recordDecl() AST matcher

[Manuel Klimek via cfe-commits]

Feel free to rename the AST nodes :)

On Mon, Sep 14, 2015, 2:44 PM Daniel Jasper <djasper at google.com> wrote:

> 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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