On v1.20.5, if String() method of a prometheus.Desc returned from NewInvalidDesc called, it will cause a nil pointer derefence, which will result in an incorrectly formatted string

type Desc struct { //... variableLabels *compiledLabels //... } func NewInvalidDesc(err error) *Desc { return &Desc{ err: err,	} } func (d *Desc) String() string { //... vlStrings := make([]string, 0, len(d.variableLabels.names)) for _, vl := range d.variableLabels.names { //... }

NewInvalidDesc will create a Desc with variableLabels equal to nil, therefore accessing its field names will cause a nil-pointer dereference.

The result is: descriptor %!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference) is invalid: test

The consequence of that is an unpleasant error message in prometheus.Register