Extract Elm language into an extension (#10432)

Release Notes:

- Extracted Elm language support into an extension

Co-authored-by: Marshall <marshall@zed.dev>

extensions/elm/src/elm.rs

@@ -0,0 +1,113 @@
+use std::{env, fs};
+use zed::{
+    serde_json::{self, Value},
+    settings::LspSettings,
+};
+use zed_extension_api::{self as zed, Result};
+
+const SERVER_PATH: &str = "node_modules/@elm-tooling/elm-language-server/out/node/index.js";
+const PACKAGE_NAME: &str = "@elm-tooling/elm-language-server";
+
+struct ElmExtension {
+    did_find_server: bool,
+}
+
+impl ElmExtension {
+    fn server_exists(&self) -> bool {
+        fs::metadata(SERVER_PATH).map_or(false, |stat| stat.is_file())
+    }
+
+    fn server_script_path(&mut self, server_id: &zed::LanguageServerId) -> Result<String> {
+        let server_exists = self.server_exists();
+        if self.did_find_server && server_exists {
+            return Ok(SERVER_PATH.to_string());
+        }
+
+        zed::set_language_server_installation_status(
+            &server_id,
+            &zed::LanguageServerInstallationStatus::CheckingForUpdate,
+        );
+        let version = zed::npm_package_latest_version(PACKAGE_NAME)?;
+
+        if !server_exists
+            || zed::npm_package_installed_version(PACKAGE_NAME)?.as_ref() != Some(&version)
+        {
+            zed::set_language_server_installation_status(
+                &server_id,
+                &zed::LanguageServerInstallationStatus::Downloading,
+            );
+            let result = zed::npm_install_package(PACKAGE_NAME, &version);
+            match result {
+                Ok(()) => {
+                    if !self.server_exists() {
+                        Err(format!(
+                            "installed package '{PACKAGE_NAME}' did not contain expected path '{SERVER_PATH}'",
+                        ))?;
+                    }
+                }
+                Err(error) => {
+                    if !self.server_exists() {
+                        Err(error)?;
+                    }
+                }
+            }
+        }
+
+        self.did_find_server = true;
+        Ok(SERVER_PATH.to_string())
+    }
+}
+
+impl zed::Extension for ElmExtension {
+    fn new() -> Self {
+        Self {
+            did_find_server: false,
+        }
+    }
+
+    fn language_server_command(
+        &mut self,
+        server_id: &zed::LanguageServerId,
+        _worktree: &zed::Worktree,
+    ) -> Result<zed::Command> {
+        let server_path = self.server_script_path(server_id)?;
+        Ok(zed::Command {
+            command: zed::node_binary_path()?,
+            args: vec![
+                env::current_dir()
+                    .unwrap()
+                    .join(&server_path)
+                    .to_string_lossy()
+                    .to_string(),
+                "--stdio".to_string(),
+            ],
+            env: Default::default(),
+        })
+    }
+
+    fn language_server_workspace_configuration(
+        &mut self,
+        server_id: &zed::LanguageServerId,
+        worktree: &zed::Worktree,
+    ) -> Result<Option<Value>> {
+        // elm-language-server expects workspace didChangeConfiguration notification
+        // params to be the same as lsp initialization_options
+        let initialization_options = LspSettings::for_worktree(server_id.as_ref(), worktree)?
+            .initialization_options
+            .clone()
+            .unwrap_or_default();
+
+        Ok(Some(match initialization_options.clone().as_object_mut() {
+            Some(op) => {
+                // elm-language-server requests workspace configuration
+                // for the `elmLS` section, so we have to nest
+                // another copy of initialization_options there
+                op.insert("elmLS".into(), initialization_options);
+                serde_json::to_value(op).unwrap_or_default()
+            }
+            None => initialization_options,
+        }))
+    }
+}
+
+zed::register_extension!(ElmExtension);