Emit error for attrs with empty lists instead of just warning they are unused

I don't know if the warning rather than the hard error was intentional before. Maybe for cases using macros? idk.

But I think we should tell users what is expected for attrs user used, instead of just warning it is unused.

Code

#[repr()]
pub struct T;

Current output

warning: unused attribute
 --> src/lib.rs:1:1
  |
1 | #[repr()]
  | ^^^^^^^^^ help: remove this attribute
  |
  = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default

Desired output

like use #[repr] without list:

#[repr]
pub struct T;

output

error[E0539]: malformed `repr` attribute input
 --> src/lib.rs:1:1
  |
1 | #[repr]
  | ^^^^^^^ expected this to be a list
  |
  = note: for more information, visit <https://doc.rust-lang.org/reference/type-layout.html#representations>
help: try changing it to one of the following valid forms of the attribute
  |
1 | #[repr(<integer type>)]
  |       ++++++++++++++++
1 | #[repr(C)]
  |       +++
1 | #[repr(Rust)]
  |       ++++++
1 | #[repr(align(...))]
  |       ++++++++++++
  = and 2 other candidates

Rationale and extra context

Maybe there are some previous discussions? But I didn't find it

Other cases

Also for other attrs expected list like allow

Rust Version

Build using the Nightly version: 1.92.0-nightly

(2025-09-16 a9d0a6f15533a364816c)

Anything else?

No response