Function plpgsql_cast 

pub fn plpgsql_cast(
    env: &dyn Env,
    source: (u32, i32),
    target: u32,
    typmod: i32,
) -> Result<Query>

get_cast_hashentry: the query SELECT $1 that casts a value of the type source to the type target with the typmod typmod, in the context of PL/pgSQL, where the output and the input functions cast a value when no other cast exists.

Errors

42846 when there is no cast, as for a type that is internal.