Fix crash when LSP returns multiple nested definition ranges (#52701)

This fixes a crash when navigating to multiple definitions in a
singleton buffer (https://github.com/zed-industries/zed/pull/51461).

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -18219,6 +18219,20 @@ impl Editor {
             for ranges in locations.values_mut() {
                 ranges.sort_by_key(|range| (range.start, Reverse(range.end)));
                 ranges.dedup();
+                // Merge overlapping or contained ranges. After sorting by
+                // (start, Reverse(end)), we can merge in a single pass:
+                // if the next range starts before the current one ends,
+                // extend the current range's end if needed.
+                let mut i = 0;
+                while i + 1 < ranges.len() {
+                    if ranges[i + 1].start <= ranges[i].end {
+                        let merged_end = ranges[i].end.max(ranges[i + 1].end);
+                        ranges[i].end = merged_end;
+                        ranges.remove(i + 1);
+                    } else {
+                        i += 1;
+                    }
+                }
                 let fits_in_one_excerpt = ranges
                     .iter()
                     .tuple_windows()

crates/editor/src/editor_tests.rs

@@ -25403,6 +25403,55 @@ async fn test_goto_definition_far_ranges_open_multibuffer(cx: &mut TestAppContex
     });
 }
 
+#[gpui::test]
+async fn test_goto_definition_contained_ranges(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+    let mut cx = EditorLspTestContext::new_rust(
+        lsp::ServerCapabilities {
+            definition_provider: Some(lsp::OneOf::Left(true)),
+            ..lsp::ServerCapabilities::default()
+        },
+        cx,
+    )
+    .await;
+
+    // The LSP returns two single-line definitions on the same row where one
+    // range contains the other. Both are on the same line so the
+    // `fits_in_one_excerpt` check won't underflow, and the code reaches
+    // `change_selections`.
+    cx.set_state(
+        &r#"fn caller() {
+            let _ = ˇtarget();
+        }
+        fn target_outer() { fn target_inner() {} }
+        "#
+        .unindent(),
+    );
+
+    // Return two definitions on the same line: an outer range covering the
+    // whole line and an inner range for just the inner function name.
+    cx.set_request_handler::<lsp::request::GotoDefinition, _, _>(move |url, _, _| async move {
+        Ok(Some(lsp::GotoDefinitionResponse::Array(vec![
+            // Inner range: just "target_inner" (cols 23..35)
+            lsp::Location {
+                uri: url.clone(),
+                range: lsp::Range::new(lsp::Position::new(3, 23), lsp::Position::new(3, 35)),
+            },
+            // Outer range: the whole line (cols 0..48)
+            lsp::Location {
+                uri: url,
+                range: lsp::Range::new(lsp::Position::new(3, 0), lsp::Position::new(3, 48)),
+            },
+        ])))
+    });
+
+    let navigated = cx
+        .update_editor(|editor, window, cx| editor.go_to_definition(&GoToDefinition, window, cx))
+        .await
+        .expect("Failed to navigate to definitions");
+    assert_eq!(navigated, Navigated::Yes);
+}
+
 #[gpui::test]
 async fn test_find_all_references_editor_reuse(cx: &mut TestAppContext) {
     init_test(cx, |_| {});