`rust-2021-incompatible-closure-captures` fires in 2021 edition code

[lnicola]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=92d18abb330e3fbc8a1425f3458830da

#![warn(rust_2021_compatibility)]
use std::sync::Arc;

pub struct Warns {
    _test: Arc<String>, // Removing this Arc results in no warning
    foo: String,
}

impl Warns {
    pub fn test(self) -> std::io::Result<()> {
        let closure = move || {
            let _ = self.foo;
            Ok(())
        };
        closure()
    }
}

The current output is:

warning: changes to closure capture in Rust 2021 will affect drop order
  --> src/lib.rs:11:23
   |
11 |         let closure = move || {
   |                       ^^^^^^^
12 |             let _ = self.foo;
   |                     ---- in Rust 2018, this causes the closure to capture `self`, but in Rust 2021, it has no effect
...
16 |     }
   |     - in Rust 2018, `self` is dropped here along with the closure, but in Rust 2021 `self` is not part of the closure
   |
note: the lint level is defined here
  --> src/lib.rs:1:9
   |
1  | #![warn(rust_2021_compatibility)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   = note: `#[warn(rust_2021_incompatible_closure_captures)]` implied by `#[warn(rust_2021_compatibility)]`
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
help: add a dummy let to cause `self` to be fully captured
   |
11 ~         let closure = move || {
12 +             let _ = &self;
   |

warning: `playground` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.52s

Ideally the output should look like:

[no warnings]

The code is already being compiled for the 2021 edition, there's no need to issue any warnings about behaviour that was different in 2018.
The compiler should assume that the code is working as expected.

This might affect other warnings in the same category, I'm not sure.

Tested in 1.63, beta and nightly.

[WaffleLapkin]

@rustbot claim

[WaffleLapkin]

This might affect other warnings in the same category, I'm not sure.

So, it seems that there are 3 warnings in this category and the issue only affects the closure one:

• array_into_iter is not affected because it checks that auto-ref happens, which doesn't in rust 2021
• non_fmt_panics is not affected because the linted code simply doesn't compile in rust 2021
• rust_2021_incompatible_closure_captures is affected

[WaffleLapkin]

Arc seems to be needed, because Vec is marked with #[rustc_insignificant_dtor] and the lint wants a significant Drop impl 😅

MRE:

#![warn(rust_2021_compatibility)]

pub struct Warns {
    // `Arc` has significant drop
    _significant_drop: std::sync::Arc<()>,
    field: String,
}

pub fn test(w: Warns) {
    _ = || drop(w.field);
}

fn main() {}