Add deprecation warning (#520)

## Summary

This PR adds a deprecation warning during the initialization request if
the `ruff` executable found is `>= 0.3.5`.

Refer to
#520 (comment)
for the discussion.

### Preview

**VS Code**

It'll provide a warning notification along with logging the message:

<img width="2560" alt="Screenshot 2025-02-03 at 11 21 29 AM"
src="https://github.com/user-attachments/assets/3745399a-ea7c-4f74-9d6f-689ecbd435b5"
/>

**Neovim**

It seems like Neovim has already deprecated `ruff_lsp` if the user is
using [`nvim-lspconfig`](https://github.com/neovim/nvim-lspconfig)
(which should be most of the users):

<img width="546" alt="Screenshot 2025-02-03 at 11 31 50 AM"
src="https://github.com/user-attachments/assets/59f70e2b-a95e-4022-be61-d87a276a1a93"
/>

But, regardless, the users should receive the warning if they've
configured it to do so:

<img width="1280" alt="Screenshot 2025-02-03 at 11 30 54 AM"
src="https://github.com/user-attachments/assets/c824ee80-f836-4dba-8515-694f8b4b6048"
/>

README.md

@@ -1,3 +1,16 @@
+> [!WARNING]
+>
+> **`ruff-lsp` is deprecated. Please switch to the native language server
+> (`ruff server`). Refer to the [setup
+> guide](https://docs.astral.sh/ruff/editors/setup/) on how to set up the
+> native language server and the [migration
+> guide](https://docs.astral.sh/ruff/editors/migration/) on how to migrate the
+> settings.**
+>
+> **Feel free to comment on the [GitHub
+> discussion](https://github.com/astral-sh/ruff/discussions/15991) to ask
+> questions or share feedback.**
+
 # ruff-lsp
 
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)

ruff_lsp/server.py

@@ -151,6 +151,9 @@ class VersionModified(NamedTuple):
 VERSION_REQUIREMENT_OUTPUT_FORMAT = SpecifierSet(">=0.0.291")
 # Version requirement after which Ruff avoids writing empty output for excluded files.
 VERSION_REQUIREMENT_EMPTY_OUTPUT = SpecifierSet(">=0.1.6")
+# Version requirement to display the deprecation warning for `ruff-lsp` and start
+# recommending `ruff server` instead.
+VERSION_REQUIREMENT_NATIVE_SERVER = SpecifierSet(">=0.3.5")
 
 # Arguments provided to every Ruff invocation.
 CHECK_ARGS = [
@@ -1613,6 +1616,24 @@ def initialize(params: InitializeParams) -> None:
 
     _update_workspace_settings(settings)
 
+    # Use the Ruff executable from the first workspace to determine when to show the
+    # deprecation warning.
+    executable = _find_ruff_binary(
+        next(iter(WORKSPACE_SETTINGS.values())), version_requirement=None
+    )
+
+    if VERSION_REQUIREMENT_NATIVE_SERVER.contains(executable.version, prereleases=True):
+        show_warning(
+            "`ruff-lsp` is deprecated. Please switch to the native language server "
+            "(`ruff server`). Refer to the "
+            "[setup guide](https://docs.astral.sh/ruff/editors/setup/) on how to set "
+            "up the native language server and the "
+            "[migration guide](https://docs.astral.sh/ruff/editors/migration/) on how "
+            "to migrate the settings. Feel free to comment on the "
+            "[GitHub discussion](https://github.com/astral-sh/ruff/discussions/15991) "
+            "to ask questions or share feedback."
+        )
+
 
 def _supports_code_action_resolve(capabilities: ClientCapabilities) -> bool:
     """Returns True if the client supports codeAction/resolve request for edits."""
@@ -1995,6 +2016,12 @@ def show_error(message: str) -> None:
     LSP_SERVER.show_message(message, MessageType.Error)
 
 
+def show_warning(message: str) -> None:
+    """Show a pop-up with a warning."""
+    LSP_SERVER.show_message_log(message, MessageType.Warning)
+    LSP_SERVER.show_message(message, MessageType.Warning)
+
+
 def log_warning(message: str) -> None:
     LSP_SERVER.show_message_log(message, MessageType.Warning)
     if os.getenv("LS_SHOW_NOTIFICATION", "off") in ["onWarning", "always"]: