Function merge 

pub fn merge(
    elements: impl IntoIterator<Item = impl Into<ExprRef>>,
    nullability: Nullability,
) -> ExprRef

Creates an expression that merges struct expressions into a single struct.

Combines fields from all input expressions. If field names are duplicated, later expressions win. Fields are not recursively merged.

let expr = merge([get_item("a", root()), get_item("b", root())], Nullability::NonNullable);