adt: Allow repr attribute on single variant enum.

[luqmana]

Fixes #33202.

[rust-highfive]

r? @nrc

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[nrc]

Why can't we let repr override the behaviour of single variant structs having no discriminant? That seems more user-friendly than having an error here.

cc @rust-lang/lang

[aturon]

Nominating for discussion at the next meeting, if we don't reach consensus before then.

[bors]

☔️ The latest upstream changes (presumably #33425) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

Discussed in @rust-lang/lang meeting. I think we all agree it seems like permitting this annotation might be nice, but if it's awkward to implement, than an error is fine too. @luqmana what say you?

[luqmana]

@nikomatsakis Ok, I made it an error because there was an asset in trans to that effect. I can change it to allow this.

[luqmana]

Ok, updated trans::adt to allow such cases.

[nrc]

@bors: r+

[bors]

📌 Commit f3e1d57 has been approved by nrc

[bors]

⌛️ Testing commit f3e1d57 with merge 33a5c9d...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild