Add support for C-famaily languages

[eddieh]

Add support for C-family languages and did some cosmetic cleanup to
match lsp-clangd.

[danielmartin]

Thanks for the patch! I'll give it a try this weekend. I think we need to add a bit of documentation about how to handle C++/Obj-C files when both sourcekit-lsp and clangd are installed.

For example, in a hybrid project Obj-C/Swift I'd expect lsp-sourcekit to handle the workspace. In a C++ project, I'd expect lsp-clangd to handle the workspace.

I think it should be possible to achieve this by tweaking the priorities of the lsp-clients from a .dir-locals.el file. We should mention how to do that in the documentation because it's a common scenario.

[danielmartin]

Instead of tweaking client priorities (which are not meant to be configurable per project), we can suggest configuring lsp-enabled-clients per project instead.

[eddieh]

I'll take a look at lsp-enabled-clients. I wonder if detecting the .xcodeproj file would be sufficient for picking sourcekit-lsp over clangd? That's what I was going to try in my personal configuration.

[danielmartin]

I'm thinking that, instead of using :major-modes, we could use :activation-fn with a custom function. Inside that custom function we can be more precise for when sourcekit-lsp should activate:

Require that the major mode is either swift-mode, c-mode, c++-mode, or objc-mode AND that the project contains a Package.swift or .xcodeproj/.xcworkspace file. You can use Emacs's locate-dominating-file function to do this.

I think this will do the right thing in almost all cases, without the user having to configure existing or new projects that contain C++ code. What do you think?

[eddieh]

That sounds like the best option. Thanks for the pointers.