Function merge_union_completion_items

src/completion/builder.rs:736–811

Partition and sort completion items by union membership. When a variable has a union type (`num_candidates > 1`), members present on **all** candidate types (intersection members) are more likely to be type-safe. This function: 1. Partitions items into intersection and branch-only based on `occurrence_count` vs `num_candidates`. 2. Sorts each partition alphabetically by `filter_text` / `label`.

(
    items: Vec<CompletionItem>,
    occurrence_count: HashMap<String, usize>,
    num_candidates: usize,
)

/// When `num_candidates <= 1`, returns `items` unchanged (the items
/// already have correct `sort_text` from [`build_completion_items`]).
pub(crate) fn merge_union_completion_items(
    items: Vec<CompletionItem>,
    occurrence_count: HashMap<String, usize>,
    num_candidates: usize,
) -> Vec<CompletionItem> {
    if num_candidates <= 1 {
        return items;
    }

    let sort_key = |item: &CompletionItem| -> (u8, String) {
        (
            kind_sort_tier(item.kind),
            item.filter_text
                .as_deref()
                .unwrap_or(&item.label)
                .to_lowercase(),
        )
    };

    let mut intersection: Vec<CompletionItem> = Vec::new();
    let mut branch_only: Vec<CompletionItem> = Vec::new();

    for item in items {
        let count = occurrence_count.get(&item.label).copied().unwrap_or(1);
        if count >= num_candidates {
            intersection.push(item);
        } else {
            branch_only.push(item);
        }
    }

    intersection.sort_by_key(|item| sort_key(item));
    branch_only.sort_by_key(|item| sort_key(item));

    // Assign sort_text: "0_NNNNN" for intersection, "1_NNNNN" for
    // branch-only.
    let mut result = Vec::with_capacity(intersection.len() + branch_only.len());

    for (i, mut item) in intersection.into_iter().enumerate() {
        item.sort_text = Some(format!("0_{:05}", i));
        // Update description to show all contributing class names
        // (the initial description only has the first candidate).
        if let Some(class_names) = class_names_from_data(&item) {
            if let Some(ref mut ld) = item.label_details {
                ld.description = Some(class_names);
            } else {
                item.label_details = Some(CompletionItemLabelDetails {
                    detail: None,
                    description: Some(class_names),
                });
            }
        }
        result.push(item);
    }

    for (i, mut item) in branch_only.into_iter().enumerate() {
        item.sort_text = Some(format!("1_{:05}", i));
        // Add label_details showing the originating class(es) so the