fix(lsp): add fallback for range completion edits

Fix off-by-one error with LSP completion edits beyond document end. Add
fallback to handle incorrect ranges and prevent index out of bounds
panic. Default to correct range first, retry with `end - 1` if
necessary.

helix-lsp/src/lib.rs

@@ -330,14 +330,24 @@ pub mod util {
         };
 
         let text = doc.slice(..);
-        let (removed_start, removed_end) = completion_range(
+        let (start, end) = completion_range(
             text,
             edit_offset,
             replace_mode,
             selection.primary().cursor(text),
         )
         .expect("transaction must be valid for primary selection");
-        let removed_text = text.slice(removed_start..removed_end);
+
+        // Some LSPs are sending completion edits beyond the end of the document. For example, pyright for python.
+        //
+        // In https://github.com/helix-editor/helix/pull/10279, there was the addition of a `+ 1` to fix an off by one
+        // error, but this has now ran into another for some LSP's that don't report the proper range, leading to an
+        // index out of bounds panic. The "solution" here is to default to the "correct" way first and if that too is
+        // out of bounds, it tries again with the `end - 1`, hopefully bringing it back into the correct range. If even
+        // that isn't correct, we panic as before.
+        let removed_text = text
+            .get_slice(start..end)
+            .map_or_else(|| text.slice(start..end - 1), |replaced| replaced);
 
         let (transaction, mut selection) = Transaction::change_by_selection_ignore_overlapping(
             doc,