pkg/proc: Prevent program crash when called meanless expression (#1934)

If we call one expression which is the fake method of meanless
string, `delve` will panic. Strengthen the inspection of boundary
conditions when supporting function calls on non-struct types.

Update: #1871

pkg/proc/eval.go

@@ -1041,10 +1041,16 @@ func (scope *EvalScope) evalStructSelector(node *ast.SelectorExpr) (*Variable, e
 	if err != nil {
 		return nil, err
 	}
+
 	// Prevent abuse, attempting to call "nil.member" directly.
 	if xv.Addr == 0 && xv.Name == "nil" {
 		return nil, fmt.Errorf("%s (type %s) is not a struct", xv.Name, xv.TypeString())
 	}
+	// Prevent abuse, attempting to call "\"fake\".member" directly.
+	if xv.Addr == 0 && xv.Name == "" && xv.DwarfType == nil && xv.RealType == nil {
+		return nil, fmt.Errorf("%s (type %s) is not a struct", xv.Value, xv.TypeString())
+	}
+
 	rv, err := xv.findMethod(node.Sel.Name)
 	if err != nil {
 		return nil, err

service/test/variables_test.go

@@ -1205,6 +1205,8 @@ func TestCallFunction(t *testing.T) {
 		{"x.CallMe()", nil, nil},
 		{"x2.CallMe(5)", []string{":int:25"}, nil},
 
+		{"\"delve\".CallMe()", nil, errors.New("\"delve\" (type string) is not a struct")},
+
 		// Nested function calls tests
 
 		{`onetwothree(intcallpanic(2))`, []string{`:[]int:[]int len: 3, cap: 3, [3,4,5]`}, nil},