macros(like vec!) cannot parse const { 1 } block as valid expressions.

[Neutron3529]

I tried this code:

let _ = vec![const { 1 }; 0];

I expected to see this happen: Compiles.

Instead, this happened:

error: no rules expected the token const
--> src/main.rs:3:18
|
3 | let _ = vec![const { 1 }; 0];
| ^^^^^ no rules expected this token in macro call
|
= note: while trying to match end of macro

Meta

rustc --version --verbose:

rustc 1.82.0-nightly (0d634185d 2024-08-29)
binary: rustc
commit-hash: 0d634185dfddefe09047881175f35c65d68dcff1
commit-date: 2024-08-29
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 19.1.0

It seems that, macro could not recognize const { 1 } as a block expression, which leads to the error.

What's more, with adding an unnecessary bracket, let _ = vec![(const { 1 }); 0];, the code compiles, but an warning about unnecessary bracket generated, which suggests me delete the unnecessary bracket and facing the error directly.

[zirconium-n]

Kind of duplicate of #126459.

Edit: It works in edition 2024 and it's a breaking change to make it work in edition 2021. So the appropriate action is suppress the unused paren lint in edition 2021 I guess. See #126457

[ehuss]

Indeed this looks like a duplicate of the two above linked issues, so closing in favor of them. If there is a reason to keep this open separately, let us know! Thanks!