From 695d30bd54af2978dc99f214dda34b568348cf86 Mon Sep 17 00:00:00 2001
From: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
Date: Thu, 23 Feb 2023 05:11:12 +0000
Subject: [PATCH] Clean up marker generics for systems (#7789)

# Objective

While we use `#[doc(hidden)]` to try and hide marker generics from the user, these types reveal themselves in compiler errors, adding visual noise and confusion.

## Solution

Replace the `AlreadyWasSystem` marker generic with `()`, to reduce visual noise in error messages. This also makes it possible to return `impl Condition<()>` from combinators.

For function systems, use their function signature as the marker type. This should drastically improve the legibility of some error messages.
The `InputMarker` type has been removed, since it is unnecessary.
---
 crates/bevy_ecs/src/schedule/condition.rs              |  6 ++----
 .../bevy_ecs/src/system/exclusive_function_system.rs   |  8 ++++----
 crates/bevy_ecs/src/system/function_system.rs          | 10 +++-------
 3 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/crates/bevy_ecs/src/schedule/condition.rs b/crates/bevy_ecs/src/schedule/condition.rs
index 040d53d6ca0bb..96ca767bd442c 100644
--- a/crates/bevy_ecs/src/schedule/condition.rs
+++ b/crates/bevy_ecs/src/schedule/condition.rs
@@ -137,7 +137,7 @@ pub mod common_conditions {
         event::{Event, EventReader},
         prelude::{Component, Query, With},
         schedule::{State, States},
-        system::{In, IntoPipeSystem, ReadOnlySystem, Res, Resource},
+        system::{In, IntoPipeSystem, Res, Resource},
     };
 
     /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
@@ -373,9 +373,7 @@ pub mod common_conditions {
     /// #
     /// # fn my_system() { unreachable!() }
     /// ```
-    pub fn not<Marker>(
-        condition: impl Condition<Marker>,
-    ) -> impl ReadOnlySystem<In = (), Out = bool> {
+    pub fn not<Marker>(condition: impl Condition<Marker>) -> impl Condition<()> {
         condition.pipe(|In(val): In<bool>| !val)
     }
 }
diff --git a/crates/bevy_ecs/src/system/exclusive_function_system.rs b/crates/bevy_ecs/src/system/exclusive_function_system.rs
index 4ba4a15418655..1fde9619e2436 100644
--- a/crates/bevy_ecs/src/system/exclusive_function_system.rs
+++ b/crates/bevy_ecs/src/system/exclusive_function_system.rs
@@ -4,8 +4,8 @@ use crate::{
     component::ComponentId,
     query::Access,
     system::{
-        check_system_change_tick, ExclusiveSystemParam, ExclusiveSystemParamItem, In, InputMarker,
-        IntoSystem, System, SystemMeta,
+        check_system_change_tick, ExclusiveSystemParam, ExclusiveSystemParamItem, In, IntoSystem,
+        System, SystemMeta,
     },
     world::{World, WorldId},
 };
@@ -181,7 +181,7 @@ pub trait ExclusiveSystemParamFunction<Marker>: Send + Sync + 'static {
 macro_rules! impl_exclusive_system_function {
     ($($param: ident),*) => {
         #[allow(non_snake_case)]
-        impl<Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<((), Out, $($param,)*)> for Func
+        impl<Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<fn($($param,)*) -> Out> for Func
         where
         for <'a> &'a mut Func:
                 FnMut(&mut World, $($param),*) -> Out +
@@ -209,7 +209,7 @@ macro_rules! impl_exclusive_system_function {
             }
         }
         #[allow(non_snake_case)]
-        impl<Input, Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<(Input, Out, $($param,)* InputMarker)> for Func
+        impl<Input, Out, Func: Send + Sync + 'static, $($param: ExclusiveSystemParam),*> ExclusiveSystemParamFunction<fn(In<Input>, $($param,)*) -> Out> for Func
         where
         for <'a> &'a mut Func:
                 FnMut(In<Input>, &mut World, $($param),*) -> Out +
diff --git a/crates/bevy_ecs/src/system/function_system.rs b/crates/bevy_ecs/src/system/function_system.rs
index aa87b75a29282..fe6701df605c2 100644
--- a/crates/bevy_ecs/src/system/function_system.rs
+++ b/crates/bevy_ecs/src/system/function_system.rs
@@ -325,10 +325,8 @@ pub trait IntoSystem<In, Out, Marker>: Sized {
     fn into_system(this: Self) -> Self::System;
 }
 
-pub struct AlreadyWasSystem;
-
 // Systems implicitly implement IntoSystem
-impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, AlreadyWasSystem> for Sys {
+impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, ()> for Sys {
     type System = Sys;
     fn into_system(this: Self) -> Sys {
         this
@@ -362,8 +360,6 @@ impl<In, Out, Sys: System<In = In, Out = Out>> IntoSystem<In, Out, AlreadyWasSys
 /// }
 /// ```
 pub struct In<In>(pub In);
-#[doc(hidden)]
-pub struct InputMarker;
 
 /// The [`System`] counter part of an ordinary function.
 ///
@@ -611,7 +607,7 @@ pub trait SystemParamFunction<Marker>: Send + Sync + 'static {
 macro_rules! impl_system_function {
     ($($param: ident),*) => {
         #[allow(non_snake_case)]
-        impl<Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<((), Out, $($param,)*)> for Func
+        impl<Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn($($param,)*) -> Out> for Func
         where
         for <'a> &'a mut Func:
                 FnMut($($param),*) -> Out +
@@ -638,7 +634,7 @@ macro_rules! impl_system_function {
         }
 
         #[allow(non_snake_case)]
-        impl<Input, Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<(Input, Out, $($param,)* InputMarker)> for Func
+        impl<Input, Out, Func: Send + Sync + 'static, $($param: SystemParam),*> SystemParamFunction<fn(In<Input>, $($param,)*) -> Out> for Func
         where
         for <'a> &'a mut Func:
                 FnMut(In<Input>, $($param),*) -> Out +