evaluator: infinite loop on cycle with embeddings

[perelo]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.5.0-rc.1

go version go1.20.2
      -buildmode exe
       -compiler gc
       -trimpath true
     CGO_ENABLED 0
          GOARCH amd64
            GOOS linux
         GOAMD64 v1

Also loops on tags v0.6.0-beta.1 and v0.6.0-rc.1, but not v0.4.3.

Bisected to 887f5649

What did you do?

x
x: {
	y
	y: x
}

What did you expect to see?

Something like
x.y: structural cycle

What did you see instead?

Apparently a non terminating evaluation.

[mvdan]

Thank you - as far as I can tell, this hang hasn't been reported before.

[mvdan]

This appears resolved on evalv3 as of v0.13.0-alpha.4. evalv2 still has this hang, but we're no longer developing the old evaluator at this point.

I'll close this with a regression test.