String.init(validatingUTF8:)是Swift标准库中的一个方法，在CString.swift中实现：

public init?(validatingUTF8 cString: UnsafePointer<CChar>) {
    let len = UTF8._nullCodeUnitOffset(in: cString)
    guard let str = cString.withMemoryRebound(to: UInt8.self, capacity: len, {
      String._tryFromUTF8(UnsafeBufferPointer(start: $0, count: len))
    })
    else { return nil }

    self = str
  }

String.init(utf8String:)在NSStringAPI中实现。

/// Creates a string by copying the data from a given
  /// C array of UTF8-encoded bytes.
  public init?(utf8String bytes: UnsafePointer<CChar>) {
    if let str = String(validatingUTF8: bytes) {
      self = str
      return
    }
    if let ns = NSString(utf8String: bytes) {
      self = String._unconditionallyBridgeFromObjectiveC(ns)
    } else {
      return nil
    }
  }

并且是基础NSString初始化器的Swift覆盖

- (nullable instancetype)initWithUTF8String:(const char *)nullTerminatedCString

而后者又在swift-corelibs-foundation/Sources/Foundation/NSString中的非Apple平台上实现。swift作为

public convenience init?(utf8String nullTerminatedCString: UnsafePointer<Int8>) {
    guard let str = String(validatingUTF8: nullTerminatedCString) else { return nil }
    self.init(str)
}

因此，这两个方法转换C字符串的方式没有区别，但是String.init(utf8String:)需要import Foundation，而String.init(validatingUTF8:)不需要额外的导入。