Function build_where_predicates 

fn build_where_predicates(
    trait_where_clause: &Option<WhereClause>,
    extra_bounds: &ExtraBounds,
) -> Vec<TokenStream>

Build combined where predicates from the trait’s own where clause and computed extra bounds.

Merges the original trait-level where clause predicates with the extra bounds computed from method signatures (e.g., IntoR for return types, TryFromSexp for parameters containing trait type params).

Returns a flat list of predicates suitable for use in a where clause.