SystemParam: lifetime may not live long enough

[mbolt35]

Bevy version

v0.10.0

What you did

Upgrading a bevy 0.9 project to the latest and greatest. I have a few #[derive(SystemParam)] implementations that were working great with 0.9, but now give me an error: lifetime may not live long enough which appears to all occur within the SystemParam attribute itself. Here's the struct definition in question:

/// TileEntities is a helper system parameter that provides pulling
/// component references directly from the tile positions.
#[allow(clippy::complexity)]
#[derive(SystemParam)]
pub struct TileEntities<'w, 's, T: EntityRef + Component>
    where <T as EntityRef>::ComponentType: Component,
{
    references: ParamSet<'w, 's, (
        Query<'w, 's, &'static mut T>,
        Query<'w, 's, &'static T>)>,
    component: ParamSet<'w, 's, (
        Query<'w, 's, &'static mut T::ComponentType>,
        Query<'w, 's, &'static T::ComponentType>)>,
    map: TileQuery<'w, 's, SimulationLayer>,
}

The TileQuery itself is also a SystemParam:

#[derive(SystemParam)]
pub struct TileQuery<'w, 's, T: Component + Layer> {
    map: Query<'w, 's, &'static TileStorage, With<T>>,
    world_manager: Res<'w, WorldManager>,
    tile_mapper: TileMapper<'w>,
}

/// TileMapper can be included in systems and used to map tile positions back into world
/// coordinates and other tile related conversions.
#[derive(SystemParam)]
pub struct TileMapper<'w> {
    config: Res<'w, MapConfig>
}

The EntityRef trait is defined as:

/// EntityRef is a trait used to reference a single entity from multiple tiles
pub trait EntityRef {
    /// The `ComponentType` refers to the internal type of component the `Entity`
    /// refers to. This is especially useful information to have for building
    /// generic bevy `Query` parameters for systems.
    type ComponentType;

    /// This method returns the `UVec2` tile position of the entity set.
    fn tile(&self) -> UVec2;

    /// This method returns the `Entity` identifier.
    fn get(&self) -> Entity;
}

What went wrong

I'm getting 4 nearly identical errors all pointing to the #[derive(SystemParam)].

Here's the first error, followed by the 3 differences

error: lifetime may not live long enough
   --> src\tiles\tile_entities.rs:155:10
    |
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |          |
    |          lifetime `'w` defined here
    |          lifetime `'w2` defined here
    |          associated function was supposed to return data with lifetime `'w2` but it is returning data with lifetime `'w`
    |
    = help: consider adding the following bound: `'w: 'w2`
    = note: requirement occurs because of the type `tile_entities::TileEntities<'_, '_, T>`, which makes the generic argument `'_` invariant
    = note: the struct `tile_entities::TileEntities<'w, 's, T>` is invariant over the parameter `'w`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
    = note: this error originates in the derive macro `SystemParam` (in Nightly builds, run with -Z macro-backtrace for more info)

Second:

...
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |          |
    |          lifetime `'s` defined here
    |          lifetime `'s2` defined here
    |          associated function was supposed to return data with lifetime `'s2` but it is returning data with lifetime `'s`
    |
    = help: consider adding the following bound: `'s: 's2`
...

Third:

...
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |          |
    |          lifetime `'s2` defined here
    |          lifetime `'s` defined here
    |          associated function was supposed to return data with lifetime `'s` but it is returning data with lifetime `'s2`
    |
    = help: consider adding the following bound: `'s2: 's`
...

Fourth:

...
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |          |
    |          lifetime `'w2` defined here
    |          lifetime `'w` defined here
    |          associated function was supposed to return data with lifetime `'w` but it is returning data with lifetime `'w2`
    |
    = help: consider adding the following bound: `'w2: 'w`
...

Additional information

I will work to see if I can get a reproducible case with less dependencies than the current project I am working in. Apologies if this is unclear in anyway. Will be happy to assist anyway possible!

Thanks for an incredible open source project

[alice-i-cecile]

What happens if you change those lifetimes to 'static?

[mbolt35]

What happens if you change those lifetimes to 'static?

@alice-i-cecile Just to be clear, do you mean a 'static constraint on 'w and 's? Or do you mean dropping the 'w and 's in my type and replacing with 'static? (I'm assuming there could be a difference there do to the way the param parses the struct body)

Will give this a shot ASAP! Thanks for the response

[joseph-gio]

Ah, seems like this issue pops up when a SystemParam has invariant lifetimes. This used to work, but it must have regressed during the 0.9-0.10 cycle (likely my fault) since none of our tests use invariant lifetimes.

Here's a minimum reproducible example:

#[derive(SystemParam)]
pub struct InvariantParam<'w, 's> {
    set: ParamSet<'w, 's, (Query<'w, 's, ()>,)>,
}

Seems like this will be fixed by #8030.

[mbolt35]

@alice-i-cecile Adding the 'static constraint on 'w and 's did seem to solve 2 of the errors, but looks as if now it's expecting a lifetime to outlive static:

error[E0478]: lifetime bound not satisfied
   --> src\tiles\tile_entities.rs:155:10
    |
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |
note: lifetime parameter instantiated with the lifetime `'_w` as defined here
   --> src\tiles\tile_entities.rs:155:10
    |
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    = note: but lifetime parameter must outlive the static lifetime
    = note: this error originates in the derive macro `SystemParam` 

and

error[E0478]: lifetime bound not satisfied
   --> src\tiles\tile_entities.rs:155:10
    |
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    |
note: lifetime parameter instantiated with the lifetime `'_s` as defined here
   --> src\tiles\tile_entities.rs:155:10
    |
155 | #[derive(SystemParam)]
    |          ^^^^^^^^^^^
    = note: but lifetime parameter must outlive the static lifetime
    = note: this error originates in the derive macro `SystemParam`

@JoJoJet Thanks for chiming in here, do you have any insight on when we might expect #8030 ? Possible there will be a patch release, or are we talking 0.11?

[joseph-gio]

#8030 has a breaking change, so in order for the fix to land in a patch release I'd have to make a new PR with just that fix and nothing else. I can look into that later but I'm not sure how that would work out.

As a temporary workaround, try changing just the lifetimes inside of the ParamSet to 'static:

Meaning if you have a struct like this: ParamSet<'w, 's, Query<'w, 's, ...>>
Change it to this: ParamSet<'w, 's, Query<'static, 'static, ...>>

Changing the lifetimes in this specific way seems to make it compile.

[mbolt35]

Meaning if you have a struct like this: ParamSet<'w, 's, Query<'w, 's, ...>> Change it to this: ParamSet<'w, 's, Query<'static, 'static, ...>>

Changing the lifetimes in this specific way seems to make it compile.

@JoJoJet Thank you, that works. Appreciate the help 👍

[joseph-gio]

Happy I could help :)