recordDecl() AST matcher

[Daniel Jasper via cfe-commits]

Ok. I am happy with this then.

(Just personally grumpy having to write
cxxRecordDecl(has(cxxConstructorDecl(..))) in the future ;-) ).

On Mon, Sep 14, 2015 at 11:41 PM, Manuel Klimek <klimek at google.com> wrote:

>
>
> On Mon, Sep 14, 2015 at 2:29 PM Aaron Ballman <aaron at aaronballman.com>
> wrote:
>
>> On Mon, Sep 14, 2015 at 4:38 PM, Manuel Klimek <klimek at google.com> wrote:
>> >
>> >
>> > On Mon, Sep 14, 2015 at 12:26 PM Aaron Ballman <aaron at aaronballman.com>
>> > wrote:
>> >>
>> >> On Mon, Sep 14, 2015 at 2:45 PM, Daniel Jasper <djasper at google.com>
>> wrote:
>> >> > By this point, I see that change might be profitable overall.
>> However,
>> >> > lets
>> >> > completely map this out. Changing just cxxRecordDecl() can actually
>> >> > increase
>> >> > confusion in other areas. Right now, not a single AST matcher has the
>> >> > cxx
>> >> > prefix (although a total of 28 stand for the corresponding CXX.. AST
>> >> > node).
>> >> > This is consistent and people knowing this will never try to write
>> >> > cxxConstructExpr(). As soon as people have used cxxRecordDecl(), the
>> >> > chance
>> >> > of them trying cxxConstructExpr() increases. You have spent a long
>> time
>> >> > figuring out that recordDecl means cxxRecordDecl(), which is one
>> >> > datapoint,
>> >> > but I am not aware of anyone else having this specific issue. And we
>> >> > could
>> >> > make this less bad with better documentation, I think.
>> >> >
>> >> > So, for me, the questions are:
>> >> > 1) Do we want/need this change?
>> >>
>> >> We definitely need *a* change because there currently is no way to
>> >> match a C struct or union when compiling in C mode. I discovered this
>> >> because I was trying to write a new checker for clang-tidy that
>> >> focuses on C code and it would fail to match when compiling in C mode.
>> >> Whether we decide to go with cxxRecordDecl vs recordDecl vs structDecl
>> >> (etc) is less important to me than the ability to write clang-tidy
>> >> checks for C code.
>> >>
>> >> > 2) Do we want to be consistent and change the other 27 matchers as
>> well?
>> >>
>> >> I'm on the fence about this for all the reasons you point out.
>> >>
>> >> > A fundamental question is whether we want AST matchers to match AST
>> >> > nodes
>> >> > 1:1 or whether they should be an abstraction from some implementation
>> >> > details of the AST.
>> >>
>> >> I absolutely agree that this is a fundamental question. I think a
>> >> higher priority fundamental question that goes along with it is: are
>> >> we okay with breaking a lot of user code (are these meant to be stable
>> >> APIs like the LLVM C APIs)? If we want these APIs to be stable, that
>> >> changes the answer of what kind of mapping we can have.
>> >
>> >
>> > I think the AST matchers are so closely coupled to the AST that it
>> trying to
>> > be more stable than the AST doesn't help. Basically all uses of AST
>> matchers
>> > do something with the AST nodes afterwards, which will break anyway.
>>
>> I can get behind that logic. So we're okay with breaking their code
>> because there's no way around it -- it's tied to the AST, so users
>> cannot rely on the AST APIs remaining the same from release to release
>> anyway.
>>
>
> We might even *want* the code to break, as the use of the AST node might
> now be incorrect on a semantic level.
>
>
>>
>> >
>> >>
>> >> > And this is not an easy question to answer. There are
>> >> > many places where we don't follow a strict 1:1 mapping. Mostly node
>> >> > matchers, but also in traversal matchers, e.g. isDerivedFrom().
>> >> >
>> >> > Personally, I'd really hate to have the cxx Prefix everywhere, but
>> >> > that's
>> >> > just my personal opinion. I would even prefer matchers like record()
>> and
>> >> > method(), but I think somebody convinced me that that would be a very
>> >> > bad
>> >> > idea ;-).
>> >>
>> >> My personal opinion is that (1) breaking code is fine, but we should
>> >> avoid doing it without very clear benefit, and (2) the mapping between
>> >> AST node identifiers and AST matcher identifiers needs to be
>> >> incredibly obvious, but perhaps not slavishly 1:1. If we instead
>> >> decide we want a 1:1 mapping, then I think we need to start seriously
>> >> considering auto-generating the AST node (and type) matchers from
>> >> tablegen so that the AST nodes *cannot* get out of sync with the AST
>> >> matchers, otherwise we'll be right back here again in a few years when
>> >> we modify the name of an AST node.
>> >
>> >
>> > I do think we want to auto-generate the matchers, but I don't think
>> tablegen
>> > is the right approach (I think an ast-matcher based tool is ;)
>> > That said, auto-generating all the matchers is a) a lot of effort and
>> b) the
>> > code-size and compile time of matchers already matters, so it's unclear
>> > which ones we would want to generate, especially for traversal matchers
>> :(
>>
>> Oh, that's an excellent point (I'm talking about (b), I already knew
>> (a) was a lot of work). Thank you for pointing that out!
>>
>> >
>> >>
>> >> My definition of "incredibly obvious" is: if the AST has a prefixed
>> >> and unprefixed version, or two different prefixes, we should mimic
>> >> that directly with the matchers. Otherwise, existing AST matchers
>> >> without prefix shenanigans can remain as they are, and new AST
>> >> matchers should prefix as-required. If we decide we're okay breaking
>> >> code, then I don't see a problem with changing ctorInitializer() into
>> >> cxxCtorInitializer() when C adds constructors. ;-)
>> >
>> >
>> > I think the main things is cost for developers who try to write
>> matchers and
>> > work from the -ast-dump. Figuring out that there *is* a matcher with an
>> > unprefixed node can take a while.
>>
>> Hmm, yes, but "take a while" should be relatively short, I would
>> think. In that use-case, the user does an -ast-dump, sees
>> "CXXFrobbleGnasher", they go to the AST matcher reference and they
>> search for "CXXFrobberGnasher." The first hit won't be
>> cxxFrobbleGnasher, but the entry for frobbleGnasher (which is still
>> the first hit when searching from the top of the document due to the
>> way we position node matchers) will have a parameter of
>> CXXFrobbleGnasher, so they will find still get to the right matcher on
>> the first hit. If someone doesn't read the documentation at all,
>> they're going to try cxxFrobbleGnasher() and get a compile error/no
>> known matcher. Then they'll go look at ASTMatchers.h and figure out
>> it's called frobbleGnasher by searching there instead of the
>> documentation.
>>
>
> The problem is that I've learned that sometimes people try to make things
> work in ways that I couldn't even imagine, and they lose more time than I
> could ever imagine them using :) Also, I agree the time is probably on
> average not that large, but we pay it over a long time in the future, and
> it tends to add up.
>
>
>> That's compared to having the matcher name always be the same as the
>> AST node, where the user writes cxxFrobbleGnasher and it just works,
>> which is definitely a mark in favor of making everything consistent. I
>> just don't think the current approach is too onerous in the case where
>> the matcher is at least *provided* for the user with a relatively sane
>> name.
>>
>> >> I should be clear, I'm not opposed to just having a 1:1 mapping. I'm
>> >> just not certain the benefits of being strict about that outweigh the
>> >> costs to broken code. cxxCtorInitializer will break someone's code,
>> >> but I don't think it adds any clarity to their code, so I don't see
>> >> the benefit of forcing the change.
>> >
>> > Well, I think there's the cost of broken code *once* now, vs. the
>> (smaller)
>> > cost for users in all future.
>> > I'm still strongly in favor of breaking now, and having a simpler model
>> > going forward.
>>
>> I'm definitely in favor of breaking now in the case of RecordDecl vs
>> CXXRecordDecl. I think having recordDecl match CXXRecordDecl is a bug
>> given that there's no way to match a RecordDecl.
>>
>> I would also be totally in favor of being consistent if we were
>> starting from scratch. I'm very, very weakly opposed to breaking more
>> user's code than we have to in order to get usable matchers because it
>> seems gratuitous. Breaking code to get something that works seems
>> reasonable. Breaking code that already works just to change the name
>> for consistency elsewhere, I'm a bit less keen on. But the fact that
>> we already can break user's code at-will because of the reliance on
>> the AST nodes makes me think it may be the right approach for the best
>> API, since that's what I would want if we were starting from scratch.
>>
>> Okay, I'm convinced. I think we should rename the type and node
>> matchers (not traversal and narrowing matchers) to match the AST node
>> names in all cases. We can document the breakage in the release notes,
>> but (hopefully) only have to do this dance one time instead of
>> spreading the pain out as it happens to eventually get to the same
>> place anyway.
>>
>
> Yea, people who want more stability do use releases anyway.
>
>
>> Daniel, is this something you would be okay with? (I'm assuming
>> Richard finds it acceptable based on previous comments from Manuel,
>> but Richard, feel free to chime in.)
>>
>
> Offline conversation with Richard says that he is convinced.
>
>
>>
>> ~Aaron
>>
>
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