False positive of `unused_braces` when using methods inside of const-generic inside enum variant

[datdenkikniet]

Code

#![allow(dead_code)]
use core::num::NonZeroUsize;

const TEST: NonZeroUsize = NonZeroUsize::new(1).unwrap();

struct Test<const N: usize> {
    value: [u8; { TEST.get() }],
}

Current output

Compiling playground v0.0.1 (/playground)
warning: unnecessary braces around const expression
 --> src/lib.rs:7:17
  |
7 |     value: [u8; { TEST.get() }],
  |                 ^^          ^^
  |
  = note: `#[warn(unused_braces)]` on by default
help: remove these braces
  |
7 -     value: [u8; { TEST.get() }],
7 +     value: [u8; TEST.get()],
  |

warning: `playground` (lib) generated 1 warning (run `cargo fix --lib -p playground` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.76s

Desired output

Compiling playground v0.0.1 (/playground)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.59s

Rationale and extra context

Applying the suggestion results in a compilation error as it is no longer valid.

I've tried my best to find duplicates, but have not managed to find any.

Other cases

This is also broken in the latest nightly:

rustc 1.91.0-nightly (809200ec9 2025-08-24)
binary: rustc
commit-hash: 809200ec956983fce4ae178b87dada69f01d0820
commit-date: 2025-08-24
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Rust Version

rustc 1.89.0 (29483883e 2025-08-04)
binary: rustc
commit-hash: 29483883eed69d5fb4db01964cdf2af4d86e9cb2
commit-date: 2025-08-04
host: x86_64-unknown-linux-gnu
release: 1.89.0
LLVM version: 20.1.7

Anything else?

No response

[GrigorenkoPV]

The version without {} seems to compile just fine (playground).

[datdenkikniet]

🤦 Ah, I had not correctly looked at this and you are absolutely right.

It's the expansion of a proc-macro (postcard::experimental::max_size::MaxSize) that adds an additional {} pair (as safeguard, perhaps?) which causes rustc to generate this warning.

I was too hasty with reporting this. I'll update the issue soon with more detailed (and useful) info, and likely close it because it's on the proc-macro to fix the problem.

Actually relevant code for those interested, plus expansion

Input code:

use postcard::experimental::max_size::MaxSize;
///
#[derive(MaxSize)]
struct Test {
    // False positive for unused braces.
    test: heapless::Vec<u8, { CHUNK_LEN.get() }>,
}

The warning-generating expansion:

    impl ::postcard::experimental::max_size::MaxSize for Test {
        const POSTCARD_MAX_SIZE: usize = 0
            + <Vec<
                u8,
                { { CHUNK_LEN.get() } },
            > as ::postcard::experimental::max_size::MaxSize>::POSTCARD_MAX_SIZE;
    }

[datdenkikniet]

Ahh crap, yeah, this was entirely our own fault. An update to postcard fixed this problem. I just got myself twisted up in the fact that the warning showed up right in the struct definition, but it's nothing the compiler was doing.

Closing, and thank you for pointing out my mistake so quickly so I had to re-check this report @GrigorenkoPV :)