Feature gate `&Void`'s uninhabitedness.

[canndrew]

Here's a totally crazy PR which should never be merged.

[canndrew]

Ping @nikomatsakis @arielb1

[rust-highfive]

r? @arielb1

(rust_highfive has picked a reviewer for you, use r? to override)

[arielb1]

@bors r+ rollup

[bors]

📌 Commit 2c6bc18 has been approved by arielb1

[arielb1]

The non-reference parts seem to be mostly uncontroversial, except for the issue that you can't have a match expression in a macro without risking unreachable code warnings. Not sure what is the best way to handle that. Macro reachability hygiene?

[Ericson2314]

This will continue to work, right?

match void_ref_result {
    Err(e) => ... ,
    Ok(&r) => match r { },
}

[Ericson2314]

Not sure what is the best way to handle that. Macro reachability hygiene?

allow unreachable annotation on arm?

[arielb1]

This will continue to work, right?

If it worked before the matchck changes, it should continue working.

allow unreachable annotation on arm?

The problem is that the change is an hygiene hazard - every match of a generic-containing enum in a macro has to be careful not to cause spurious unreachable arm errors. Maybe just disabling unreachable arm errors in cross-crate macros or something would work as a basic form of hygiene.

[brson]

@bors r+ p=1 beta fix

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: #38842

[bors]

📌 Commit 2c6bc18 has been approved by brson

[brson]

@bors p=1

[Ericson2314]

@arielb1 yeah, the current "unhygenic" approach is somewhat analogous to doing a post-monomorphization warning with generic functions. I feel like maybe you don't always want that to be ignored, but then a use-site "no---really----please warn/error" would suffice.

[brson]

This patch does not apply to beta - the inhabitedness mod does not exist. Is this really a beta backport?

[nikomatsakis]

@canndrew I just wanted to say thanks for turning this around so quickly.

[arielb1]

@brson

No. it's a 1.16 issue only.