internal/core/export: always export required fields

Required fields are either printed (in case of a definition) or
erroneous (in case of export). We now always print them and
leave error reporting of required fields, if desired, up to the user.

Fixes #2305

Change-Id: Id440a20a5f4b247a52ce4c74555903df39c7214f
Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552169
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

internal/core/export/testdata/main/def.txtar

@@ -6,7 +6,12 @@ c: [string]: int
 #d: {
 	e!: string
 	e?: =~"a"
+
+	f?: 1
 }
+
+// Issue #2305
+g: #d
 -- out/definition --
 a:  int | *2
 b?: 4 | 5
@@ -15,19 +20,34 @@ c: {
 }
 #d: {
 	e!: =~"a"
+	f?: 1
 }
+
+// Issue #2305
+g: #d
 -- out/doc --
 []
 [a]
 [b]
 [c]
 [#d]
 [#d e]
+[#d f]
+[g]
+- Issue #2305
+
+[g e]
+[g f]
 -- out/value --
 == Simplified
 {
 	a: *2 | int
 	c: {}
+
+	// Issue #2305
+	g: {
+		e!: =~"a"
+	}
 }
 == Raw
 {
@@ -36,12 +56,22 @@ c: {
 	c: {}
 	#d: {
 		e!: =~"a"
+		f?: 1
+	}
+
+	// Issue #2305
+	g: {
+		e!: =~"a"
+		f?: 1
 	}
 }
 == Final
 {
 	a: 2
 	c: {}
+	g: {
+		e!: =~"a"
+	}
 }
 == All
 {
@@ -50,6 +80,13 @@ c: {
 	c: {}
 	#d: {
 		e!: =~"a"
+		f?: 1
+	}
+
+	// Issue #2305
+	g: {
+		e!: =~"a"
+		f?: 1
 	}
 }
 == Eval
@@ -59,5 +96,10 @@ c: {
 	c: {}
 	#d: {
 		e!: =~"a"
+		f?: 1
+	}
+	g: {
+		e!: =~"a"
+		f?: 1
 	}
 }

internal/core/export/value.go

@@ -74,11 +74,9 @@ func (e *exporter) vertex(n *adt.Vertex) (result ast.Expr) {
 
 	case *adt.Bottom:
 		switch {
-		case n.IsConstraint():
-			// Constraints may always be the original value.
-			// TODO: this was included for backwards compatibility. But
-			// should we show the error here? It signifies that this field
-			// may not be used.
+		case n.ArcType == adt.ArcOptional:
+			// Optional fields may always be the original value.
+
 		case e.cfg.ShowErrors && x.ChildError:
 			// TODO(perf): use precompiled arc statistics
 			if len(n.Arcs) > 0 && n.Arcs[0].Label.IsInt() && !e.showArcs(n) && attrs == nil {
@@ -438,12 +436,14 @@ func (e *exporter) structComposite(v *adt.Vertex, attrs []*ast.Attribute) ast.Ex
 			continue
 		}
 
-		if isOptional := arc.IsConstraint(); isOptional {
-			if !p.ShowOptional {
-				continue
-			}
-			internal.SetConstraint(f, arc.ArcType.Token())
+		if arc.ArcType == adt.ArcOptional && !p.ShowOptional {
+			continue
 		}
+		// TODO: report an error for required fields in Final mode?
+		// This package typically does not create errors that did not result
+		// from evaluation already.
+
+		internal.SetConstraint(f, arc.ArcType.Token())
 
 		f.Value = e.vertex(arc)