undo an optimization 😭

MIRI is buggy so we can't use fn pointers for comparisons

Author: joseph-gio

crates/bevy_utils/src/label.rs

@@ -81,6 +81,7 @@ macro_rules! define_label {
         $(#[$id_attr])*
         #[derive(Clone, Copy)]
         pub struct $id_name {
+            ty: ::std::any::TypeId,
             data: u64,
             f: fn(u64, &mut ::std::fmt::Formatter) -> ::std::fmt::Result,
         }
@@ -98,7 +99,7 @@ macro_rules! define_label {
             #[inline]
             fn as_label(&self) -> $id_name {
                 let data = self.data();
-                $id_name { data, f: Self::fmt }
+                $id_name { data, ty: ::std::any::TypeId::of::<Self>(), f: Self::fmt }
             }
             /// Returns a number used to distinguish different labels of the same type.
             fn data(&self) -> u64;
@@ -128,26 +129,23 @@ macro_rules! define_label {
         impl PartialEq for $id_name {
             #[inline]
             fn eq(&self, rhs: &Self) -> bool {
-                (self.f as usize) == (rhs.f as usize) && self.data() == rhs.data()
+                self.ty == rhs.ty && self.data() == rhs.data()
             }
         }
         impl Eq for $id_name {}
 
 
         impl std::hash::Hash for $id_name {
             fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
-                (self.f as usize).hash(state);
+                self.ty.hash(state);
                 self.data().hash(state);
             }
         }
 
         impl $id_name {
             /// Returns true if this label was constructed from an instance of type `L`.
             pub fn is<L: $label_name>(self) -> bool {
-                // FIXME: This is potentially incorrect, due to the
-                // compiler unifying identical functions. We'll likely
-                // have to store some kind of hash of the TypeId.
-                (self.f as usize) == (<L as $label_name>::fmt as usize)
+                self.ty == ::std::any::TypeId::of::<L>()
             }
             /// Attempts to downcast this label to type `L`.
             ///