM Aaron Ballman <aaron at aaronballman.com> wrote:
>>
>> On Tue, Sep 8, 2015 at 5:40 AM, Manuel Klimek <klimek at google.com> wrote:
>> > Yea, we had that discussion a few times, and I can never remember why we
>> > ended up in the state we're in.
>> > We definitely had a time where we switched to just using the exact same
>> > name
>> > as the node's class name for the matchers.
>> > I *think* we didn't do it for cxxRecordDecl, because Richard said that's
>> > a
>> > relic we should get rid of anyway, but I'm not sure.
>>
>> FWIW, I think the state we're in is the worst of all worlds. It's not
>> intuitive that recordDecl() doesn't match a struct in C mode, and as
>> it stands, there is no way to match a struct or union declaration in C
>> at all.
>
>
> Agreed. Best intentions. Worst possible outcome. That's software development
> :)
>
>> >
>> > On Fri, Sep 4, 2015 at 8:32 PM Aaron Ballman <aaron at aaronballman.com>
>> > wrote:
>> >>
>> >> It turns out that the recordDecl() AST matcher doesn't match
>> >> RecordDecl objects; instead, it matches CXXRecordDecl objects. This
>> >> is... unfortunate... as it makes writing AST matchers more complicated
>> >> because of having to translate between recordDecl()/CXXRecordDecl. It
>> >> also makes it impossible to match a struct or union declaration in C
>> >> or ObjC. However, given how prevalent recordDecl()'s use is in the
>> >> wild (I'm guessing), changing it at this point would be a Bad Thing.
>> >>
>> >> For people trying to write AST matchers for languages like C or ObjC,
>> >> I would like to propose adding:
>> >>
>> >> structDecl()
>> >> unionDecl()
>> >> tagDecl()
>> >>
>> >> These will match nicely with the existing enumDecl() AST matcher.
>> >>
>> >> Additionally, I would like to add cxxRecordDecl() to match
>> >> CXXRecordDecl objects. While it duplicates the functionality exposed
>> >> by recordDecl(), it more clearly matches the intention of which AST
>> >> node it corresponds to.
>> >>
>> >> Finally, I would like to undocument recordDecl() and change our
>> >> existing documentation and AST matcher uses to use
>> >> cxxRecordDecl/structDecl() instead. Maybe someday we can deprecate
>> >> recordDecl() more officially.
>> >>
>> >> I'm open to other ideas if there are better ways to move forward. If
>> >> you think changing the meaning of recordDecl() is acceptable, I can
>> >> also go that route (though I would still propose adding unionDecl()
>> >> and cxxRecordDecl() in that case).
>> >
>> >
>> > I think changing recordDecl is acceptable. I believe very few tools will
>> > actually start doing wrong things because of it. I'd like more opinions
>> > first, though :)
>>
>> I was giving this more thought over the long weekend, and I think you
>> may be right. I think changing recordDecl() to mean RecordDecl will
>> fix more code than it breaks, so long as we take a holistic approach
>> to the change and see which narrowing and traversal matchers we need
>> to fix up at the same time. When I tried to think of AST matchers that
>> mean CXXRecordDecl but *not* RecordDecl, they were horribly contrived
>> because you usually are matching on additional selection criteria that
>> is specific to C++ (such as hasMethod() or isDerivedFrom()) which
>> would cause the match to continue to fail, as expected. Code that uses
>> recordDecl() to mean RecordDecl will suddenly start to match in more
>> cases, but that's likely to be a bug fix more than a breaking change.
>> To the best of my understanding, the only breaking cases would be
>> where you wrote recordDecl(), meant CXXRecordDecl, had no further
>> narrowing or traversal matchers, and were compiling in C mode; with
>> the result being additional unexpected matches.
>
>
> Ah, there's one thing that can break: the compile can break:
> recordDecl(hasMethod(...)) will *not* compile (it'll work in the dynamic
> matchers and fail as you suggest, but the in-C++ DSL does more static type
> checking).
> I don't think that's super bad though.
>
>>
>> So perhaps it would make sense to:
>>
>> 1) Make recordDecl() mean RecordDecl
>> 2) Do a comprehensive review of matchers that take a CXXRecordDecl and
>> see if they should instead take a RecordDecl
>> 3) Add unionDecl() as a node matcher (or should we add isUnion() and
>> isStruct() as narrowing matchers?)
>> 4) Add tagDecl() as a node matcher, but not add cxxRecordDecl()
>
>
> Why not add cxxRecordDecl()? I think we need it if we want narrowing
> matchers on CXXRecordDecl?

If Richard thinks CXXRecordDecl is an anachronism, I figured we didn't
want to expose it. Instead, we could make hasMethod (et al) accept a
RecordDecl and do the type checking for the caller. Then
recordDecl(hasMethod(...)) continues to compile and work, and when
hasMethod is given a RecordDecl instead of a CXXRecordDecl, it simply
matches nothing. But you bring up a good point about the C++ DSL being
a problem still, I hadn't considered that.

~Aaron

>
>>
>>
>> ~Aaron
>>
>> >
>> >>
>> >>
>> >> Thanks!
>> >>
>> >> ~Aaron