cue: Value API should support looking at pattern constraints on structs

[mvdan]

Given a CUE value such as:

foo: [string]: #Entry

one can grab the cue.Value for foo and do val.LookupPath(cue.MakePath(cue.AnyString)) and obtain the reference value for #Entry.

However, given:

foo: [=~"^prefix]: #Entry

it is currently not possible to do the same; a lookup of "any string" returns an error because any string does not match the constraint.

In the more general case, it's impossible to inspect a CUE value such as

foo: {
    [string]: #SchemaAnyString
    [=~"^x/"]: #SchemaX
    [=~"^y/"]: #SchemaY

    [int]: #SchemaAnyInt
}

and look at the pattern constraints being applied via the public Go API. Note that some may overlap, for example foo: "x/one": {} would unify with #SchemaAnyString & #SchemaX.

Also note that, per the spec, default constraints like foo: {...string} could also apply, but are not currently implemented.

One idea could be:

type PatternConstraint struct {
    Pattern    Value
    Constraint Value
}

func (Value) Patterns() ([]PatternConstraint, error)

Marcel also briefly mentioned in a recent exchange that he had been toying with the idea of adapting the existing struct field iterator API (https://pkg.go.dev/cuelang.org/go/cue#Value.Fields) to support pattern constraints as well. For that to work, we might need two new APIs to enable iterating over pattern constraints, and then seeing their pattern values, given that Iterator.Selector is not enough:

func Patterns(include bool) Option

func (*Iterator) Pattern() Value

[myitcv]

Just flagging that we should also bear in mind #3312 when considering this API.

[mvdan]

I should also note that @cuematthew mentioned that trim needed this. He ended up using the internal APIs because the public APIs were missing features like this one.