support embedded error interface in source mode (#445)

Fixes: #446

mockgen/internal/tests/aux_imports_embedded_interface/faux/faux.go

@@ -3,6 +3,7 @@ package faux
 type Foreign interface {
 	Method() Return
 	Embedded
+	error
 }
 
 type Embedded interface{}

mockgen/internal/tests/import_embedded_interface/bugreport.go

@@ -27,6 +27,7 @@ import (
 type Source interface {
 	ersatz.Embedded
 	faux.Foreign
+	error
 }
 
 func CallForeignMethod(s Source) {

mockgen/model/model.go

@@ -468,3 +468,19 @@ func impPath(imp string) string {
 	}
 	return imp
 }
+
+// ErrorInterface represent built-in error interface.
+var ErrorInterface = Interface{
+	Name: "error",
+	Methods: []*Method{
+		{
+			Name: "Error",
+			Out: []*Parameter{
+				{
+					Name: "",
+					Type: PredeclaredType("string"),
+				},
+			},
+		},
+	},
+}

mockgen/parse.go

@@ -272,14 +272,24 @@ func (p *fileParser) parseInterface(name, pkg string, it *ast.InterfaceType) (*m
 			// Embedded interface in this package.
 			ei := p.auxInterfaces[pkg][v.String()]
 			if ei == nil {
-				if ei = p.importedInterfaces[pkg][v.String()]; ei == nil {
+				ei = p.importedInterfaces[pkg][v.String()]
+			}
+
+			var eintf *model.Interface
+			if ei != nil {
+				var err error
+				eintf, err = p.parseInterface(v.String(), pkg, ei)
+				if err != nil {
+					return nil, err
+				}
+			} else {
+				// This is built-in error interface.
+				if v.String() == model.ErrorInterface.Name {
+					eintf = &model.ErrorInterface
+				} else {
 					return nil, p.errorf(v.Pos(), "unknown embedded interface %s", v.String())
 				}
 			}
-			eintf, err := p.parseInterface(v.String(), pkg, ei)
-			if err != nil {
-				return nil, err
-			}
 			// Copy the methods.
 			// TODO: apply shadowing rules.
 			intf.Methods = append(intf.Methods, eintf.Methods...)