Create a LSP server for Ante

[jfecher]

Ante needs a Language Server Protocol server to give errors and warnings as users write their programs in LSP compatible editors.

This is obviously quite a large task so it can be broken up into as many smaller tasks as desired. Here's some examples:

• Create a LSP server skeleton that runs whenever Ante code is opened in an editor
• Show errors & warnings when a file is saved
• Implement go to definition
• Implement go to declaration
• Implement "hover" action to show a variable's type.

To limit scope somewhat, I can help with any changes needed in the compiler. I also think the LSP's design should start as simple as possible, e.g. only updating when a file is saved to avoid worrying about partially correct programs for now. Ante's current parser is not recoverable so it will not work well on partial files.

[ehllie]

Should that be done inside this repository as a separate workspace, or would you prefer having the language server in a separate repository completely? In terms of implementing the language server, I think using tower-lsp and the tower-lsp-boilerplate should offer the quickest way to get a functional language server. Are you ok with using a library like that to abstract the lower levels of the language server protocol?

[jfecher]

@ehllie I think a separate repository might be best. I'm absolutely in favor of using any libraries for the LSP to make things easier. You can take a look at https://github.com/jfecher/ante-lsp for a possible good starting point (and place to merge things into). It uses tower-lsp and was forked from the boilerplate repo. I haven't looked at it in a while though so I doubt it's in a working state.

[ehllie]

I've noticed a couple things that would need to be done for the language server to be effective:

• Make name resolution and type checking steps produce a list of errors, warnings and notes rather than printing them directly to stderr or panicking.
• Make the AST not require a lifetime parameter. The lifetime of AST being tied to the lifetime of the Path reference makes it awkward to work with in a language server context, since the filename is created from the request, which only lives inside the body of a function that receives it. If we want the AST to live longer in the cache it needs to take ownership of the path.
• Fix the unsafe impl Send for ... lines? They are marked with a comment saying TODO: Remove, and it's usually not the best idea to lie to the compiler 😛. Not sure how much of an issue it could be though.