Add language_server_workspace_configuration to extension API (#10212)

This PR adds the ability for extensions to implement
`language_server_workspace_configuration` to provide workspace
configuration to the language server.

We've used the Dart extension as a motivating example for this, pulling
it out into an extension in the process.

Release Notes:

- Removed built-in support for Dart, in favor of making it available as
an extension. The Dart extension will be suggested for download when you
open a `.dart` file.

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

extensions/dart/src/dart.rs

@@ -0,0 +1,123 @@
+use zed::lsp::CompletionKind;
+use zed::settings::LspSettings;
+use zed::{CodeLabel, CodeLabelSpan};
+use zed_extension_api::{self as zed, serde_json, Result};
+
+struct DartExtension;
+
+impl zed::Extension for DartExtension {
+    fn new() -> Self {
+        Self
+    }
+
+    fn language_server_command(
+        &mut self,
+        _language_server_id: &zed::LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        let path = worktree
+            .which("dart")
+            .ok_or_else(|| "dart must be installed from dart.dev/get-dart".to_string())?;
+
+        Ok(zed::Command {
+            command: path,
+            args: vec!["language-server".to_string(), "--protocol=lsp".to_string()],
+            env: Default::default(),
+        })
+    }
+
+    fn language_server_workspace_configuration(
+        &mut self,
+        _language_server_id: &zed::LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<Option<serde_json::Value>> {
+        let settings = LspSettings::for_worktree("dart", worktree)
+            .ok()
+            .and_then(|lsp_settings| lsp_settings.settings.clone())
+            .unwrap_or_default();
+
+        Ok(Some(serde_json::json!({
+            "dart": settings
+        })))
+    }
+
+    fn label_for_completion(
+        &self,
+        _language_server_id: &zed::LanguageServerId,
+        completion: zed::lsp::Completion,
+    ) -> Option<CodeLabel> {
+        let arrow = " → ";
+
+        match completion.kind? {
+            CompletionKind::Class => Some(CodeLabel {
+                filter_range: (0..completion.label.len()).into(),
+                spans: vec![CodeLabelSpan::literal(
+                    completion.label,
+                    Some("type".into()),
+                )],
+                code: String::new(),
+            }),
+            CompletionKind::Function | CompletionKind::Constructor | CompletionKind::Method => {
+                let mut parts = completion.detail.as_ref()?.split(arrow);
+                let (name, _) = completion.label.split_once('(')?;
+                let parameter_list = parts.next()?;
+                let return_type = parts.next()?;
+                let fn_name = " a";
+                let fat_arrow = " => ";
+                let call_expr = "();";
+
+                let code =
+                    format!("{return_type}{fn_name}{parameter_list}{fat_arrow}{name}{call_expr}");
+
+                let parameter_list_start = return_type.len() + fn_name.len();
+
+                Some(CodeLabel {
+                    spans: vec![
+                        CodeLabelSpan::code_range(
+                            code.len() - call_expr.len() - name.len()..code.len() - call_expr.len(),
+                        ),
+                        CodeLabelSpan::code_range(
+                            parameter_list_start..parameter_list_start + parameter_list.len(),
+                        ),
+                        CodeLabelSpan::literal(arrow, None),
+                        CodeLabelSpan::code_range(0..return_type.len()),
+                    ],
+                    filter_range: (0..name.len()).into(),
+                    code,
+                })
+            }
+            CompletionKind::Property => {
+                let class_start = "class A {";
+                let get = " get ";
+                let property_end = " => a; }";
+                let ty = completion.detail?;
+                let name = completion.label;
+
+                let code = format!("{class_start}{ty}{get}{name}{property_end}");
+                let name_start = class_start.len() + ty.len() + get.len();
+
+                Some(CodeLabel {
+                    spans: vec![
+                        CodeLabelSpan::code_range(name_start..name_start + name.len()),
+                        CodeLabelSpan::literal(arrow, None),
+                        CodeLabelSpan::code_range(class_start.len()..class_start.len() + ty.len()),
+                    ],
+                    filter_range: (0..name.len()).into(),
+                    code,
+                })
+            }
+            CompletionKind::Variable => {
+                let name = completion.label;
+
+                Some(CodeLabel {
+                    filter_range: (0..name.len()).into(),
+                    spans: vec![CodeLabelSpan::literal(name, Some("variable".into()))],
+                    code: String::new(),
+                })
+            }
+            _ => None,
+        }
+    }
+}
+
+zed::register_extension!(DartExtension);