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

[Zalathar]

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

[Zalathar]

(Given that this works in 2024, I'm not sure whether it's worth the effort of a specific hint, but I figured I might as well file an issue for it.)

[Zalathar]

See also #126457, where the compiler sometimes inappropriately warns about the necessary parens.

[Neutron3529]

Currently, even in edition 2024, vec![const { vec![] }]; failed to compile (at least for now)
playground

[ehuss]

Currently, even in edition 2024, vec![const { vec![] }]; failed to compile (at least for now)

The relevant edition is the one used for building the standard library itself. So the relevant edition will be the one where the macro is defined, not where it is called.

[jendrikw]

Can we add another macro pattern for edition 2021 to match this?