cmd/doc: fix strange indentation artifacts with unexported fields

The NamePos value was not being set, and would default to a value
of zero. This would cause the printing logic to get confused as
to where exactly to place the "Has unexported fields" string.

A trivial package changes from

<
type A struct {
	A int // A
	B int
			// B
	// Has unexported fields.
}
>

to

<
type A struct {
	A int // A
	B int // B
	// Has unexported fields.
}
>

Fixes #12971

Change-Id: I53b7799a1f1c0ad7dcaddff83d9aaeb1d6b7823e
Reviewed-on: https://go-review.googlesource.com/16286
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/doc_test.go

@@ -219,7 +219,8 @@ var tests = []test{
 		[]string{
 			`Comment about exported type`, // Include comment.
 			`type ExportedType struct`,    // Type definition.
-			`Comment before exported field.*\n.*ExportedField +int`,
+			`Comment before exported field.*\n.*ExportedField +int` +
+				`.*Comment on line with exported field.`,
 			`Has unexported fields`,
 			`func \(ExportedType\) ExportedMethod\(a int\) bool`,
 			`const ExportedTypedConstant ExportedType = iota`, // Must include associated constant.
@@ -263,6 +264,40 @@ var tests = []test{
 		nil,
 	},
 
+	// Interface.
+	{
+		"type",
+		[]string{p, `ExportedInterface`},
+		[]string{
+			`Comment about exported interface`, // Include comment.
+			`type ExportedInterface interface`, // Interface definition.
+			`Comment before exported method.*\n.*ExportedMethod\(\)` +
+				`.*Comment on line with exported method`,
+			`Has unexported methods`,
+		},
+		[]string{
+			`unexportedField`,               // No unexported field.
+			`Comment about exported method`, // No comment about exported method.
+			`unexportedMethod`,              // No unexported method.
+			`unexportedTypedConstant`,       // No unexported constant.
+		},
+	},
+	// Interface -u with unexported methods.
+	{
+		"type with unexported methods and -u",
+		[]string{"-u", p, `ExportedInterface`},
+		[]string{
+			`Comment about exported interface`, // Include comment.
+			`type ExportedInterface interface`, // Interface definition.
+			`Comment before exported method.*\n.*ExportedMethod\(\)` +
+				`.*Comment on line with exported method`,
+			`unexportedMethod\(\).*Comment on line with unexported method.`,
+		},
+		[]string{
+			`Has unexported methods`,
+		},
+	},
+
 	// Method.
 	{
 		"method",

src/cmd/doc/pkg.go

@@ -504,7 +504,14 @@ func trimUnexportedFields(fields *ast.FieldList, what string) *ast.FieldList {
 		return fields
 	}
 	unexportedField := &ast.Field{
-		Type: ast.NewIdent(""), // Hack: printer will treat this as a field with a named type.
+		Type: &ast.Ident{
+			// Hack: printer will treat this as a field with a named type.
+			// Setting Name and NamePos to ("", fields.Closing-1) ensures that
+			// when Pos and End are called on this field, they return the
+			// position right before closing '}' character.
+			Name:    "",
+			NamePos: fields.Closing - 1,
+		},
 		Comment: &ast.CommentGroup{
 			List: []*ast.Comment{{Text: fmt.Sprintf("// Has unexported %s.\n", what)}},
 		},

src/cmd/doc/testdata/pkg.go

@@ -60,7 +60,7 @@ func internalFunc(a int) bool
 // Comment about exported type.
 type ExportedType struct {
 	// Comment before exported field.
-	ExportedField   int
+	ExportedField   int // Comment on line with exported field.
 	unexportedField int // Comment on line with unexported field.
 }
 
@@ -87,6 +87,13 @@ func ExportedTypeConstructor() *ExportedType {
 
 const unexportedTypedConstant ExportedType = 1 // In a separate section to test -u.
 
+// Comment about exported interface.
+type ExportedInterface interface {
+	// Comment before exported method.
+	ExportedMethod()   // Comment on line with exported method.
+	unexportedMethod() // Comment on line with unexported method.
+}
+
 // Comment about unexported type.
 type unexportedType int