A half-hour to learn Rust

Re:A half-hour to learn Rust by kzrdude on Saturday February 29, 2020 @07:50AM (#22450295)

I don't agree, because we can try the two following programs, and one of them does not compile. It compiles with underscore.

    let x = String::new();
    let _ = x;
    println!("{}", x);

    let x = String::new();
    x;
    println!("{}", x);   // ERROR: Use of moved value x.

And this is why I said that let _ = x; is a no-op. :)

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Re:A half-hour to learn Rust by fasterthanlime on Saturday February 29, 2020 @12:00PM (#22451517)

Huh, TIL! Had no idea `x;` actually moved `x`.

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Re:A half-hour to learn Rust by AnimalMuppet on Saturday February 29, 2020 @12:38PM (#22451772)

But where is it moved to? Who owns it now? Or is it just lost?

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Re:A half-hour to learn Rust by kzrdude on Saturday February 29, 2020 @01:15PM (#22451993)

yes, it is dropped

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Re:A half-hour to learn Rust by runarberg on Saturday February 29, 2020 @03:00PM (#22452663)

Ahh, I remember a while back reading a blog where the author talked about `std::mem::drop` being their favorite standard library function because they used this effect. It is literately defined as:

    pub fn drop<T>(_x: T) { }

https://doc.rust-lang.org/std/mem/fn.drop.html

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Re:A half-hour to learn Rust by kzrdude on Saturday February 29, 2020 @05:10PM (#22453516)

Right, so drop is useless :) We can drop things fine without it. Since of course it's scope/ownership based and not something that's explicitly called.

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Re:A half-hour to learn Rust by AnimalMuppet on Saturday February 29, 2020 @07:12PM (#22454181)

Well,

  drop(x);

is a lot clearer than

x;

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Re:A half-hour to learn Rust by e_proxus on Sunday March 01, 2020 @06:13AM (#22456363)

So is drop like free, or is it something else?

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Re:A half-hour to learn Rust by cyphar on Sunday March 01, 2020 @08:07AM (#22456706)

Yes, it's effectively like free -- though C++'s "auto" is probably a closer idea.

The main difference to C's free is that you simply cannot double-free a value (without using unsafe). That's because a value is only dropped when it falls out of scope -- and since it values only have one owner (and references must not outlive values) there cannot be any dangling references after it's dropped.

The documentation for std::mem::drop is explaining that dropping a value is a property of the language and scoping rules -- thus there is nothing for the function to do other than take a value and let it go out of scope.

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Re:A half-hour to learn Rust by pixel_fcker on Sunday March 01, 2020 @12:13PM (#22457812)

It’s more like delete in c++ because you can implement the Drop trait for a type in order to run code when it is dropped in order to clean up resources etc.

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Re:A half-hour to learn Rust by kzrdude on Sunday March 01, 2020 @04:29PM (#22459499)

Drop is just what runs when a value goes out of scope. Like destructors in C++.

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Re:A half-hour to learn Rust by kzrdude on Saturday February 29, 2020 @01:56PM (#22452233)

When you put it like that, I see that it's a rather minor detail.

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Re:A half-hour to learn Rust by Tehnix on Saturday February 29, 2020 @01:44PM (#22452151)

I think no-op is a bit misleading, and throwing away is more precise, since Rust has strict evaluation.

    let _ = timeConsumingFunction();

Still needs to spend time performing timeConsumingFunction(). It might not move, but it still did the work and produced a result, you are just not using it (and hence not need to move it).

If you you had lazy evaluation I would agree with your argument for the no-op terminology.

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Re:A half-hour to learn Rust by kzrdude on Saturday February 29, 2020 @01:57PM (#22452241)

That statement is different, the rhs is not a variable's name, so I agree with you, all of that is not a no-op.

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