String value to UnsafePointer<UInt8> function parameter behavior

Question

I found the following code compiles and works:

func foo(p:UnsafePointer<UInt8>) {
    var p = p
    for p; p.memory != 0; p++ {
        print(String(format:"%2X", p.memory))
    }
}

let str:String = "今日"
foo(str)

This prints E4BB8AE697A5 and that is a valid UTF8 representation of 今日

As far as I know, this is undocumented behavior. from the document:

When a function is declared as taking a UnsafePointer argument, it can accept any of the following:

• nil, which is passed as a null pointer
• An UnsafePointer, UnsafeMutablePointer, or AutoreleasingUnsafeMutablePointer value, which is converted to UnsafePointer if necessary
• An in-out expression whose operand is an lvalue of type Type, which is passed as the address of the lvalue
• A [Type] value, which is passed as a pointer to the start of the array, and lifetime-extended for the duration of the call

In this case, str is non of them.

Am I missing something?

---

ADDED:

And it doesn't work if the parameter type is UnsafePointer<UInt16>

func foo(p:UnsafePointer<UInt16>) {
    var p = p
    for p; p.memory != 0; p++ {
        print(String(format:"%4X", p.memory))
    }
}
let str:String = "今日"
foo(str)
//  ^ 'String' is not convertible to 'UnsafePointer<UInt16>'

Even though the internal String representation is UTF16

let str = "今日"
var p = UnsafePointer<UInt16>(str._core._baseAddress)
for p; p.memory != 0; p++ {
    print(String(format:"%4X", p.memory)) // prints 4ECA65E5 which is UTF16 今日
}

Answer 1

This is working because of one of the interoperability changes the Swift team has made since the initial launch - you're right that it looks like it hasn't made it into the documentation yet. String works where an UnsafePointer<UInt8> is required so that you can call C functions that expect a const char * parameter without a lot of extra work.

Look at the C function strlen, defined in "shims.h":

size_t strlen(const char *s);

In Swift it comes through as this:

func strlen(s: UnsafePointer<Int8>) -> UInt

Which can be called with a String with no additional work:

let str = "Hi."
strlen(str)
// 3

Look at the revisions on this answer to see how C-string interop has changed over time: https://stackoverflow.com/a/24438698/59541