cmd/doc: trim unexported methods from interfaces

Fixes #10856.

Change-Id: I5de65b8dd94eec3451ee0ba9c75698cdd88f5fea
Reviewed-on: https://go-review.googlesource.com/10088Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/doc/pkg.go

@@ -345,7 +345,7 @@ func (pkg *Package) symbolDoc(symbol string) {
 		}
 		decl := typ.Decl
 		spec := pkg.findTypeSpec(decl, typ.Name)
-		trimUnexportedFields(spec)
+		trimUnexportedElems(spec)
 		// If there are multiple types defined, reduce to just this one.
 		if len(decl.Specs) > 1 {
 			decl.Specs = []ast.Spec{spec}
@@ -366,22 +366,26 @@ func (pkg *Package) symbolDoc(symbol string) {
 	}
 }
 
-// trimUnexportedFields modifies spec in place to elide unexported fields (unless
-// the unexported flag is set). If spec is not a structure declartion, nothing happens.
-func trimUnexportedFields(spec *ast.TypeSpec) {
+// trimUnexportedElems modifies spec in place to elide unexported fields from
+// structs and methods from interfaces (unless the unexported flag is set).
+func trimUnexportedElems(spec *ast.TypeSpec) {
 	if *unexported {
-		// We're printing all fields.
-		return
+		return fields
 	}
-	// It must be a struct for us to care. (We show unexported methods in interfaces.)
-	structType, ok := spec.Type.(*ast.StructType)
-	if !ok {
-		return
+	switch typ := spec.Type.(type) {
+	case *ast.StructType:
+		typ.Fields = trimUnexportedFields(typ.Fields, "fields")
+	case *ast.InterfaceType:
+		typ.Methods = trimUnexportedFields(typ.Methods, "methods")
 	}
+}
+
+// trimUnexportedFields returns the field list trimmed of unexported fields.
+func trimUnexportedFields(fields *ast.FieldList, what string) *ast.FieldList {
 	trimmed := false
-	list := make([]*ast.Field, 0, len(structType.Fields.List))
-	for _, field := range structType.Fields.List {
-		// Trims if any is unexported. Fine in practice.
+	list := make([]*ast.Field, 0, len(fields.List))
+	for _, field := range fields.List {
+		// Trims if any is unexported. Good enough in practice.
 		ok := true
 		for _, name := range field.Names {
 			if !isExported(name.Name) {
@@ -394,19 +398,23 @@ func trimUnexportedFields(spec *ast.TypeSpec) {
 			list = append(list, field)
 		}
 	}
-	if trimmed {
-		unexportedField := &ast.Field{
-			Type: ast.NewIdent(""), // Hack: printer will treat this as a field with a named type.
-			Comment: &ast.CommentGroup{
-				List: []*ast.Comment{
-					&ast.Comment{
-						Text: "// Has unexported fields.\n",
-					},
+	if !trimmed {
+		return fields
+	}
+	unexportedField := &ast.Field{
+		Type: ast.NewIdent(""), // Hack: printer will treat this as a field with a named type.
+		Comment: &ast.CommentGroup{
+			List: []*ast.Comment{
+				&ast.Comment{
+					Text: fmt.Sprintf("// Has unexported %s.\n", what),
 				},
 			},
-		}
-		list = append(list, unexportedField)
-		structType.Fields.List = list
+		},
+	}
+	return &ast.FieldList{
+		Opening: fields.Opening,
+		List:    append(list, unexportedField),
+		Closing: fields.Closing,
 	}
 }