[cfe-dev] Apple's investment into Clangd and refactoring tooling

[Alex L via cfe-dev]

Hi all,

We at Apple have decided to switch focus from supporting the libclang-based
tooling infrastructure in order to join forces on the Clangd development
efforts. We believe that Clangd is the preferred solution for interactive
Clang-based tooling. There has been great work on Clangd already, and we're
going to start investing effort as well to make Clangd faster, more
capable, and more efficient.

We have some high-level requirements for Clangd in order to make it
first-class on our platform and integrate it with our cross-language IDE
features:

- We need to support a completely different transport layer protocol in
Clangd. We would like to split the implementation of LSP into two layers:
the logical LSP layer and the JSON-RPC transport layer. This would allow us
to add support for Apple's XPC [1] technology as an alternative to the
existing LSP's JSON-RPC transport layer. The new transport layer will be
supported on Darwin only. We intend to carry the LSP payloads over XPC.

- We need to support extensions to the protocol beyond LSP's current
specification. The extension mechanism should allow us to add new protocol
entries and to extend the information provided in the existing requests and
responses. These LSP extensions will be supported in both the XPC and the
JSON-RPC transport layers.

This proposal presents an outline of our current plan for how these
requirements can be satisfied. We welcome the feedback that you might have
for us!

## Our Vision for the Transport Layer

The introduction of a new transport layer protocol is a big change to
Clangd's codebase.

In the long term, we envision a solution where Clangd has a clean and an
efficient split between the transport layers and LSP itself. This
separation would also allow other parties to adopt their own transport
layers in a straightforward manner. We would like to come up with a design
for transport layer separation in Clangd in the upcoming months. In the
meantime, we would love to receive the feedback that you might have for us.

## Initial XPC Bring-up

In the short term, in order to bring up the XPC transport layer more
quickly, we plan on utilizing the existing JSON-RPC serialization
infrastructure.  We'll overlay JSON on top of XPC, serializing the JSON
objects to/from XPC objects.  This is less efficient but will get us to
something testable more quickly.

## Initial investment efforts

We would like to invest our time into implementing some of the missing
features in Clangd and extending some of the existing features. Our initial
use-case will utilize a limited subset of LSP and we plan on focusing on
that subset.

We'll also need to ensure that Clangd works with our cross-language
indexer, which drives the IDE integration for global refactoring
operations. We'll go back to and continue upstreaming our Clang-based
refactoring infrastructure that we introduced in Xcode last year and will
ensure that it will be usable from Clangd. We intend to make the
refactorings (including global refactorings) work with both Clangd's
builtin indexer and our standalone cross-language indexer.

Best regards,
Alex

References:

[1]: https://developer.apple.com/documentation/xpc

P.S. Catch me and/or Jan Korous at EuroLLVM today if you'd like to discuss
this in person :)
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