fix parse array with the external const correctly

mockgen/internal/tests/const_array_length/input.go

@@ -1,10 +1,13 @@
 package const_length
 
+import "math"
+
 //go:generate mockgen -package const_length -destination mock.go -source input.go
 
 const C = 2
 
 type I interface {
 	Foo() [C]int
 	Bar() [2]int
+	Baz() [math.MaxInt8]int
 }

mockgen/parse.go

@@ -22,8 +22,10 @@ import (
 	"fmt"
 	"go/ast"
 	"go/build"
+	"go/importer"
 	"go/parser"
 	"go/token"
+	"go/types"
 	"io/ioutil"
 	"log"
 	"path"
@@ -409,8 +411,19 @@ func (p *fileParser) parseType(pkg string, typ ast.Expr) (model.Type, error) {
 			case (*ast.BasicLit):
 				value = val.Value
 			case (*ast.Ident):
-				// when the length is a const
+				// when the length is a const defined locally
 				value = val.Obj.Decl.(*ast.ValueSpec).Values[0].(*ast.BasicLit).Value
+			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 nil, p.errorf(v.Len.Pos(), "unknown package in array length: %v", err)
+				}
+				ev, err := types.Eval(token.NewFileSet(), usedPkg, token.NoPos, val.Sel.Name)
+				if err != nil {
+					return nil, p.errorf(v.Len.Pos(), "unknown constant in array length: %v", err)
+				}
+				value = ev.Value.String()
 			}
 
 			x, err := strconv.Atoi(value)

mockgen/parse_test.go

@@ -116,24 +116,31 @@ func Benchmark_parseFile(b *testing.B) {
 
 func TestParseArrayWithConstLength(t *testing.T) {
 	fs := token.NewFileSet()
+	srcDir := "internal/tests/const_array_length/input.go"
 
-	file, err := parser.ParseFile(fs, "internal/tests/const_array_length/input.go", nil, 0)
+	file, err := parser.ParseFile(fs, srcDir, nil, 0)
 	if err != nil {
 		t.Fatalf("Unexpected error: %v", err)
 	}
 
 	p := fileParser{
-		fileSet: fs,
+		fileSet:            fs,
+		imports:            make(map[string]importedPackage),
+		importedInterfaces: make(map[string]map[string]*ast.InterfaceType),
+		auxInterfaces:      make(map[string]map[string]*ast.InterfaceType),
+		srcDir:             srcDir,
 	}
 
 	pkg, err := p.parseFile("", file)
 	if err != nil {
 		t.Fatalf("Unexpected error: %v", err)
 	}
 
-	expect := "[2]int"
-	got := pkg.Interfaces[0].Methods[0].Out[0].Type.String(nil, "")
-	if got != expect {
-		t.Fatalf("got %v; expected %v", got, expect)
+	expects := []string{"[2]int", "[2]int", "[127]int"}
+	for i, e := range expects {
+		got := pkg.Interfaces[0].Methods[i].Out[0].Type.String(nil, "")
+		if got != e {
+			t.Fatalf("got %v; expected %v", got, e)
+		}
 	}
 }