Suggest 2021 edition upgrade on attempt to use disjoint capture

[jruderman]

Given the following code, compiled using Rust 2018 edition:

fn main() {
    let mut x = ("a".to_string(), "b".to_string());
    let mut f = || x.0 += "...";
    x.1 += "...";
    f();
}

The current output is:

error[E0499]: cannot borrow `x.1` as mutable more than once at a time
 --> src/main.rs:4:5
  |
3 |     let mut f = || x.0 += "...";
  |                 -- --- first borrow occurs due to use of `x` in closure
  |                 |
  |                 first mutable borrow occurs here
4 |     x.1 += "...";
  |     ^^^ second mutable borrow occurs here
5 |     f();
  |     - first borrow later used here

For more information about this error, try `rustc --explain E0499`.

Ideally the output should look include:

Note: disjoint capture is available in Rust 2021 edition; see https://doc.rust-lang.org/edition-guide/rust-2021/disjoint-capture-in-closures.html

[leonardo-m]

Is a good idea to generate a similar note if the programmer tries to use try_from without importing it?

[JakobDegen]

I'll start working on the suggestion from @leonardo-m (I've asked on zulip whether this is something we even want). I don't expect to work on the main point brought up in this issue though