[PATCH] D153114: [clangd] [C++20] [Modules] Support C++20 modules for clangd

[Sam McCall via Phabricator via cfe-commits]

sammccall added a comment.

Thanks for putting this together, I'm going to study it carefully and try it out!

That said, there are two large issues that I think should be addressed in the design (though not necessarily *implemented* now).
I'll be upfront: these are things without which $EMPLOYER will not be able to use this at all.
Since there isn't really room for multiple modules implementations in clangd, I'd like to make sure these fit into the design, or can be bolted on.

In the past, we've been reasonably successful at finding extension points that let clangd scale to unreasonable codebase sizes, while doing the right thing for smaller projects too. (Index, build system integration, VFS support, etc). To quote from the bug:

> you will need an alternative for them (luckily most of them already are willing to invest in custom tooling), but scanning all project files should still be fine for the vast majority of clangd users

For historical context: clangd **was** that custom tooling designed to scale to huge monorepos. This isn

---

**support for clang header modules**

Google has a large deployment of clangd, serving a codebase that builds significant parts as clang header modules for performance reasons. There is no near-term plan of adopting C++20 modules (multiple reasons, which I probably can't represent well).
We've been disabling header modules for clangd & other tools for a long time. But it's come to a head, and we expect to get someone working on enabling modules in clangd in ~2 months.
As I understand it, Meta are in a similar situation (though their solution is to version-lock clangd with the toolchain, keep modules enabled, and accept that some things are broken).

Our specific build system setup is that all module-build actions and inputs explicit (-fmodule-file=, -fmodule-map-file=). The build system will not produce usable PCMs due to version skew.
I know that Meta folks *would* like to make use of available PCMs from the build system.

---

**support for large projects**

We have multiple codebases large enough that touching every file to discover dependencies isn't feasible. 
The largest internal one had 2B LOC 7 years ago, and is now much larger. But even Chrome for example has 20M. Such projects are prone to adopt modules (in some form) for build scalability.
Apart from the concrete scanning of deps, keeping the full project module graph in memory won't always be possible. (It's a perfectly reasonable default implementation though).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153114/new/

https://reviews.llvm.org/D153114