cmd/doc: handle embedded interfaces properly

Changes made:
* Disallow star expression on interfaces as this is not possible.
* Show an embedded "error" in an interface as public similar to
how godoc does it.
* Properly handle selector expressions in both structs and interfaces.
This is possible since a type may refer to something defined in
another package (e.g. io.Reader).

Before:
<<<
$ go doc runtime.Error
type Error interface {

    // RuntimeError is a no-op function but
    // serves to distinguish types that are run time
    // errors from ordinary errors: a type is a
    // run time error if it has a RuntimeError method.
    RuntimeError()
    // Has unexported methods.
}

$ go doc compress/flate Reader
doc: invalid program: unexpected type for embedded field
doc: invalid program: unexpected type for embedded field
type Reader interface {
    io.Reader
    io.ByteReader
}
>>>

After:
<<<
$ go doc runtime.Error
type Error interface {
    error

    // RuntimeError is a no-op function but
    // serves to distinguish types that are run time
    // errors from ordinary errors: a type is a
    // run time error if it has a RuntimeError method.
    RuntimeError()
}

$ go doc compress/flate Reader
type Reader interface {
    io.Reader
    io.ByteReader
}
>>>

Fixes #16567

Change-Id: I272dede971eee9f43173966233eb8810e4a8c907
Reviewed-on: https://go-review.googlesource.com/25365
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 32 insertions, 4 deletions

diff --git a/src/cmd/doc/doc_test.go b/src/cmd/doc/doc_test.go
index 5cb1ec990e..bfb9099dd2 100644
--- a/src/cmd/doc/doc_test.go
+++ b/src/cmd/doc/doc_test.go
@@ -221,6 +221,7 @@ var tests = []test{
 			`func \(ExportedType\) ExportedMethod\(a int\) bool`,
 			`const ExportedTypedConstant ExportedType = iota`, // Must include associated constant.
 			`func ExportedTypeConstructor\(\) \*ExportedType`, // Must include constructor.
+			`io.Reader.*Comment on line with embedded Reader.`,
 		},
 		[]string{
 			`unexportedField`,                // No unexported field.
@@ -228,6 +229,7 @@ var tests = []test{
 			`Comment about exported method.`, // No comment about exported method.
 			`unexportedMethod`,               // No unexported method.
 			`unexportedTypedConstant`,        // No unexported constant.
+			`error`,                          // No embedded error.
 		},
 	},
 	// Type -u with unexported fields.
@@ -243,6 +245,8 @@ var tests = []test{
 			`\*ExportedEmbeddedType.*Comment on line with exported embedded \*field.`,
 			`unexportedType.*Comment on line with unexported embedded field.`,
 			`\*unexportedType.*Comment on line with unexported embedded \*field.`,
+			`io.Reader.*Comment on line with embedded Reader.`,
+			`error.*Comment on line with embedded error.`,
 			`func \(ExportedType\) unexportedMethod\(a int\) bool`,
 			`unexportedTypedConstant`,
 		},
@@ -274,6 +278,8 @@ var tests = []test{
 			`type ExportedInterface interface`, // Interface definition.
 			`Comment before exported method.*\n.*ExportedMethod\(\)` +
 				`.*Comment on line with exported method`,
+			`io.Reader.*Comment on line with embedded Reader.`,
+			`error.*Comment on line with embedded error.`,
 			`Has unexported methods`,
 		},
 		[]string{
@@ -293,6 +299,8 @@ var tests = []test{
 			`Comment before exported method.*\n.*ExportedMethod\(\)` +
 				`.*Comment on line with exported method`,
 			`unexportedMethod\(\).*Comment on line with unexported method.`,
+			`io.Reader.*Comment on line with embedded Reader.`,
+			`error.*Comment on line with embedded error.`,
 		},
 		[]string{
 			`Has unexported methods`,
diff --git a/src/cmd/doc/pkg.go b/src/cmd/doc/pkg.go
index efd681d514..eec9f1e803 100644
--- a/src/cmd/doc/pkg.go
+++ b/src/cmd/doc/pkg.go
@@ -494,14 +494,19 @@ func trimUnexportedElems(spec *ast.TypeSpec) {
 	}
 	switch typ := spec.Type.(type) {
 	case *ast.StructType:
-		typ.Fields = trimUnexportedFields(typ.Fields, "fields")
+		typ.Fields = trimUnexportedFields(typ.Fields, false)
 	case *ast.InterfaceType:
-		typ.Methods = trimUnexportedFields(typ.Methods, "methods")
+		typ.Methods = trimUnexportedFields(typ.Methods, true)
 	}
 }
 
 // trimUnexportedFields returns the field list trimmed of unexported fields.
-func trimUnexportedFields(fields *ast.FieldList, what string) *ast.FieldList {
+func trimUnexportedFields(fields *ast.FieldList, isInterface bool) *ast.FieldList {
+	what := "methods"
+	if !isInterface {
+		what = "fields"
+	}
+
 	trimmed := false
 	list := make([]*ast.Field, 0, len(fields.List))
 	for _, field := range fields.List {
@@ -511,12 +516,23 @@ func trimUnexportedFields(fields *ast.FieldList, what string) *ast.FieldList {
 			// Nothing else is allowed.
 			switch ident := field.Type.(type) {
 			case *ast.Ident:
+				if isInterface && ident.Name == "error" && ident.Obj == nil {
+					// For documentation purposes, we consider the builtin error
+					// type special when embedded in an interface, such that it
+					// always gets shown publicly.
+					list = append(list, field)
+					continue
+				}
 				names = []*ast.Ident{ident}
 			case *ast.StarExpr:
 				// Must have the form *identifier.
-				if ident, ok := ident.X.(*ast.Ident); ok {
+				// This is only valid on embedded types in structs.
+				if ident, ok := ident.X.(*ast.Ident); ok && !isInterface {
 					names = []*ast.Ident{ident}
 				}
+			case *ast.SelectorExpr:
+				// An embedded type may refer to a type in another package.
+				names = []*ast.Ident{ident.Sel}
 			}
 			if names == nil {
 				// Can only happen if AST is incorrect. Safe to continue with a nil list.
diff --git a/src/cmd/doc/testdata/pkg.go b/src/cmd/doc/testdata/pkg.go
index 5f79414b33..9c5cf8f557 100644
--- a/src/cmd/doc/testdata/pkg.go
+++ b/src/cmd/doc/testdata/pkg.go
@@ -66,6 +66,8 @@ type ExportedType struct {
 	*ExportedEmbeddedType     // Comment on line with exported embedded *field.
 	unexportedType            // Comment on line with unexported embedded field.
 	*unexportedType           // Comment on line with unexported embedded *field.
+	io.Reader                 // Comment on line with embedded Reader.
+	error                     // Comment on line with embedded error.
 }
 
 // Comment about exported method.
@@ -96,6 +98,8 @@ type ExportedInterface interface {
 	// Comment before exported method.
 	ExportedMethod()   // Comment on line with exported method.
 	unexportedMethod() // Comment on line with unexported method.
+	io.Reader          // Comment on line with embedded Reader.
+	error              // Comment on line with embedded error.
 }
 
 // Comment about unexported type.