[clangd-dev] RFC: expected types in clangd completions

Ilya Biryukov via clangd-dev clangd-dev at lists.llvm.org
Thu Aug 9 00:56:38 PDT 2018

We planned to start with hard-coding the most common cases. This is
obviously hard to generalize, but should work nicely for the common things
(mostly STL, maybe other libs if we're good at detecting those, but I'm not
Annotating the type in the header would be perfect for the tools, but I
wonder if people are willing to change the code for the sole purpose of
improved tooling (you could argue this also serves documentation purposes).
There will always be code that cannot be annotated, because users don't own

Alternative approaches that come to mind:
- Detecting constructor functions during indexing. (By looking at the
function bodies or the most common functions used to "create" variables of
some type).
  Pros: does not require any actions from the user.
  Cons: requires preprocessing of the code base to work, might be hard to
make it work with index-while-build.
- Having external annotations (in addition to in-code annotations?),
something that could give the tools enough info to do completions, e.g.
'{ type: "std::unique_ptr<$T>", construction: { insert_snippet:
"std::make_unique<$T>($0)",  required_include: "<memory>" }' (the syntax
can obviously be something else, e.g. C++ with attributes or YAML)
  Pros: do not need to change the code, potentially allows per-project
  Cons: needs design (e.g. should C++ lang standards be taken into
account?), agreement on where to put those configs, how to load them, etc.

We could also do both alternatives if we have infinite time: external
annotations and ways to infer them from the code while indexing.

On Thu, Aug 9, 2018 at 1:39 AM Alex L <arphaman at gmail.com> wrote:

> Thanks for putting this together!
> I have one question:
> In the example where an 'std::make_unique<A>' completion was added based
> on function argument type, how would Clang know that `std::unique_ptr` is
> constructed using `std::make_unique`? Would that be hardcoded into Clang,
> or would there be some way to annotate the type in the header (e.g. using
> an attribute) so that Clang can deduce this (e.g. for supporting
> `llvm::make_unique`, etc.).
> Thanks,
> Alex
> On Wed, 8 Aug 2018 at 02:58, Ilya Biryukov via clangd-dev <
> clangd-dev at lists.llvm.org> wrote:
>> Hi clangd-dev,
>> I have just posted a design doc about adding the expected types to the
>> C++ completions:
>> https://docs.google.com/document/d/1_BSeTIr2aLA6wKKCRCajSI0AdnjhvgK4BXXJQX-Dsro/edit#
>> It serves two purposes:
>> - List possible improvements to code completion that can be gained by
>> using more semantic information available in Clang.
>> - Discuss the design of type-based ranking in presence of the index, i.e.
>> when some completions are not from Sema.
>> The first half of the document attempts to enumerate interesting cases
>> where completions can be improved by using type information. It would be
>> great to get ideas from anyone interested in improvements to ClangD
>> completions in general,  the 'Anticipated Improvements' is there to collect
>> a raw list of nice-to-have cases, feel free to contribue!
>> The second half is targeted at a narrower audience, specifically anyone
>> who's interested in support of type-based ranking with index enabled in
>> ClangD. If you're interested, please take a look.
>> Feedback and suggestions are very welcome!
>> --
>> Regards,
>> Ilya Biryukov
>> _______________________________________________
>> clangd-dev mailing list
>> clangd-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/clangd-dev

Ilya Biryukov
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