Inline `const` expression inside `vec!`/macros should suggest adding parentheses under edition <2024

Code

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];

    // 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];
}

Current output

error: no rules expected the token `const`
  --> src/main.rs:12:36
   |
12 |     let _: Vec<Vec<String>> = vec![const { vec![] }];
   |                                    ^^^^^ no rules expected this token in macro call
   |
   = note: while trying to match end of macro

error: no rules expected the token `const`
  --> src/main.rs:13:36
   |
13 |     let _: Vec<Vec<String>> = vec![const { vec![] }; 10];
   |                                    ^^^^^ no rules expected this token in macro call
   |
   = note: while trying to match end of macro

Desired output

help: consider surrounding the const expression with parentheses
   |
12 -     let _: Vec<Vec<String>> = vec![const { vec![] }];
12 +     let _: Vec<Vec<String>> = vec![(const { vec![] })];
   |

Rationale and extra context

For compatibility reasons, macros under edition <2024 do not treat const as the start of an expr, so a layer of parentheses is needed to convince the macro to treat inline-const as an expression.

Other cases

No response

Rust Version

(Stable Rust 1.79.0 on playground.)

Anything else?

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

Start with the reproducer in src/main.rs, focusing on the vec! macro calls containing inline const expressions under edition 2021. Compare the failing cases with the parenthesized working case and the edition 2024 behavior. Done means the failing forms produce a help suggestion to surround the const expression with parentheses.