simplify the uses of obfuscatedTypesPackage (#284)

obfuscatedTypesPackage is now only useful for one scenario: a type which
is used with reflection in a dependency, so neither its name nor any of
its potential struct field names are obfuscated.

This is because we can only detect the use of reflection with go/ast,
which we don't have for dependencies.

As such, we only need obfuscatedTypesPackage in two places - when
considering to obfuscate a field or a type name.

There were two other calls to the function, which we remove.

The first was used for linkname directives. Those directives only work
for variables and functions, neither of which is affected by the
reflection detection.

The second was used for all identifiers, at the very end of the
transformGo inner func. It's entirely unnecessary right now, as it never
triggers anymore. It's possible it was necessary some time ago when we
still didn't obfuscate assembly functions.

While at it, improve some comments and add a few TODOs for edge cases
which do not have code coverage.

main.go

@@ -21,7 +21,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"reflect"
 	"runtime"
 	"runtime/debug"
 	"strconv"
@@ -757,10 +756,6 @@ func (tf *transformer) handleDirectives(comments []*ast.CommentGroup) {
 			if !lpkg.Private {
 				continue // ignore non-private symbols
 			}
-			obfPkg := obfuscatedTypesPackage(pkgPath)
-			if obfPkg != nil && obfPkg.Scope().Lookup(name) != nil {
-				continue // the name exists and was not obfuscated
-			}
 
 			// The name exists and was obfuscated; replace the
 			// comment with the obfuscated name.
@@ -1089,33 +1084,32 @@ func (tf *transformer) transformGo(file *ast.File) *ast.File {
 
 		// log.Printf("%#v %T", node, obj)
 		parentScope := obj.Parent()
-		switch x := obj.(type) {
+		switch obj := obj.(type) {
 		case *types.Var:
 			if parentScope != nil && parentScope != pkg.Scope() {
-				// identifiers of non-global variables never show up in the binary
+				// Identifiers of non-global variables never show up in the binary.
 				return true
 			}
 
-			// if the struct of this field was not obfuscated, do not obfuscate
-			// any of that struct's fields
-			if parentScope != tf.pkg.Scope() && x.IsField() && !x.Embedded() {
+			// If the struct of this field was not obfuscated, do not obfuscate
+			// any of that struct's fields.
+			if parentScope != tf.pkg.Scope() && obj.IsField() && !obj.Embedded() {
 				parent, ok := cursor.Parent().(*ast.SelectorExpr)
 				if !ok {
 					break
 				}
-				parentType := tf.info.TypeOf(parent.X)
-				if parentType == nil {
-					break
-				}
-				named := namedType(parentType)
+				named := namedType(tf.info.TypeOf(parent.X))
 				if named == nil {
-					break
+					break // TODO(mvdan): add a test
 				}
 				if name := named.Obj().Name(); strings.HasPrefix(name, "_Ctype") {
 					// A field accessor on a cgo type, such as a C struct.
 					// We're not obfuscating cgo names.
 					return true
 				}
+				// If the type originates from an indirect import,
+				// it's possible for obfPkg to be nil here.
+				// TODO(mvdan): add a test and think how to fix this
 				if obfPkg := obfuscatedTypesPackage(path); obfPkg != nil {
 					if obfPkg.Scope().Lookup(named.Obj().Name()) != nil {
 						recordStruct(named, tf.ignoreObjects)
@@ -1125,22 +1119,22 @@ func (tf *transformer) transformGo(file *ast.File) *ast.File {
 			}
 		case *types.TypeName:
 			if parentScope != pkg.Scope() {
-				// identifiers of non-global types never show up in the binary
+				// Identifiers of non-global types never show up in the binary.
 				return true
 			}
 
-			// if the type was not obfuscated in the package were it was defined,
-			// do not obfuscate it here
+			// If the type was not obfuscated in the package were it was defined,
+			// do not obfuscate it here.
 			if parentScope != tf.pkg.Scope() {
-				named := namedType(x.Type())
+				named := namedType(obj.Type())
 				if named == nil {
-					break
+					break // TODO(mvdan): add a test
 				}
-				if obfPkg := obfuscatedTypesPackage(path); obfPkg != nil {
-					if obfPkg.Scope().Lookup(x.Name()) != nil {
-						recordStruct(named, tf.ignoreObjects)
-						return true
-					}
+				// The type is directly referenced by name,
+				// so obfuscatedTypesPackage can't return nil.
+				if obfuscatedTypesPackage(path).Scope().Lookup(obj.Name()) != nil {
+					recordStruct(named, tf.ignoreObjects)
+					return true
 				}
 			}
 		case *types.Func:
@@ -1159,20 +1153,6 @@ func (tf *transformer) transformGo(file *ast.File) *ast.File {
 			return true // we only want to rename the above
 		}
 
-		obfPkg := obfuscatedTypesPackage(path)
-		// Check if the imported name wasn't obfuscated.
-		// If the object returned from the obfuscated package's scope has a
-		// different type as the object we're searching for, they are
-		// most likely two separate objects with the same name, so ok to
-		// obfuscate.
-		if obfPkg == nil {
-			// TODO(mvdan): This is probably a bug.
-			// Add a test case where an indirect package has a name
-			// that we did not obfuscate.
-		} else if o := obfPkg.Scope().Lookup(obj.Name()); o != nil && reflect.TypeOf(o) == reflect.TypeOf(obj) {
-			return true
-		}
-
 		origName := node.Name
 		_ = origName // used for debug prints below