Message suggesting prefixing with underscore is unhelpful

[Kixunil]

I believe that in the vast majority of cases when an unused variable is present, the author of the code intended to use it, but forgot. Thus the message suggesting to effectively turn off the warning is unhelpful.

I suggest changing it to something like:

help: you probably wanted to use it somehow, check the code around to see if it's the case
help: if you really intend to create the variable and not use it, prefix it with underscore: `_var`

While this doesn't concern me much personally, I believe it'd be much more helpful to novice programmers. To put it in extreme, imagine this kind of lint message: "Reference outlives borrowed value. Consider putting #![allow(undefined_behavior)] at the top of your module."

And I must admit the current warning feels a bit strange to me too. ("Dear compiler, the content of your message is completely useless, but its presence saved my ass twice today, so thank you for that!")

Disclaimer: this is entirely based on my personal experience and may not be true in general. Feel free to close if there's a good evidence that I'm broken, not the compiler. :)

This issue has been assigned to @loris-intergalactique via this comment.

[loris-intergalactique]

Hi, I'm a novice and would like to help ! I feel the same way about this message :)

[loris-intergalactique]

What do you think about mentioning the lifetime of the variable ?, like the following snippet from the Rust book :

warning: unused variable `i` in the lifetime starting at 1:1 and ending at 5:1
 --> src/main.rs:2:9
1 |  / fn main() {
...  |
2 |  |    let i = 3; // Lifetime for `i` starts. ────────────────┐
3 |  |     //                                                    │
...  |
4 |  |     //                                                    │
5 |  | }   // Lifetime ends. ────────────────────────────────────┘  
  |  |_^
  |  help: if the variable is meant to be unused, consider prefixing with an underscore: `_i`
  |
  = note: `#[warn(unused_variables)]` on by default

Or maybe something like :

note: ...the reference/variable is valid for the lifetime 'a as defined on the impl at 27:1...
  --> src\lib.rs:27:1
   |
27 | / impl<'a> MyImpl<'a>{
28 | |
29 | |     pub fn myfunc(i: &String) -> MyImpl{
30 | |         ...
...  |
89 | |     }
90 | | }
   | |_^

What's your opinion ? Am I too novice ?

[soumyalahiri]

Hello! I'm new to this repo and I'd like to take up this issue.

[Kixunil]

@loris-intergalactique what does it have to do with lifetimes?

[loris-intergalactique]

@Kixunil When I wrote the message, I didn't think about encapsulated lifetimes, my bad - This could have made the hints maybe even less helpful. What I think could give an insight to programmers about what they tried to accomplish is actually mentionning the function name, not the lifetime.

I think that in a huge code base this could make save some time, and help checking the code around.

I imagine that if you had something like :

warning: unused variable `bark` in the function new_dog()
--> src/main.rs:2:9
1 |  / fn new_dog() {
...  |
2 |  |    let bark = Bark::from("bark"); 
  |  |        ^^^^ help: you probably wanted to use it somehow, check the code around to see if it's the case
  |  |             help: if the variable is meant to be unused, consider prefixing with an underscore: `_bark`
  |
  = note: `#[warn(unused_variables)]` on by default

You'd have more hints about what's going on. With only weeks of practicing Rust, I don't know every use cases yet. But I think that combining the nice message that you gave, as well as another hint on where this variable is could be nice++

What do you think about that ?

[Kixunil]

Ah I think you mean to mention the scope in which the variable is introduced, right? That would be very interesting, I think.

[loris-intergalactique]

Yes, that's right !

I'm gonna try implenting it and testing it. I'm happy to make my first contribution ! :)
@rustbot claim

Hi, just a quick edit to say that I am still on it. I have more work than usual, but I do want to make this contrib ! :)