Confusing message (empty attribute) in double_must_use

[ChrisJefferson]

Summary

The double_must_use message is:

warning: this function has an empty `#[must_use]` attribute, but returns a type already marked as `#[must_use]`

However, I don't know what an 'empty attribute' is. I tried googling for the term, and nothing came up.

Reproducer

No response

Version

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-unknown-linux-gnu
release: 1.79.0
LLVM version: 18.1.7

Additional Labels

No response

[y21]

In the context of this lint, #[must_use] is empty, #[must_use = "message"] is not.
The lint only warns on the former since must_use with a message can add additional context for that specific function that the type's #[must_use] message does not have, in which case it would not be redundant.

Maybe it could say "this function has a #[must_use] attribute with no message, but returns a type already marked as #[must_use]"