[cfe-dev] Reflection/metaparsing implem

[David Rector via cfe-dev]

> On Nov 11, 2019, at 4:09 PM, David Blaikie <dblaikie at gmail.com> wrote:
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> 
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> On Mon, Nov 11, 2019 at 1:01 PM James Y Knight <jyknight at google.com <mailto:jyknight at google.com>> wrote:
> On Mon, Nov 11, 2019 at 3:03 PM David Blaikie via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote: 
> I think that'd still be a pretty hard sell - Clang's APIs and AST are volatile & intentionally so. People writing against the Clang APIs themselves with things like clang-tidy are aware of that. If we sold this as a feature to general C++ users, I don't think there would be a way to sufficiently communicate how volatile this will be - and end up backing ourselves into a corner & limiting the ability to change Clang.
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> Does Clang's AST API _need_ to be unstable in order to keep up with language changes, or is it just a convenience?
> 
> I imagine with sufficient heroics the AST could be maintained for a particular version - and /probably/ version-over-version (so the C++23 AST was backwards compatible with the C++20 AST, etc). In the same way that the language itself is mostly backwards compatible version over version.
>  
> Is this concern indicative of problems that a standardized AST API would also have, or is unique to Clang's current API?
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> The concern is mostly indicative of a very high fidelity API - the lower level/more bare the API, the more brittle it'll be to changes (both refactoring improvements to Clang, and necessary changes to accommodate new language features). Standardizing something a little higher level could provide a lot more flexibility under the hood.
>  

Volatility issues could be managed. E.g. I’ve already implem’d so that CLANG_NO_REFLECT turns off reflection for any AST public const method or field — but note that it is a crutch; any public const method is either a) eventually going to be asked to be reflected or b) probably shouldn’t be a public const method.  (Reflection is an opportunity to look in the mirror — better to hit the gym than to design a funhouse mirror to fix the issue.)

I imagine Other AST attributes — e.g. CLANG_REFLECT_DEPRECATED — could be defined so that the tool generates warnings and fixits whenever they are used.
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