DbTxn must_use doesn't work

[kayabaNerve]

This is presumably somewhat a bug in Rust itself. I just need to make a MRE and evaluate it.

Identified due to #561.

[kayabaNerve]

rust-lang/rust#102238 (comment) for my comment on the relevant issue.

[kayabaNerve]

#[must_use]
trait X {}

fn y<Z: X>(z: Z) -> Z {
  z
}

fn a<Z: X>(z: Z) {
    y(z);
}

#[must_use]
struct A;
impl X for A {}

fn main() {
  a(A);
}

for the MRE.