From 2a2ce6e0badf4e18e084fc38d2133063baf0d0c7 Mon Sep 17 00:00:00 2001
From: lcnr <rust@lcnr.de>
Date: Mon, 14 Apr 2025 21:55:31 +0200
Subject: [PATCH] remove reliance on a trait solver inference bug (#18840)

The parameter `In` of `call_inner` is completely unconstrained by its
arguments and return type. We are only able to infer it by assuming that
the only associated type equal to `In::Param<'_>` is `In::Param<'_>`
itself. It could just as well be some other associated type which only
normalizes to `In::Param<'_>`. This will change with the next-generation
trait solver and was encountered by a crater run
https://github.com/rust-lang/rust/pull/133502-

cc
https://github.com/rust-lang/trait-system-refactor-initiative/issues/168

I couldn't think of a cleaner alternative here. I first tried to just
provide `In` as an explicit type parameter. This is also kinda ugly as I
need to provide a variable number of them and `${ignore(..)}` is
currently still unstable https://github.com/rust-lang/rust/issues/83527.

Sorry for the inconvenience. Also fun that this function exists to avoid
a separate solver bug in the first place :sweat_smile:
---
 crates/bevy_ecs/src/system/exclusive_function_system.rs | 3 ++-
 crates/bevy_ecs/src/system/function_system.rs           | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/crates/bevy_ecs/src/system/exclusive_function_system.rs b/crates/bevy_ecs/src/system/exclusive_function_system.rs
index 22ce54bb5c..15027d2aef 100644
--- a/crates/bevy_ecs/src/system/exclusive_function_system.rs
+++ b/crates/bevy_ecs/src/system/exclusive_function_system.rs
@@ -284,6 +284,7 @@ macro_rules! impl_exclusive_system_function {
                 // without using this function. It fails to recognize that `func`
                 // is a function, potentially because of the multiple impls of `FnMut`
                 fn call_inner<In: SystemInput, Out, $($param,)*>(
+                    _: PhantomData<In>,
                     mut f: impl FnMut(In::Param<'_>, &mut World, $($param,)*) -> Out,
                     input: In::Inner<'_>,
                     world: &mut World,
@@ -292,7 +293,7 @@ macro_rules! impl_exclusive_system_function {
                     f(In::wrap(input), world, $($param,)*)
                 }
                 let ($($param,)*) = param_value;
-                call_inner(self, input, world, $($param),*)
+                call_inner(PhantomData::<In>, self, input, world, $($param),*)
             }
         }
     };
diff --git a/crates/bevy_ecs/src/system/function_system.rs b/crates/bevy_ecs/src/system/function_system.rs
index 4e05ef4dae..b0bbe187ed 100644
--- a/crates/bevy_ecs/src/system/function_system.rs
+++ b/crates/bevy_ecs/src/system/function_system.rs
@@ -972,6 +972,7 @@ macro_rules! impl_system_function {
             #[inline]
             fn run(&mut self, input: In::Inner<'_>, param_value: SystemParamItem< ($($param,)*)>) -> Out {
                 fn call_inner<In: SystemInput, Out, $($param,)*>(
+                    _: PhantomData<In>,
                     mut f: impl FnMut(In::Param<'_>, $($param,)*)->Out,
                     input: In::Inner<'_>,
                     $($param: $param,)*
@@ -979,7 +980,7 @@ macro_rules! impl_system_function {
                     f(In::wrap(input), $($param,)*)
                 }
                 let ($($param,)*) = param_value;
-                call_inner(self, input, $($param),*)
+                call_inner(PhantomData::<In>, self, input, $($param),*)
             }
         }
     };