尝试分配时出现Swift错误：newString = oldString.removeLast（Int）

Question

我没有看到一些非常基本的东西w /分配Swift字符串方法的结果（Swift 4，Xcode 9.1 playground）。 anyString.removeLast（X）可以自行正常工作，最后用x少字符修改anyString，但是如果我尝试将结果赋值给声明的变量，比如：newString = anyString.removeLast（X），我得到一个错误。以下是结果示例或注释失败：

var address = "St. James St."
var searchedFor = "St."
var replaceWith = "Street"
var standardAddress = ""
if address.hasSuffix(searchedFor) {
    address.removeLast(searchedFor.count)
    standardAddress = address + replaceWith
}
print(standardAddress)
// prints: St. James Street

但这失败了：

var address = "St. James St."
var searchedFor = "St."
var replaceWith = "Street"
var standardAddress = ""
if address.hasSuffix(searchedFor) {
    standardAddress = address.removeLast(searchedFor.count) + replaceWith
    // error: type 'inout String' does not conform to protocol 'BidirectionalCollection' standardAddress = address.removeLast(searchedFor.count) + replaceWith
}
print(standardAddress)

这也失败了，不同的Xcode游乐场错误：

var address = "St. James St."
var searchedFor = "St."
var replaceWith = "Street"
var standardAddress = ""
if address.hasSuffix(searchedFor) {
    standardAddress = address.removeLast(searchedFor.count)
    // error: cannot assign value of type '()' to type 'String' standardAddress = address.removeLast(searchedFor.count)
}
print(standardAddress)

感谢您帮助我了解上述差异。