Limiting what can be moved

community
1

Is there anyway to get the compiler / linter to enforce the following pattern:

{ // move start

  let x = ..;
  let y = ...;
  
  move | ... | { ... }



} // move end

Then, the move is only allowed to move things within the move begin / move end block. The goal here is that I want to be explicit about what is being moved.

2 Likes
2

By default all values are moved. Exceptions are, when its type implement Copy.

Ref: Expressions - The Rust Reference

3

To be pedantic, I think it is "all values USED IN THE CLOSURE are moved"

Sometimes, in large functions, it is hard to see what is / is not moved.

This I want to put a {} block around a move and somehow have the linter / compiler tell me if something else is being moved. Is there a way to lint for this?

1 Like
4

Rust doesn't have any features for listing the captured variables. You can stop things from moving by using them after the closure, possibly in a let a = a or drop(a) way.

1 Like
5

Yes you can be explicit it is just a matter of adding:

fn foo() { // move start

  let x = ..;
  let y = ...;
  
  move | ... | { ... }

} // move end

I am kidding of course, if only slightly. If you are concerned about what is moved thats probably because there is too much complexity therefore the extra function boundary might be a good idea.

1 Like
6 
{
    fn closure(x: T, y: U) -> impl Fn(...) -> ... {
        move |...| { ... }
    }

    let x = ..;
    let y = ..;

    closure(x, y)
}
4 Likes
7

@sinkuu: This is pedantic -- and I'm willing to pay the runtime cost, but is there anyway to get the compiler to compile the closure(x,y) call away at compile time?

8

It will probably be inlined already, but you can hint further with #[inline] or #[inline(always)] on the function.

9

Does anybody know if it's feasible to write a clippy lint for this - does clippy have enough context to recognize which names are captured (vs. imports, for example)?

10

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