Support for not() builtin

[cueckoo]

Originally opened by @myitcv in cuelang/cue#571

Noting an offline conversation with @mpvl and @rogpeppe

Is your feature request related to a problem? Please describe.

Consider:

Steps: [name=string]: #Step | 
    { [#LangCode]: #Step } | 
    { [#ScenarioName]: #Step } | 
    { [#LangCode]: [#ScenarioName]: #Step } | 
    { [#ScenarioName]: [#LangCode]: #Step }

#LangCode = "en" | "fr" // etc

Scenarios: [name=XYZ]: #Scenario

#ScenarioName: or([ for name, _ in Scenarios {name}])

The declaration of Steps as written is only possible if we can constrain a #Scenario name in some way (shown with XYZ here) to be any string that is not a valid #LangCode.

Describe the solution you'd like

Potentially something like:

not(or(#LangCode))

Describe alternatives you've considered

A negative specification using _|_ does work:

Scenarios: [#LangCode]: _|_

However, it leads to more misleading error messages.

Additional context

[cueckoo]

Original reply by @myitcv in cuelang/cue#571 (comment)

Noting an offline conversation: that not() is effectively syntactic sugar around constrain():

x: not(y)

// is equivalent to

x: constrain(x & y == _|_)

Which might mean that not() is not required (assuming "nice" error messages can be inferred from such negative constraints).

[myitcv]

As part of the recent issue garden, we are focussing on non-feature requests. As such, I'm removing the milestone on this feature request. We will revisit feature requests in a later pass, at which point we will start to milestone and prioritise new features (in addition to those that we are already working on).

Noting however that this is covered (almost entirely) by #943.