Using SpawnIter results in E0521: borrowed data escapes outside of function

[frank prawn]

Hey all, I'm a hobby game developer and new to rust (I have only ever used it for Bevy). I took inspiration from the official Bevy examples to create a main menu using SpawnIter. However, I wanted extract the closures into their own functions. So I created create_button_bundle , which uses SpawnIter to iterate over a list of ButtonInfos and calls create_menu_bundle for each element.

However, this leads to the lifetime error below. The passed in ButtonInfo has shorter lifetime than static, but I don't see why it needs to have static lifetime. The information in each ButtonInfo is cloned into the Bundle, so how is the data escaping create_menu_bundle? The suggested fix from rustc --explain E0521 is to not type annotate my closures, but I am not using closures.

Any tips, either to fix this issue, or to restructure my code, would be appreciated.

   --> src/main.rs:100:17
    |
82  |       button_infos: &[ButtonInfo],
    |       ------------  - let's call the lifetime of this reference `'1`
    |       |
    |       `button_infos` is a reference that is only valid in the function body
...
100 | /                 Children::spawn(SpawnIter(
101 | |                     button_infos.into_iter().map(create_button_bundle)
102 | |                 )),
    | |                  ^
    | |                  |
    | |__________________`button_infos` escapes the function body here
    |                    argument requires that `'1` must outlive `'static`

For more information about this error, try `rustc --explain E0521`.

[royal bane]

fn create_button_bundle(
    button_info: &ButtonInfo
) -> impl Bundle + use<>

[frank prawn]

That doesn't seem to make a difference for me

[royal bane]

did you put only on the one that i sent or on all of the -> impl Bundle?

[frank prawn]

Oh, I just updated the one in create_button_bundle. Adding it to create_menu_bundle gives me an error saying I must declare all the type parameters used, so I do

fn create_menu_bundle<T: Component>(
    menu_tag: T,
    button_infos: &[ButtonInfo],
) -> impl Bundle + use<T>

which removes the new error, but I still have the old error.

[royal bane]

the you probably need to pass Vec<ButtonInfo> instead of &[ButtonInfo]

[frank prawn]

That works, but why? Is it because create_menu_bundle now takes ownership? Why would that change the lifetime of the data?

[royal bane]

SpawnIter is lazy, it will only run after the system ends, so it can't hold references

[frank prawn]

Oh yeah, I also had to change .map(create_button_bundle) to map(|button_info| {return create_button_bundle(&button_info);}), for anyone having this issue later.

[frank prawn]

So the Vec, unlike the &[] (array reference?) is stored on the heap, so when create_menu_bundle gets ownership, it can transfer this ownership to SpawnIter, and have it live for however long it needs to (because it is on the heap). And SpawnIter needs the vector to have static lifetime because it runs lazily?

Is there a non-lazy SpawnIter?

[royal bane]

not that i know of

[silk vale]

Just a small clarification -- a slice (like &[ButtonInfo]) makes no claims about where the data it refers to lives. It could be an array on the stack, or it could be a portion of a vector stored in the heap.

But since the vector is fully owned (i.e. it's not borrowing anything, which would impose lifetimes on it), you can freely pass it around without triggering the borrow checker.