Dashboard examples should be run using the default edition for their book

[adpaco-aws]

Contrary to what we had thought, dashboard examples should be run using Rust 2015 edition by default, unless otherwise noted:
https://doc.rust-lang.org/edition-guide/rust-2015/index.html

Since it's the default edition, there's no way to port your code to Rust 2015; it just is. You'll be transitioning away from 2015, but never really to 2015. As such, there's not much else to say about it!

This is a bit counter-intuitive since only 4 examples out of the ~700 ones that we run specify the 2015 edition as a requirement. The rest do not specify the edition. Take for example Rust by Example/Functions/Closures/Examples in std/Iterator::any/22.rs, which is currently failing while using edition 2021:

stderr:
            
error[E0308]: mismatched types
--> src/test/dashboard/books/Rust by Example/Functions/Closures/Examples in std/Iterator::any/22.rs:19:57
|
19 |     println!("2 in array2: {}", array2.into_iter().any(|&x| x == 2));
|                                                         ^--
|                                                         ||
|                                                         |expected due to this
|                                                         expected integer, found reference
|                                                         help: did you mean `x`: `&{integer}`
|
= note:   expected type `{integer}`
found reference `&_`

error: aborting due to previous error

This does not happen when using the 2015 edition. So it is probably the best to:

1. Not use any edition by default (this is essentially defaulting to 2015).
2. Use an edition only if specified in the example (such cases do not exist at the moment except for the 4 mentioned examples, which use Rust 2015 anyway).

[adpaco-aws]

When testing the proposed changes I found this other failure:

error[E0670]: `async fn` is not permitted in Rust 2015
--> src/test/dashboard/books/The Rust Reference/Items/Functions/230.rs:3:1
|
3 | async fn regular_example() { }
| ^^^^^ to use `async fn`, switch to Rust 2018 or later
|
= help: set `edition = "2021"` in `Cargo.toml`
= note: for more on editions, read https://doc.rust-lang.org/edition-guide

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found keyword `unsafe`
--> src/test/dashboard/books/The Rust Reference/Items/Functions/230.rs:4:7
|
4 | async unsafe fn unsafe_example() { }
|       ^^^^^^ expected one of 8 possible tokens

error: aborting due to 2 previous errors

The example clearly needs to use edition 2018 or later but this is not specified. Moreover, the Rust Playground seems to use the 2021 edition by default. I am bit confused about this, so I am going to ask in Zulip.

[adpaco-aws]

Turns out each book defines a default edition on their own. The information is included in book.toml, and if it does not appear then the default edition is Rust 2015: https://rust-lang.zulipchat.com/#narrow/stream/237824-t-lang.2Fdoc