为什么String :: find不是＆str的方法？

Question

I recently noticed that String::find is actually a method on an owned String .

But I can't see why it wouldn't just be a method on &str instead, making it useful in more cases (and still being just as useful for String ). Am I missing a reason for why it's like this, or is it just a historical accident?

Answer 1

Apparently, the documentation confused you. This method is listed under this section:

So it is not even implemented for String , but indeed just for &str .

Answer 2

Actually it's only available for String because it Deref s to str :

Methods from Deref<Target=str>

You won't find it in the source for String , but in the source for str .

Answer 3

Actually... you are wrong: it is not a String method.

What you are looking at is str::find .

It just so happens that the Rust documentation automatically includes on the String page the methods brought in by the fact that String implements Deref<Target=str> as can be seen here .

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Why does the documentation includes the methods that can be called on the target of Deref ?

Because you can actually call them directly on a String object, since the compiler will automatically follow Deref if it does not find the method you are calling, recursively.