[cfe-dev] Adding Support for APINotes

James Y Knight via cfe-dev cfe-dev at lists.llvm.org
Fri Oct 16 07:27:48 PDT 2020

On Thu, Oct 15, 2020 at 11:59 PM Chris Lattner <clattner at nondot.org> wrote:

> On Oct 14, 2020, at 6:49 PM, James Y Knight via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>>> If such a hook mechanism is added, it then seems entirely plausible that
>>> SemaAPINotes.cpp
>>> <https://github.com/apple/llvm-project/blob/apple/main/clang/lib/Sema/SemaAPINotes.cpp>
>>>  and lib/APINotes/*
>>> <https://github.com/apple/llvm-project/tree/apple/main/clang/lib/APINotes> can
>>> be moved from the clang-fork into swift itself without requiring
>>> significant modifications of the code, or any change in functionality of
>>> the feature.
>> Well, the APINotes also provide nullability information for ObjC
>> interfaces, which is not part of Swift, but rather part of ObjC, so that
>> still needs to be in clang.  That is, the functionality is useful for ObjC,
>> and that doesn't make sense to be in Swift.
> Theoretically one could imagine doing so -- but TTBOMK, it's never
> actually been used like this. All nullability annotations used by objc
> compilations live in the headers themselves, and the apinotes command-line
> options are passed only by Swift, for creating a Swift-ObjC-bridge module.
> Nullability isn’t just useful for swift, it is also useful to the clang
> static analyzer and other tools that want to reason about C API semantics.

Nullability is being used in ObjC to emit warnings, but apinotes is not.
For ObjC compiles (and static analyzer), the nullability information comes
from header files, _Nullable and _Nonnull keywords,
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