Change Rust language server to rust-analyzer and drop RLS support

[skangas]

As discussed in #800.

rust-analyzer is the officially blessed Language Server for Rust:
rust-lang/rfcs#2912

Also drop the special support code for RLS.

• eglot.el (eglot-server-programs): Change rust-mode language server
  from rls to rust-analyzer.
  (eglot-rls, jsonrpc-connection-ready-p)
  (eglot-handle-notification): Delete.
• README.md: Update references for RLS to point to rust-analyzer.

[skangas]

@joaotavora WDYT?

[joaotavora]

Seems very good, but not enough. There is a test in eglot-tests.el based on rls that would have to be changed to rely on some other server (not necessarily rust).

[joaotavora]

Also, we should somehow make sure that rust-analyzer IS supported. I'm reasonably sure I've seen issues around here doing that and confirming check, but we should double check.

[theothornhill]

I've used rust-analyzer with eglot reliably at work

[skangas]

@theothornhill That's useful information, thanks.

Updating the tests was pretty trivial, see the updated PR. Everything passes here, of course.

[nemethf]

At first glance, it seems the deleted code is not strictly necessary for rls to work to some extent. So maybe it should be (eglot-alternatives '("rust-analyzer" "rls"))? Then the change is a more graceful deprecation. But I haven't actually tried what happens when eglot doesn't wait for the indexing to finish. So, to contradict myself, maybe half supporting rls isn't worth the trouble.

[joaotavora]

But I haven't actually tried what happens when eglot doesn't wait for the indexing to finish.

If I remember correctly, typing something quickly and then waiting for completions failed, because the completion provided wasn't aware of the things you had typed in the meantime, because indexing wasn't done. Seems a problem most other LSP servers would have, and RLS was the only one that had that particular quirk (which they solved in their VSCode extension, too).

(yes they had a VSCode "reference implementation"... so much for LSP 🤷 )

[skangas]

IIUC, rls would need this workaround because they compile everything first, and then use that data to provide LSP completions, etc. Whereas rust-analyzer compiles things on the fly. So if we want to keep rls around, I think we should also keep that extra supporting code as well.

[nemethf]

Having removed the rls specific code from Eglot I opened a very simple rust file and xref-find-definitions worked. So I think keeping rls at the end of the alternatives list is somewhat useful in some cases even without extra supporting code. But I'm also OK with completely removing the rls support.

[skangas]

I've pushed a new version which keeps rls in eglot-server-programs, but removes the support code.

[joaotavora]

I think this is good to go. We should to something about the rls -using test later. But I don't think that test relies on the RLS's support code either.

[skangas]

I think this is good to go. We should to something about the rls -using test later. But I don't think that test relies on the RLS's support code either.

Unless I'm missing something, I've changed those tests to use rust-analyzer.

[joaotavora]

Unless I'm missing something, I've changed those tests to use rust-analyzer.

Clearly, I was missing something :-D