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documented run_system_cached issue#21617 #21707

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6254ce5
documented run_system_cached #21617
DylanD42 Oct 31, 2025
257f5a7
Merge branch 'main' into main
DylanD42 Nov 5, 2025
241977f
fixed run_system_cached_with documentation
DylanD42 Nov 5, 2025
3f43a33
Merge branch 'main' of https://github.com/DylanD42/Group5Bevy
DylanD42 Nov 5, 2025
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25 changes: 24 additions & 1 deletion crates/bevy_ecs/src/system/system_registry.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -500,6 +500,16 @@ impl World {

/// Runs a cached system, registering it if necessary.
///
/// # Type Inference Note
/// If the system returns `()`, you may need to explicitly constrain the output
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@chescock chescock Nov 7, 2025

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I think inference actually works for system functions that return plain (), and we only get errors for systems that return Result.

Suggested change
/// If the system returns `()`, you may need to explicitly constrain the output
/// If the system returns `Result`, you may need to explicitly constrain the output

/// type for error handling:
///
/// ```rust
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@chescock chescock Nov 7, 2025

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You'll need to declare world and my_system for the doc tests to compile, like

Suggested change
/// ```rust
/// ```rust
/// # use bevy_ecs::prelude::*;
/// # fn my_system() -> Result { Ok(()) }
/// # let mut world = World::new();

And you'll also need input for the run_system_cached test.

/// () = world.run_system_cached(my_system)?;
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@chescock chescock Nov 7, 2025

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I think we should also recommend using a turbofish, both here and below. It is more verbose, but it's idiomatic rust, while () = e; is an unusual construction.

Suggested change
/// () = world.run_system_cached(my_system)?;
/// // Either constrain the output type
/// () = world.run_system_cached(my_system)?;
/// // or supply the type parameter explicitly
/// world.run_system_cached::<(), _, _>(my_system)?;

/// ```
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@chescock chescock Nov 7, 2025

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And since you're using ?, you'll need a trailing Ok expression for rustdoc to make the test fallible:

Suggested change
/// ```
/// # Ok::<(), BevyError>(())
/// ```

///
/// Without this, Rust may fail to infer the system’s output type and produce
/// a `IntoResult<!>` inference error.
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@chescock chescock Nov 7, 2025

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Huh, I thought the error was "cannot infer type of the type parameter O declared on the method run_system_cached".

... Oh, I see, world.run_system_cached(my_system).unwrap() gives the helpful "cannot infer type" message, but world.run_system_cached(my_system)? gives the unhelpful "the trait function_system::IntoResult<!> is not implemented for core::result::Result<(), bevy_error::BevyError>". I wonder why that is. ... Huh, and if my_system returns !, then unwrap() still gives "cannot infer type", but ? actually compiles. So the compiler really assumes it's ! for some reason.

Anyway, it might be helpful to give a little more detail on the inference error, like:

Suggested change
/// a `IntoResult<!>` inference error.
/// an error either of the form
///
/// ```text
/// the trait `IntoResult<!>` is not implemented for `Result<(), BevyError>`
/// ```
///
/// or
///
/// ```text
/// cannot infer type of the type parameter `O` declared on the method `run_system_cached`
/// ```

/// See [`World::register_system_cached`] for more information.
pub fn run_system_cached<O: 'static, M, S: IntoSystem<(), O, M> + 'static>(
&mut self,
Expand All @@ -508,8 +518,21 @@ impl World {
self.run_system_cached_with(system, ())
}

/// Runs a cached system with an input, registering it if necessary.
/// Runs a cached system with the provided input, registering it if necessary.
///
/// This is a more general version of [`World::run_system_cached`], allowing
/// callers to supply an explicit system input.
///
/// # Type Inference Note
/// If the system returns `()`, you may need to explicitly constrain the
/// output type for proper error inference:
///
/// ```rust
/// () = world.run_system_cached_with(my_system, input)?;
/// ```
///
/// Without this, Rust may fail to infer the system’s output type and produce
/// a `IntoResult<!>` inference error.
/// See [`World::register_system_cached`] for more information.
pub fn run_system_cached_with<I, O, M, S>(
&mut self,
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