trim_to_len in prover does not actually trim anything.

[qope]

The code quotient_poly.trim_to_len(quotient_degree) in plonk/prover.rs does not actually trim anything because the length of quotient_poly is always equal to quotient_degree.

https://github.com/mir-protocol/plonky2/blob/eebdd02972fc98c3b0c6082f51d7a807cd8fcdf5/plonky2/src/plonk/prover.rs#L143

Therefore, even if the vanishing polynomial is not divisible by Z_H, the assertion in trim_to_len is ignored and an invalid proof will be generated.

Something like,

quotient_poly.assert_tail_zeros(common_data.quotient_degree_factor).expect(
     "Quotient has failed, the vanishing polynomial is not divisible by Z_H",
);

and

pub fn assert_tail_zeros(&self, n: usize) -> Result<()> {
    ensure!(self.coeffs.iter().rev().take(n).all(F::is_zero));
    Ok(())
}

is desirable.