Use packages instead of importer

mockgen/parse.go

@@ -21,7 +21,6 @@ import (
 	"fmt"
 	"go/ast"
 	"go/build"
-	"go/importer"
 	"go/parser"
 	"go/token"
 	"go/types"
@@ -32,6 +31,8 @@ import (
 	"strconv"
 	"strings"
 
+	"golang.org/x/tools/go/packages"
+
 	"go.uber.org/mock/mockgen/model"
 )
 
@@ -640,11 +641,12 @@ func (p *fileParser) parseArrayLength(expr ast.Expr) (string, error) {
 		return val.Obj.Decl.(*ast.ValueSpec).Values[0].(*ast.BasicLit).Value, nil
 	case (*ast.SelectorExpr):
 		// when the length is a const defined in an external package
-		usedPkg, err := importer.Default().Import(fmt.Sprintf("%s", val.X))
-		if err != nil {
-			return "", p.errorf(expr.Pos(), "unknown package in array length: %v", err)
+		loadConfig := &packages.Config{
+			Mode: packages.NeedName | packages.NeedTypes,
 		}
-		ev, err := types.Eval(token.NewFileSet(), usedPkg, token.NoPos, val.Sel.Name)
+		loadConfig.Fset = token.NewFileSet()
+		pkgs, err := packages.Load(loadConfig, fmt.Sprintf("%s", val.X))
+		ev, err := types.Eval(token.NewFileSet(), pkgs[0].Types, token.NoPos, val.Sel.Name)
 		if err != nil {
 			return "", p.errorf(expr.Pos(), "unknown constant in array length: %v", err)
 		}