Reorganize system modules (#8419)

# Objective

Follow-up to #8377.

As the system module has been refactored, there are many types that no
longer make sense to live in the files that they do:
- The `IntoSystem` trait is in `function_system.rs`, even though this
trait is relevant to all kinds of systems. Same for the `In<T>` type.
- `PipeSystem` is now just an implementation of `CombinatorSystem`, so
`system_piping.rs` no longer needs its own file.

## Solution

- Move `IntoSystem`, `In<T>`, and system piping combinators & tests into
the top-level `mod.rs` file for `bevy_ecs::system`.
- Move `PipeSystem` into `combinator.rs`.

Author: joseph-gio

crates/bevy_ecs/src/system/combinator.rs

@@ -237,3 +237,65 @@ where
     B: ReadOnlySystem,
 {
 }
+
+/// A [`System`] created by piping the output of the first system into the input of the second.
+///
+/// This can be repeated indefinitely, but system pipes cannot branch: the output is consumed by the receiving system.
+///
+/// Given two systems `A` and `B`, A may be piped into `B` as `A.pipe(B)` if the output type of `A` is
+/// equal to the input type of `B`.
+///
+/// Note that for [`FunctionSystem`](crate::system::FunctionSystem)s the output is the return value
+/// of the function and the input is the first [`SystemParam`](crate::system::SystemParam) if it is
+/// tagged with [`In`](crate::system::In) or `()` if the function has no designated input parameter.
+///
+/// # Examples
+///
+/// ```
+/// use std::num::ParseIntError;
+///
+/// use bevy_ecs::prelude::*;
+///
+/// fn main() {
+///     let mut world = World::default();
+///     world.insert_resource(Message("42".to_string()));
+///
+///     // pipe the `parse_message_system`'s output into the `filter_system`s input
+///     let mut piped_system = parse_message_system.pipe(filter_system);
+///     piped_system.initialize(&mut world);
+///     assert_eq!(piped_system.run((), &mut world), Some(42));
+/// }
+///
+/// #[derive(Resource)]
+/// struct Message(String);
+///
+/// fn parse_message_system(message: Res<Message>) -> Result<usize, ParseIntError> {
+///     message.0.parse::<usize>()
+/// }
+///
+/// fn filter_system(In(result): In<Result<usize, ParseIntError>>) -> Option<usize> {
+///     result.ok().filter(|&n| n < 100)
+/// }
+/// ```
+pub type PipeSystem<SystemA, SystemB> = CombinatorSystem<Pipe, SystemA, SystemB>;
+
+#[doc(hidden)]
+pub struct Pipe;
+
+impl<A, B> Combine<A, B> for Pipe
+where
+    A: System,
+    B: System<In = A::Out>,
+{
+    type In = A::In;
+    type Out = B::Out;
+
+    fn combine(
+        input: Self::In,
+        a: impl FnOnce(A::In) -> A::Out,
+        b: impl FnOnce(B::In) -> B::Out,
+    ) -> Self::Out {
+        let value = a(input);
+        b(value)
+    }
+}

crates/bevy_ecs/src/system/function_system.rs

@@ -10,7 +10,7 @@ use crate::{
 use bevy_utils::all_tuples;
 use std::{any::TypeId, borrow::Cow, marker::PhantomData};
 
-use super::{PipeSystem, ReadOnlySystem};
+use super::{In, IntoSystem, ReadOnlySystem};
 
 /// The metadata of a [`System`].
 #[derive(Clone)]
@@ -308,79 +308,6 @@ impl<Param: SystemParam> FromWorld for SystemState<Param> {
     }
 }
 
-/// Conversion trait to turn something into a [`System`].
-///
-/// Use this to get a system from a function. Also note that every system implements this trait as
-/// well.
-///
-/// # Examples
-///
-/// ```
-/// use bevy_ecs::prelude::*;
-///
-/// fn my_system_function(a_usize_local: Local<usize>) {}
-///
-/// let system = IntoSystem::into_system(my_system_function);
-/// ```
-// This trait has to be generic because we have potentially overlapping impls, in particular
-// because Rust thinks a type could impl multiple different `FnMut` combinations
-// even though none can currently
-pub trait IntoSystem<In, Out, Marker>: Sized {
-    type System: System<In = In, Out = Out>;
-    /// Turns this value into its corresponding [`System`].
-    fn into_system(this: Self) -> Self::System;
-
-    /// Pass the output of this system `A` into a second system `B`, creating a new compound system.
-    ///
-    /// The second system must have `In<T>` as its first parameter, where `T`
-    /// is the return type of the first system.
-    fn pipe<B, Final, MarkerB>(self, system: B) -> PipeSystem<Self::System, B::System>
-    where
-        B: IntoSystem<Out, Final, MarkerB>,
-    {
-        let system_a = IntoSystem::into_system(self);
-        let system_b = IntoSystem::into_system(system);
-        let name = format!("Pipe({}, {})", system_a.name(), system_b.name());
-        PipeSystem::new(system_a, system_b, Cow::Owned(name))
-    }
-}
-
-// Systems implicitly implement IntoSystem
-impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, ()> for Sys {
-    type System = Sys;
-    fn into_system(this: Self) -> Sys {
-        this
-    }
-}
-
-/// Wrapper type to mark a [`SystemParam`] as an input.
-///
-/// [`System`]s may take an optional input which they require to be passed to them when they
-/// are being [`run`](System::run). For [`FunctionSystems`](FunctionSystem) the input may be marked
-/// with this `In` type, but only the first param of a function may be tagged as an input. This also
-/// means a system can only have one or zero input parameters.
-///
-/// # Examples
-///
-/// Here is a simple example of a system that takes a [`usize`] returning the square of it.
-///
-/// ```
-/// use bevy_ecs::prelude::*;
-///
-/// fn main() {
-///     let mut square_system = IntoSystem::into_system(square);
-///
-///     let mut world = World::default();
-///     square_system.initialize(&mut world);
-///     assert_eq!(square_system.run(12, &mut world), 144);
-/// }
-///
-/// fn square(In(input): In<usize>) -> usize {
-///     input * input
-/// }
-/// ```
-pub struct In<In>(pub In);
-
 /// The [`System`] counter part of an ordinary function.
 ///
 /// You get this by calling [`IntoSystem::into_system`]  on a function that only accepts