recordDecl() AST matcher

[Aaron Ballman via cfe-commits]

On Mon, Sep 14, 2015 at 5:47 PM, Daniel Jasper <djasper at google.com> wrote:
> Btw, I think generating them, potentially into several different headers to
> work around the compile time issue isn't such a bad idea.

I'm not going to start with this approach, but think it may be worth
exploring at some point. ;-)

~Aaron

>
> On Mon, Sep 14, 2015 at 11:45 PM, Manuel Klimek <klimek at google.com> wrote:
>>
>> 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
>>>
>>>
>