Function duckdb_expression_fold 

pub unsafe extern "C" fn duckdb_expression_fold(
    context: duckdb_client_context,
    expr: duckdb_expression,
    out_value: *mut duckdb_value,
) -> duckdb_error_data

Folds an expression creating a folded value.

@param context The client context. @param expr The expression. Must be foldable. @param out_value The folded value, if folding was successful. Must be destroyed with duckdb_destroy_value. @return The error data. Must be destroyed with duckdb_destroy_error_data.