dart: Respect LSP binary settings (#17494)

Enable configuring Dart's LSP from other means of installation types.

Some users don't install the `dart` binary, but uses version manager.

In the example, I uses [FVM](https://fvm.app/) (short for "Flutter
Version Manager").

I have tested this with "Install Dev Extensions".

Release Notes:

- N/A

cc other maintainer: @agent3bood @flo80

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/languages/dart.md

@@ -5,6 +5,23 @@ Dart support is available through the [Dart extension](https://github.com/zed-in
 - Tree Sitter: [UserNobody14/tree-sitter-dart](https://github.com/UserNobody14/tree-sitter-dart)
 - Language Server: [dart language-server](https://github.com/dart-lang/sdk)
 
+## Configuration
+
+The `dart` binary can be configured in a Zed settings file with:
+
+```json
+{
+  "lsp": {
+    "dart": {
+      "binary": {
+        "path": "/opt/homebrew/bin/fvm",
+        "arguments": ["dart", "language-server", "--protocol=lsp"]
+      }
+    }
+  }
+}
+```
+
 <!--
 TBD: Document Dart. pubspec.yaml
 - https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/tool/lsp_spec/README.md

extensions/dart/extension.toml

@@ -3,7 +3,7 @@ name = "Dart"
 description = "Dart support."
 version = "0.0.3"
 schema_version = 1
-authors = ["Abdullah Alsigar <abdullah.alsigar@gmail.com>", "Flo <flo80@users.noreply.github.com>"]
+authors = ["Abdullah Alsigar <abdullah.alsigar@gmail.com>", "Flo <flo80@users.noreply.github.com>", "ybbond <hi@ybbond.id>"]
 repository = "https://github.com/zed-industries/zed"
 
 [language_servers.dart]

extensions/dart/src/dart.rs

@@ -3,8 +3,47 @@ use zed::settings::LspSettings;
 use zed::{CodeLabel, CodeLabelSpan};
 use zed_extension_api::{self as zed, serde_json, Result};
 
+struct DartBinary {
+    pub path: String,
+    pub args: Option<Vec<String>>,
+}
+
 struct DartExtension;
 
+impl DartExtension {
+    fn language_server_binary(
+        &mut self,
+        _language_server_id: &zed::LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<DartBinary> {
+        let binary_settings = LspSettings::for_worktree("dart", worktree)
+            .ok()
+            .and_then(|lsp_settings| lsp_settings.binary);
+        let binary_args = binary_settings
+            .as_ref()
+            .and_then(|binary_settings| binary_settings.arguments.clone());
+
+        if let Some(path) = binary_settings.and_then(|binary_settings| binary_settings.path) {
+            return Ok(DartBinary {
+                path,
+                args: binary_args,
+            });
+        }
+
+        if let Some(path) = worktree.which("dart") {
+            return Ok(DartBinary {
+                path,
+                args: binary_args,
+            });
+        }
+
+        Err(
+            "dart must be installed from dart.dev/get-dart or pointed to by the LSP binary settings"
+                .to_string(),
+        )
+    }
+}
+
 impl zed::Extension for DartExtension {
     fn new() -> Self {
         Self
@@ -12,16 +51,16 @@ impl zed::Extension for DartExtension {
 
     fn language_server_command(
         &mut self,
-        _language_server_id: &zed::LanguageServerId,
+        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())?;
+        let dart_binary = self.language_server_binary(language_server_id, worktree)?;
 
         Ok(zed::Command {
-            command: path,
-            args: vec!["language-server".to_string(), "--protocol=lsp".to_string()],
+            command: dart_binary.path,
+            args: dart_binary.args.unwrap_or_else(|| {
+                vec!["language-server".to_string(), "--protocol=lsp".to_string()]
+            }),
             env: Default::default(),
         })
     }