Write migration guide for 2024 edition panicking closure inference regression (#2092)

Co-authored-by: JaySpruce <[email protected]>

Author: alice-i-cecile

release-content/0.16/migration-guides/_guides.toml

@@ -1017,4 +1017,10 @@ file_name = "18326_Use_4byte_LightmapSlabIndex_for_batching_instead_of_16byte.md
 title = "Use `target_abi = \"sim\"` instead of `ios_simulator` feature"
 prs = [17702]
 areas = []
-file_name = "17702_Use_target_abi__sim_instead_of_ios_simulator_feature.md"
+file_name = "17702_Use_target_abi__sim_instead_of_ios_simulator_feature.md"
+
+[[guides]]
+title = "Closures which panic now need to explicitly specify a `()` return type"
+prs = [17967]
+areas = ["ECS"]
+file_name = "panicking_closures.md"

release-content/0.16/migration-guides/panicking_closures.md

@@ -0,0 +1,11 @@
+As a result of changes to [Never type fallback] in Rust 2024, closures which panic now have an inferred return type of `!`, rather than `()`. This might occur if you're using `panic!`, `todo!`, `unimplemented!`, `unreachable!` or other means of panicking.
+
+This affects Bevy users as our blanket implementations for `Command`, `IntoSystem` and `IntoObserverSystem`, which operate on all matching functions with the correct type signature, no longer cover closures of this sort.
+
+While this may be fixable on Bevy's end (see [#18778]), for now, you can work around this issue by either:
+
+1. Converting your closures to ordinary named functions, whose return type is not inferred.
+2. Explicitly specifying the return type of your closure as `()`.
+
+[Never type fallback]: https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html
+[#18778]: https://github.com/bevyengine/bevy/issues/18778