Fix panic when using set-language on a scratch buffer

This commit adds a dummy URL for scratch buffers.

* Add Document::scratch_url() to generate a dummy URL for the scratch buffer.
  The format of the URL is `file://<current working dir>/scratch<document id>`.
  The scheme is `file://` in order to confrom to LSP DocumentUri format and
  the DocumentUri on the LSP side is made unique using the document id.
* Send textDocument/didClose notification to LSP server when a scratch buffer
  is written as from that point on it will have a valid URL and the dummy
  connection is no longer needed.

helix-term/src/commands/typed.rs

@@ -193,6 +193,7 @@ fn buffer_previous(
 fn write_impl(cx: &mut compositor::Context, path: Option<&Cow<str>>) -> anyhow::Result<()> {
     let jobs = &mut cx.jobs;
     let doc = doc_mut!(cx.editor);
+    let doc_was_scratch = doc.path().is_none();
 
     if let Some(ref path) = path {
         doc.set_path(Some(path.as_ref().as_ref()))
@@ -217,6 +218,9 @@ fn write_impl(cx: &mut compositor::Context, path: Option<&Cow<str>>) -> anyhow::
 
     if path.is_some() {
         let id = doc.id();
+        if doc_was_scratch {
+            let _ = doc.no_longer_scratch();
+        }
         doc.detect_language(cx.editor.syn_loader.clone());
         let _ = cx.editor.refresh_language_server(id);
     }

helix-view/src/document.rs

@@ -506,6 +506,17 @@ impl Document {
         }
     }
 
+    // TODO: better name
+    pub fn no_longer_scratch(&mut self) -> Option<()> {
+        if let Some(language_server) = self.language_server() {
+            let scratch_url = Document::scratch_url(self.id()).ok()?;
+            let identifier = lsp::TextDocumentIdentifier::new(scratch_url);
+            tokio::spawn(language_server.text_document_did_close(identifier));
+            self.language_server = None;
+        }
+        Some(())
+    }
+
     /// Detect the programming language based on the file type.
     pub fn detect_language(&mut self, config_loader: Arc<syntax::Loader>) {
         if let Some(path) = &self.path {
@@ -902,7 +913,17 @@ impl Document {
 
     /// File path as a URL.
     pub fn url(&self) -> Option<Url> {
-        Url::from_file_path(self.path()?).ok()
+        if let Some(ref path) = self.path {
+            Url::from_file_path(path).ok()
+        } else {
+            Document::scratch_url(self.id()).ok()
+        }
+    }
+
+    fn scratch_url(doc_id: DocumentId) -> Result<Url, ()> {
+        let cwd = std::env::current_dir().map_err(|_| ())?;
+        let path = cwd.join(format!("scratch{}", doc_id));
+        Url::from_file_path(path)
     }
 
     #[inline]