[cfe-dev] [RFC] A C++ pseudo parser for tooling

[Haojian Wu via cfe-dev]

Hello everyone,

We’d like to propose a pseudo-parser which can approximately parse C++
(including broken code). It parses a file in isolation, without needing
headers, compile flags etc. Ambiguities are resolved heuristically, like
clang-format. Its output is a clang::syntax tree, which maps the token
sequence onto the C++ grammar.
Our motivation comes from wanting to add some low latency features (file
outline, refactorings etc) in clangd, but we think this is a useful
building block for other tools too.

Design is discussed in detail here:
https://docs.google.com/document/d/1eGkTOsFja63wsv8v0vd5JdoTonj-NlN3ujGF0T7xDbM/edit?usp=sharing

The proposal is based on the experience with a working prototype.
Initially, we will focus on building the foundation. We consider the first
version as experimental, and plan to use and validate it with applications
in clangd (the detailed plan is described here
<https://docs.google.com/document/d/1eGkTOsFja63wsv8v0vd5JdoTonj-NlN3ujGF0T7xDbM/edit#heading=h.mawgmexy688j>
).

As soon as we have consensus on the proposal, we plan to start this work in
the clang repository (code would be under clang/Tooling/Syntax). We hope we
can start sending out patches for review at the end of November.

Eager to hear your thoughts. Comments and suggestions are much appreciated.

Thanks,
Haojian
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