deriv(Hash) for single-variant enum should not hash discriminant

[stepancheg]

Fixes #39137

[rust-highfive]

r? @brson

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[arielb1]

r? @jseyfried

[Mark-Simulacrum]

@jseyfried This is waiting for a review from you, and possibly should get sign off from @rust-lang/libs as well - I don't think if this change is something we want to commit to. Seems fine to me, though.

[jseyfried]

Reviewed, r=me if @rust-lang/libs is OK with the new behavior (I don't have a strong opinion either way).

[alexcrichton]

@bors: r=jseyfried

Yeah libs-wise this is fine

[bors]

📌 Commit f11e40a has been approved by jseyfried

[bors]

⌛ Testing commit f11e40a with merge 5bc8941...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: jseyfried
Pushing 5bc8941 to master...

[kryptan]

I don't think this PR fixes #39137.

#39137 is about enums marked with repr(u8) (or other similar reprs). This PR affects enums with single variant regardless of their repr.

[Mark-Simulacrum]

Reopened.

[stepancheg]

@kryptan yes, sorry.