Update error message for `self`

[limira]

I did some search and find no similar issue, so here is my report:

Compile the following code:

struct Data;
impl Data {
    fn rc(self: &std::rc::Rc<Self>) {
        println!("&rc");
    }
}

Give the error:

error[E0658]: `&std::rc::Rc<Data>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
 --> src/main.rs:3:17
  |
3 |     fn rc(self: &std::rc::Rc<Self>) {
  |                 ^^^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/44874
  = help: consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`

Think the last line of the error message should be updated to match the current status: https://doc.rust-lang.org/reference/items/associated-items.html which states:

If the type of the self parameter is specified, it is limited to one of the following types:

• Self
• &Self
• &mut Self
• Box<Self>
• Rc<Self>
• Arc<Self>
• Pin<P> where P is one of the above types except Self.

[limira]

But it is better if Rust supports self: &Rc<Self>, is it possible?

[hellow554]

It's not yet possible. See the compiler output:

cannot be used as the type of self without the arbitrary_self_types feature

See the unstable book for that feature flag. So you have to use a nightly compiler and opt into that feature to use it. There's nothing werong here. The reference is referencing the nightly compiler and not the stable one.

[limira]

At least self: Rc<Self> works on stable (unless I am confused), but the error message only suggest &self, &mut self, or self: Box<Self>.

[hellow554]

Upsidaisy. you're right, my bad. Would you think you could do a PR and improve that? Should be fairly easy. Ask here if you need some guidance.

[limira]

I do a quick search and found at least 3 instances of help with the same content in wfcheck: the first here, the second on L825, the third is on L832 (all in the same file). If I do it, I want to replace them with a

    const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
                                     `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
                                     of the previous types except `Self`)";

then use .help(HELP_FOR_SELF_TYPE) in all 3 locations.

Anything else do I need to pay attention?

Hope it goes well, because I don't think I will try to compile Rust on my tiny machine!

[limira]

I create a PR #62712