Fix rustc & clippy warnings for nightly (2021-09-16)

[sharnoff]

Clippy/rustc commit rust-lang/rust@e4828d5b7

There's a few additional warnings that currently pop up on nightly (not sure exactly when they started), some from rustc itself and some from clippy.

Perhaps there's a better solution to handling the unused fields in IdentifySystem, but I'm not sure what it is right now. I think it does make sense for that struct to be left as-is, given that rust-postgres doesn't provide an IDENTIFY_SYSTEM interface.

[hlinnaka]

Perhaps there's a better solution to handling the unused fields in IdentifySystem, but I'm not sure what it is right now. I think it does make sense for that struct to be left as-is, given that rust-postgres doesn't provide an IDENTIFY_SYSTEM interface.

Why does it consider IdentifySystem as unused? It is printed here:

    let identify = identify_system(&mut rclient)?;
    info!("{:?}", identify);

Is that info!-line not enough? Perhaps we should print out the individual fields in that message. And maybe downgradi it to 'trace' while we're at it.

[sharnoff]

@hlinnaka Hmm, you're right. After looking into it a little further, it seems like it's related to this issue: rust-lang/rust#88900.

There's some debate about whether that change actually should have been made, but the tl;dr about why it's there is essentially: "a field isn't used if it's only read as part of a Debug impl". In the meantime, I've added a comment describing why we have #[allow(dead_code)] and linking to the issue.