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.

[kayabaNerve]

We can wrap the DbTxn generic in a struct which is must_use to workaround this. It's ugly but would be safe.

[kayabaNerve]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=82ba3d8b66d59ac5a9faa415032775e3

must_use only requires use on a branch, not on all branches :///

[kayabaNerve]

The struct (and the correct application of must_use) would only guarantee the txn is bound, not moved out of scope.

Opened an issue calling for a new clippy lint for an expanded interpretation of must_use: rust-lang/rust-clippy#13997