cmd/cue: support optional comments in get go

The Kuberentes ecosystem use comments to denote whether a field is
optional or required. The comment has a direct effect on the produced
OpenAPI schema, and often means otherwise correct Go "schemas" will be
represented incorrectly with CUE.

https://github.com/kubernetes/community/blob/e977e6ea355f26130ca555d1e8704893727ee024/contributors/devel/sig-architecture/api-conventions.md#optional-vs-required

In addition to the heuristics of the "omitempty" struct tag, the
"// +optional" comment will now also mark a field as optional. The
behaviour from the Kubernetes project which parses this comment as a
key/value pair (tag) is also preserved.

https://pkg.go.dev/k8s.io/gengo/types#ExtractCommentTags

Fixes #2679

Change-Id: Ieeb2e7bee61e16ed9910ea46e68cb1a3eaa47197
Signed-off-by: Thomas Way <[email protected]>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171971
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/get_go.go

@@ -1292,15 +1292,15 @@ func (e *extractor) addFields(x *types.Struct, st *cueast.StructLit) {
 		if name == "-" {
 			continue
 		}
+
+		doc := docs[i]
+
 		// TODO: check referrers
 		kind := regular
-		if e.isOptional(tag) {
-			kind = optional
-		}
-		if _, ok := f.Type().(*types.Pointer); ok {
+		if e.isOptional(f, doc, tag) {
 			kind = optional
 		}
-		field, cueType := e.makeField(name, kind, f.Type(), docs[i], count > 0)
+		field, cueType := e.makeField(name, kind, f.Type(), doc, count > 0)
 		add(field)
 
 		if s := reflect.StructTag(tag).Get("cue"); s != "" {
@@ -1395,7 +1395,18 @@ func (e *extractor) isInline(tag string) bool {
 		hasFlag(tag, "yaml", "inline", 1)
 }
 
-func (e *extractor) isOptional(tag string) bool {
+func (e *extractor) isOptional(f *types.Var, doc *ast.CommentGroup, tag string) bool {
+	if _, ok := f.Type().(*types.Pointer); ok {
+		return true
+	}
+
+	for _, line := range strings.Split(doc.Text(), "\n") {
+		before, _, _ := strings.Cut(strings.TrimSpace(line), "=")
+		if before == "+optional" {
+			return true
+		}
+	}
+
 	// TODO: also when the type is a list or other kind of pointer.
 	return hasFlag(tag, "json", "omitempty", 1) ||
 		hasFlag(tag, "yaml", "omitempty", 1)

cmd/cue/cmd/testdata/script/get_go_types.txtar

@@ -103,6 +103,35 @@ type Foozer struct {
 	Embed struct{ CustomJSON }
 
 	Unsupported map[int]string
+
+	OptionalOmitEmptyJSON string `json:"optionalOmitEmptyJSON,omitempty"`
+
+	OptionalOmitEmptyYAML string `yaml:"optionalOmitEmptyYAML,omitempty"`
+
+	// +optional
+	OptionalComment string `json:"optionalComment"`
+
+	//+optional
+	OptionalCommentNoSpace string `json:"optionalCommentNoSpace"`
+
+	// Something before
+	//
+	// +optional
+	//
+	// Something after
+	OptionalCommentExtra string `json:"optionalCommentExtra"`
+
+	// +optional=
+	OptionalCommentTag string `json:"optionalCommentTag"`
+
+	// +optional=some-value
+	OptionalCommentTagValue string `json:"optionalCommentTagValue"`
+
+	// some-prefix+optional
+	RequiredCommentPrefix string `json:"requiredCommentPrefix"`
+
+	// +optional-some-suffix
+	RequiredCommentSuffix string `json:"requiredCommentSuffix"`
 }
 
 type Identifier string
@@ -293,6 +322,33 @@ import (
 	Embed: {
 		CustomJSON: #CustomJSON
 	} @go(,struct{CustomJSON})
+	optionalOmitEmptyJSON?: string @go(OptionalOmitEmptyJSON)
+	optionalOmitEmptyYAML?: string @go(OptionalOmitEmptyYAML)
+
+	// +optional
+	optionalComment?: string @go(OptionalComment)
+
+	//+optional
+	optionalCommentNoSpace?: string @go(OptionalCommentNoSpace)
+
+	// Something before
+	//
+	// +optional
+	//
+	// Something after
+	optionalCommentExtra?: string @go(OptionalCommentExtra)
+
+	// +optional=
+	optionalCommentTag?: string @go(OptionalCommentTag)
+
+	// +optional=some-value
+	optionalCommentTagValue?: string @go(OptionalCommentTagValue)
+
+	// some-prefix+optional
+	requiredCommentPrefix: string @go(RequiredCommentPrefix)
+
+	// +optional-some-suffix
+	requiredCommentSuffix: string @go(RequiredCommentSuffix)
 }
 
 #Identifier: string // #enumIdentifier