lsp: Check server provider capabilities (#3554)

Language Servers may signal that they do not support a method in
the initialization result (server capabilities). We can check these
when making LSP requests and hint in the status line when a method
is not supported by the server. This can also prevent crashes in
servers which assume that clients do not send requests for methods
which are disabled in the server capabilities.

There is an existing pattern the LSP client module where a method
returns `Option<impl Future<Output = Result<_>>>` with `None` signaling
no support in the server. This change extends this pattern to the rest
of the client functions. And we log an error to the statusline for
manually triggered LSP calls which return `None`.

1 files changed, 19 insertions, 10 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index 5b44775b..8af5a7e3 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -3801,15 +3801,21 @@ fn format_selections(cx: &mut Context) {
 
     let range = ranges[0];
 
-    let edits = tokio::task::block_in_place(|| {
-        helix_lsp::block_on(language_server.text_document_range_formatting(
-            doc.identifier(),
-            range,
-            lsp::FormattingOptions::default(),
-            None,
-        ))
-    })
-    .unwrap_or_default();
+    let request = match language_server.text_document_range_formatting(
+        doc.identifier(),
+        range,
+        lsp::FormattingOptions::default(),
+        None,
+    ) {
+        Some(future) => future,
+        None => {
+            cx.editor
+                .set_error("Language server does not support range formatting");
+            return;
+        }
+    };
+
+    let edits = tokio::task::block_in_place(|| helix_lsp::block_on(request)).unwrap_or_default();
 
     let transaction = helix_lsp::util::generate_transaction_from_edits(
         doc.text(),
@@ -3953,7 +3959,10 @@ pub fn completion(cx: &mut Context) {
 
     let pos = pos_to_lsp_pos(doc.text(), cursor, offset_encoding);
 
-    let future = language_server.completion(doc.identifier(), pos, None);
+    let future = match language_server.completion(doc.identifier(), pos, None) {
+        Some(future) => future,
+        None => return,
+    };
 
     let trigger_offset = cursor;