Add richer LSP server example (#20017)

[shashi1687]

Summary

Adds a new “head-start” LSP server example inside crates/lsp-server/examples/. This fresh example shows how to:

• Log document edits (textDocument/didChange)
• Provide a stubbed completion handler (returns an empty list)
• Provide a stubbed formatting handler (returns no edits)

Details

1. Setup

   • Launches JSON-RPC over stdio
   • Advertises full-text sync, completion, and formatting capabilities

2. Message Loop

   • Logs each didChange URI
   • Responds to completion with {"isIncomplete":false,"items":[]}
   • Responds to formatting with []
   • Cleans up on shutdown/exit

This minimal template makes it easy to embed rust-analyzer as an LSP server with basic completion and formatting hooks.

---

Testing & Validation

1. Build

   cargo build -p rust-analyzer --example rust_analyzer_lsp_example

[ShoyuVanilla]

I think this example should be located under lib/lsp-server/examples/ like the previous one

[shashi1687]

Thanks for catching that! You’re absolutely right—I’ve now moved the example into lib/lsp-server/examples/ alongside goto_def.rs. The file is renamed to completion_and_formatting.rs, and I’ve updated the crates/lsp-server/Cargo.toml manifest accordingly. Let me know if you spot anything else!

[ShoyuVanilla]

Well, I think that the original intention of the OP in #20017 is asking opinions about adding their existing lsp-server example codes to rust-analyzer repository, not implementing new examples, though I don't have strong opinion on it.

What the example does

...
This gives anyone embedding rust-analyzer a minimal “plug-in” template for completions and formatting.

Besides that, I think this example is not that "richer" than existing goto_def.rs one because most of the scaffolding codes are almost same, and I can't really understand what is the intention of the message handling examples in this PR: It's just a personal thought but I think it would be better to be somewhat more exhaustive on different kinds of messages or more focused or detailed on a specific category of messages.
In short, I'd like to ask "What would be the proper filename for this when located in the same directory with goto_def.rs example?"

[lnicola]

Yeah, I think a single example would be better.

[shashi1687]

Well, I think that the original intention of the OP in #20017 is asking opinions about adding their existing lsp-server example codes to rust-analyzer repository, not implementing new examples, though I don't have strong opinion on it.

What the example does

...
This gives anyone embedding rust-analyzer a minimal “plug-in” template for completions and formatting.

Besides that, I think this example is not that "richer" than existing goto_def.rs one because most of the scaffolding codes are almost same, and I can't really understand what is the intention of the message handling examples in this PR: It's just a personal thought but I think it would be better to be somewhat more exhaustive on different kinds of messages or more focused or detailed on a specific category of messages. In short, I'd like to ask "What would be the proper filename for this when located in the same directory with goto_def.rs example?"

Thank you so much for raising these points—I really appreciate your insight! 🙏
You’re absolutely right that a richer sample could handle additional messages (hover, semantic tokens, etc.), and I think that would make a great follow-up PR once this one is settled.

As for the filename, would you be comfortable with completion_and_formatting.rs? It follows our snake_case style and clearly indicates the two features it covers, sitting neatly beside goto_def.rs. If you’d prefer a different name or any other tweaks, I’m more than happy to update—just let me know what you’d like!

[shashi1687]

Yeah, I think a single example would be better.

Thank you again for all your feedback! 🙏

At this point, would you prefer that I close this PR, or is there anything else you’d like me to adjust before we merge it? Please let me know what you’d recommend, and I’ll act accordingly.

[ShoyuVanilla]

@lnicola and I pondered upon this and we think in current status, this would be best to be merged into existing goto_def.rs as they share much scaffolding codes in common - of course, the filename goto_def.rs should be adjusted`.

I'll comment on some details though if you take the "merging" suggestion most of them wouldn't be needed 😅

[ShoyuVanilla]

We don't need this because the new example code is located inside examples/

[ShoyuVanilla]

"embed rust-analyzer" is not a right term here. This lsp-server is just a crate that rust-analyzer dogfoods on to handle lsp requests and responses.
So, the examples are not for rust-analyer or embedding. They just show how to use this "library"