evaluator: improve error "new field not allowed by earlier comprehension or reference cycle"

[myitcv]

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

$ cue version
cue version v0.0.0-20230926131433-a9a6965188f7

go version go1.21.1
      -buildmode exe
       -compiler gc
  DefaultGODEBUG panicnil=1
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
             vcs git
    vcs.revision a9a6965188f751fca54251b803dfe4883c1f42bc
        vcs.time 2023-09-26T13:14:33Z
    vcs.modified false

Does this issue reproduce with the latest release?

Yes

What did you do?

exec cue export x.cue

-- x.cue --
repos?: [n=string]: {
	trybot: *false | bool
	name!:  string
}

// Define the CUE repo
repos: "cue": {
	trybot: true
}

// Template that for repos which define trybot == true
// that there should be an associated -trybot repo
for n, r in repos
if r.trybot {
	repos: "\(n)-trybot": {
		trybot: false
	}
}

Playground: https://tip.cuelang.org/play/?id=obQKQuIiCIY#cue@export@cue

What did you expect to see?

Passing test, I think.

What did you see instead?

> exec cue export x.cue
[stderr]
repos: field "cue-trybot" not allowed by earlier comprehension or reference cycle
[exit status 1]
FAIL: /tmp/testscript2321507989/repro.txtar/script.txtar:1: unexpected command failure

The set of repos where trybot == true is disjoint from the set where disjoint == false. Hence it's possible to view this as not being a cycle, I think.

[myitcv]

Per offline conversation, this is working as expected:

In this case, it is growing the set of fields of the object the comprehension is iterating over. That is not allowed.

[mvdan]

David on Slack ran into this issue and suggested that the error message could be more obvious about the cause of the problem:

Also, the error message is REAALLY unclear... is not allowed should have been can't be added to a comprehension currently being iterated over or something like that. Then I would have maybe guessed that was the problem.

Reopening and re-titling this issue to track how we could make the error better.