`const { ... }` blocks cannot be written outside of function

[StackOverflowExcept1on]

I tried this code:

// error
const { assert!(size_of::<u32>() <= size_of::<usize>()) };

fn main() {
    //ok
    const { assert!(size_of::<u32>() <= size_of::<usize>()) };
}

I expected to see this happen: it compiles

Instead, this happened: it doesn't compile

error: expected item, found keyword `const`
 --> src/main.rs:2:1
  |
2 | const { assert!(size_of::<u32>() <= size_of::<usize>()) };
  | ^^^^^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

error: could not compile `app` (bin "app") due to 1 previous error

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rustc --version --verbose:

rustc 1.82.0-nightly (2cbbe8b8b 2024-07-28)
binary: rustc
commit-hash: 2cbbe8b8bb2be672b14cf741a2f0ec24a49f3f0b
commit-date: 2024-07-28
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 18.1.7

[RalfJung]

Thanks for the report! However, there is no bug here. This is expected. const { ... } is an expression, and you can't write expressions outside of functions.

If you want to just declare a constant outside a function for the purpose of checking an assertion, you can declare an anonymous const item as follows:

const _: () = assert!(size_of::<u32>() <= size_of::<usize>());

[workingjubilee]

The diagnostic should say that, then.

@rustbot label: -C-discussion +C-bug +A-diagnostics +D-papercut +D-terse

[maia-s]

It would be nice if this was allowed if the const block evaluates to (), like in this example. The _: () = syntax feels more like a workaround for missing functionality than something that's supposed to still be required now that we have const blocks.