Inappropriate `unused_parens` for inline `const` inside `vec!`/macros

[Zalathar]

https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Weird.20interaction.20between.20vec!.5B.5D.20macro.20and.20inline.20const

In this example, the inline const must be wrapped in parentheses, in order to work inside vec! under editions <2024:

fn main() {
    // Works fine, no warning.
    let _: [Vec<String>; 10] = [const { vec![] }; 10];
    // Works fine, no warning.
    let _: Vec<Vec<String>> = vec![(const { vec![] })];

    // Works, but warns about unnecessary parentheses.
    let _: Vec<Vec<String>> = vec![(const { vec![] }); 10]; // <=== THIS

    // These fail on stable79/beta80/nightly81 with edition 2021.
    // They work on nightly81 with edition 2024.
    let _: Vec<Vec<String>> = vec![const { vec![] }];
    let _: Vec<Vec<String>> = vec![const { vec![] }; 10];
}

(playground)

However, notice that in the case of vec![(const { vec![] }); 10], the compiler accepts the code but warns that the parentheses are unnecessary:

warning: unnecessary parentheses around function argument
 --> src/main.rs:8:36
  |
8 |     let _: Vec<Vec<String>> = vec![(const { vec![] }); 10];
  |                                    ^                ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
8 -     let _: Vec<Vec<String>> = vec![(const { vec![] }); 10];
8 +     let _: Vec<Vec<String>> = vec![const { vec![] }; 10];
  |

The warning is incorrect, because removing the parentheses would result in an error under edition <2024.

(The vec![(const { vec![] })] case correctly reports no warning.)

[Zalathar]

@rustbot label +A-diagnostics