Unused comparison that must be used

help
1

I found this warning today: unused comparison that must be used
I miss-typed a == instead of a = .
Example: foo==true; instead of foo=true;
Why is not this a compile error, but only a warning?
Is there scenarios when this code is actually wanted?

2

It is allowed for the same reason that you can ignore the integer returned by foo in the snippet below:

fn foo() -> i32 {
    println!("Hi!");
    10
}

fn main() {
    foo();
}
3

Use #![deny(unused_must_use)] if you want it a compiler error.

2 Likes
4

What happens if a fn returns a Result that is not used?
If an Err is returned? Is it just silently ignored?
Or it panics like unwrap?

5

It is completely ignored, but you can annotate the function to warn if you do.

6

ok.
My conclusion is that is wise to always use
#![deny(unused_must_use)].
Is this attribute only for unused variables?
Or also for unused functions and methods?
The first case can make strange complications.
The second is just a reminder, cannot complicate anything.

7

This topic was automatically closed 90 days after the last reply. We invite you to open a new topic if you have further questions or comments.