Lint request: warn when temporaries are used for values for which dropping has side effects

[PieterPenninckx]

Temporary values (that I tend to call 'anonymous values') are values for which there is no variable binding. E.g.:

let a = Vec::<u8>::new().len();

Here the value of the type Vec<u8> is a temporary value.

Temporary values are -- generally speaking -- dropped at the semi-colon that terminates the statement in which they were created. More details can be found in the reference. This is not so well known and can be confusing, see e.g. rust-lang/book#1871. This can lead to bugs when a temporary is used to store a value for which the precise point where it is dropped is important, such as a LockResult. An example can be found in rust-lang/rust#37612.

To prevent such bugs, I think it would be a good idea if clippy would warn when a 'sensitive' value is stored in a temporary. With a 'sensitive' value, I mean something like a LockResult where calling the drop() has important side effects (so not just releasing memory).

[PieterPenninckx]

I realized that my request was too vague about which datatypes have important side effects when being dropped. Since there is no way clippy could judge this, I would just work with a list. This will not capture all problems, but hopefully enough of them. This is the list I came up with:

• std::sync::LockResult<T>
• std::sync::TryLockResult<T>
• std::sync::MutexGuard<T>
• std::sync::RwLockReadGuard<T>
• std::sync::RwLockWriteGuard<T>
• std::cell::Ref<T>
• std::cell::RefMut<T>

[PieterPenninckx]

I'm working on this.
Update: I gave up a long time ago.

[academician]

Is it worth revisiting and possibly closing this issue after the changes to temporary scope in the 2024 edition?

[PieterPenninckx]

@academician , I think the issue is still there in the 2024 edition: the 2024 edition only narrowed the scope when the temporary is in a tail expression, which is a case I hadn't ever considered. If the temporary is not in a tail expression, the issue is still there.

Also, reading into the documentation again after seven years, I now read "The exact rules for temporary lifetime extension are subject to change.", which to me is a sign that storing something of which the drop order is of importance in a temporary can still lead to issues.