#[must_use] silently does nothing on type aliases

[tamird]

There are at least two problems with #[must_use]:

• it doesn't work on type aliases at all.
• for methods that return a type containing a #[must_use] type, no warning is issued if the #[must_use] value is immediately discarded by destructuring assignment at the caller.

http://is.gd/TEG45t

---

Edit: the consensus is that #[must_use] should not be usable on type aliases, but it should not just silently fail. The behaviour with destructing assignment is intentional.

[Thiez]

Performing let _ = yields_must_use(); already silences the #[must_use] warning, surely it would not make sense for let (x, _) = yields_tuple_of_u8_and_must_use(); to give a warning; it is doing the exact same thing as assigning a #[must_use] to _.

As for the type aliases, they always have the exact same behaviour as their original type. If you want different behaviour you must use a newtype. It would be inconsistent to make an exception for #[must_use].

[tamird]

Alright, let's forget about the type alias case.

I still feel that the tuple cast (or any other destructuring assignment) presents a hole in the semantics of #[must_use].

[nagisa]

I’m on the same page as @Thiez.

[Mark-Simulacrum]

The type alias problem here appears to have been discussed and decided on as intentional. The tuple problem is covered by #26291, as far as I can tell, or should be added to that issue and this issue closed.

[Mark-Simulacrum]

Just ran into #[must_use] on a type alias silently doing nothing. I understand why it doesn't make sense for it to work, but I do expect some form of warning. Should I open another issue about that?

[Mark-Simulacrum]

Rebranding this as #[must_use] silently does nothing on type aliases.

[zackmdavis]

but I do expect some form of warning

Related/likely-prerequisite: #14407.

[LunaBorowska]

I don't think it would be necessarily inconsistent to make #[must_use] work for types because unlike most attributes, #[must_use] doesn't affect code generation. Other attributes that don't affect code generation - allow, warn, deny, forbid, deprecated and doc work fine for type statements.

[jieyouxu]

As of stable 1.76.0, rustc emits the following warning:

warning: `#[must_use]` has no effect when applied to a type alias
 --> src/main.rs:4:1
  |
4 | #[must_use]
  | ^^^^^^^^^^^
  |
  = note: `#[warn(unused_attributes)]` on by default