`#[repr]` on zero-variant enums leaves the reader with questions

Code

#[repr(isize)]
enum Uninhabited {}

Current output

error[E0084]: unsupported representation for zero-variant enum
 --> src/lib.rs:1:1
  |
1 | #[repr(isize)]
  | ^^^^^^^^^^^^^^
2 | enum Uninhabited {}
  | ---------------- zero-variant enum

Desired output

error[E0084]: zero-variant enums do not support `#[repr]`
 --> src/lib.rs:1:1
  |
1 | #[repr(isize)]
  | ^^^^^^^^^^^^^^
2 | enum Uninhabited {}
  | ---------------- zero-variant enum

Rationale and extra context

The current output leaves the user questioning whether there may be some valid repr while not being told which ones are valid options. The diagnostic should probably be more clear about the fact that the attribute is generally not supported on zero-variant enums

Other cases

Rust Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-pc-windows-msvc
release: 1.86.0
LLVM version: 19.1.7

Anything else?

No response

Reproduce the diagnostic with the issue's zero-variant enum and #[repr(isize)] snippet. Find the compiler diagnostic entry for E0084, update the wording so it states that zero-variant enums do not support #[repr], and update or add the corresponding expected diagnostic test. Confirm the revised output matches the desired example.