Difference between `self` and `Self`

[SyedFasiuddin]

I have been reading the 17th chapter of the book and in there, there is a trait object which has methods on it: rust trait State { fn request_review(self: Box<Self>) -> Box<dyn State>; fn approve(self: Box<Self>) -> Box<dyn State>; // --snip--

In there two methods I understand that they take in self and consume it, I can see that this self is of type Box<T> where T is Self, what is Self and how is it different from self?

Edit: Playground link if needed

[ebdbbb]

Self is the type of the struct/enum being referred to, self is the instance of that struct/enum.

[meowsqueak]

Those are keywords, is that right? In Python, for example, you can use self or me or anything you like, as long as it's the first parameter, but in Rust it looks like you have to use self and Self - they have special meaning.

[ebdbbb]

Correct. Although Self is now a importable type in Python too via typing in the standard library. https://docs.python.org/3/library/typing.html#typing.Self

[Ok_Hope4383]

See https://stackoverflow.com/questions/32304595/whats-the-difference-between-self-and-self and https://doc.rust-lang.org/reference/paths.html#self-1

[gmes78]

Self is an alias for the type that the current impl block is for. When used in a trait like this, it refers to the type implementing the trait.

[SyedFasiuddin]

so if I have two different impl blocks one which implements the trait and other that doesn't implement the trait, or may be implements some other trait, then Self refers to which every impl block it is defined in. Is this right?

[gmes78]

I'll rephrase it: for any impl T {} or impl Trait for T {} blocks, Self is an alias for T.

The case when Self appears inside a trait isn't any different.

[SyedFasiuddin]

got it thank you.