[meta] update rust to 1.84

[sunshowers]

We're moving three versions ahead, so this is something we should do with some caution. However there's at least one important illumos-specific fix that's in 1.84 (rust-lang/rust#132984) so we should definitely move over.

I've verified that rust-lang/rust#132064 no longer affects us. To be more precise, its impact has been mostly taken care of -- there's still a small regression, see this comment, but the Rust team has said it's one that should get better in the next few months.

Depends on #7333.

[iliana]

I'm wondering if we should mark the unused lifetime lint that seems to be most of this diff as an allowed lint. It seems like it really ought to be categorized as a style lint instead IMO, and we disable style lints by default:

omicron/Cargo.toml

Lines 277 to 279 in d9a6a8b

	[workspace.lints.clippy]
	# Clippy's style nits are useful, but not worth keeping in CI.
	style = { level = "allow", priority = -1 }

(I have been meaning to go back through the list of style lints we presently have disabled and update our list of warned style lints.)

[iliana]

Suggested change

	#[allow(clippy::needless_lifetimes)]
	#[expect(clippy::needless_lifetimes)]

(1.81 release notes)

[sunshowers]

see the comment :( it didn't work

[iliana]

Oh that seems bad. I wonder if we can minimize it and file an issue.

[sunshowers]

It's a little tricky because the needless_lifetimes thing has been fixed in clippy main -- would need to find some other case where expect doesn't work.

[sunshowers]

I'm wondering if we should mark the unused lifetime lint that seems to be most of this diff as an allowed lint.

Ah -- so needless_lifetimes is actually a very old lint, but over time it's been extended to catch more cases. I think it's a valuable lint to have.