Add 'World::run_system_with_input' function + allow World::run_system to get system output (#10380)

# Objective

Allows chained systems taking an `In<_>` input parameter to be run as
one-shot systems. This API was mentioned in #8963.

In addition, `run_system(_with_input)` returns the system output, for
any `'static` output type.

## Solution

A new function, `World::run_system_with_input` allows a `SystemId<I, O>`
to be run by providing an `I` value as input and producing `O` as an
output.

`SystemId<I, O>` is now generic over the input type `I` and output type
`O`, along with the related functions and types `RegisteredSystem`,
`RemovedSystem`, `register_system`, `remove_system`, and
`RegisteredSystemError`. These default to `()`, preserving the existing
API, for all of the public types.

---

## Changelog

- Added `World::run_system_with_input` function to allow one-shot
systems that take `In<_>` input parameters
- Changed `World::run_system` and `World::register_system` to support
systems with return types beyond `()`
- Added `Commands::run_system_with_input` command that schedules a
one-shot system with an `In<_>` input parameter

Author: Nathan-Fenner

crates/bevy_ecs/src/system/commands/mod.rs

@@ -5,7 +5,7 @@ use crate::{
     self as bevy_ecs,
     bundle::Bundle,
     entity::{Entities, Entity},
-    system::{RunSystem, SystemId},
+    system::{RunSystemWithInput, SystemId},
     world::{EntityWorldMut, FromWorld, World},
 };
 use bevy_ecs_macros::SystemParam;
@@ -523,8 +523,26 @@ impl<'w, 's> Commands<'w, 's> {
     /// Running slow systems can become a bottleneck.
     ///
     /// Calls [`World::run_system`](crate::system::World::run_system).
+    ///
+    /// There is no way to get the output of a system when run as a command, because the
+    /// execution of the system happens later. To get the output of a system, use
+    /// [`World::run_system`] or [`World::run_system_with_input`] instead of running the system as a command.
     pub fn run_system(&mut self, id: SystemId) {
-        self.queue.push(RunSystem::new(id));
+        self.run_system_with_input(id, ());
+    }
+
+    /// Runs the system corresponding to the given [`SystemId`].
+    /// Systems are ran in an exclusive and single threaded way.
+    /// Running slow systems can become a bottleneck.
+    ///
+    /// Calls [`World::run_system_with_input`](crate::system::World::run_system_with_input).
+    ///
+    /// There is no way to get the output of a system when run as a command, because the
+    /// execution of the system happens later. To get the output of a system, use
+    /// [`World::run_system`] or [`World::run_system_with_input`] instead of running the system as a command.
+    pub fn run_system_with_input<I: 'static + Send>(&mut self, id: SystemId<I>, input: I) {
+        self.queue
+            .push(RunSystemWithInput::new_with_input(id, input));
     }
 
     /// Pushes a generic [`Command`] to the command queue.