Fix panic when using set-language on a scratch buffer

[zen3ger]

Skip launching a language server if a document doesn't have a valid URL.

Closes #1994

[kirawi]

Would this cause any problems with the LSP?

[zen3ger]

Yeah, you are right. It seems to require that the URI is a valid file path, which is a bit.. not good... Also the scatch buffer URIs have to be unique otherwise it seems to complain as well and get super confused about edits.

I've tried what happens if I generate a file URI as file://<current working dir>/scratch<document id> which seems to work and not cause any issues with the LSPs (tried rust-analyzer, clangd and erlang_ls).

Edit:
No, that's not entirely correct either. If you save a scratch buffer and the detected language ends up being the same as it the one set for the scratch buffer, then we won't close the dummy LSP session and won't start a new client for it, but the URI will no longer match the scratch one...

[zen3ger]

Alright, I think this covers everything. I found no protocol errors and the scratch connection is cleaned up.

2022-04-07T22:13:43.321 helix_lsp::transport [INFO] -> {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"languageId":"rust","text":"\n","uri":"file:///usr/home/zen3ger/project/helix/scratch1","version":0}}}
.
.
.
2022-04-07T22:14:06.535 helix_lsp::transport [INFO] -> {"jsonrpc":"2.0","method":"textDocument/didChange","params":{"contentChanges":[{"range":{"end":{"character":12,"line":1},"start":{"character":12,"line":1}},"text":";"}],"textDocument":{"uri":"file:///usr/home/zen3ger/project/helix/scratch1","version":25}}}
2022-04-07T22:14:20.113 helix_lsp::transport [INFO] -> {"jsonrpc":"2.0","method":"textDocument/didClose","params":{"textDocument":{"uri":"file:///usr/home/zen3ger/project/helix/scratch1"}}}
2022-04-07T22:14:20.168 helix_lsp::transport [INFO] -> {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"languageId":"c","text":"int main(void) {\n    return 0;\n}\n","uri":"file:///usr/home/zen3ger/project/helix/nothing.c","version":25}}}

[archseer]

Should we set language servers for scratch buffers or just ignore them instead until they get saved with a path?

How does vscode or neovim handle such cases? I don't think we should use fake filenames.

[smason]

Should we set language servers for scratch buffers or just ignore them instead until they get saved with a path?

ignoring until they get saved sounds better to me; the reason I tried this was to get syntax highlighting/tree-sitter and these would presumably still function without a language server

further, the scratch buffer might end up somewhere arbitrary, or only loosely related to the current session

[zen3ger]

Should we set language servers for scratch buffers or just ignore them instead until they get saved with a path?

Why didn't I think about the obvious solution...

How does vscode or neovim handle such cases?

If I'm not mistaken in neovim LSP is not started when you do syntax=<lang> or filetype to enable the highlighters unless you manually added a hook to do so.

[archseer]

Let's ignore scratch buffers then. refresh_language_server should return if the doc doesn't have a doc.url()