Corrected foreach_enumerate implementation.

src/semidiscretization/semidiscretization_coupled.jl

@@ -148,6 +148,22 @@ end
     @view u_ode[semi.u_indices[index]]
 end
 
+# Same as `foreach(enumerate(something))`, but without allocations.
+#
+# Note that compile times may increase if this is used with big tuples.
+@inline foreach_enumerate(func, collection) = foreach_enumerate(func, collection, 1)
+@inline foreach_enumerate(func, collection::Tuple{}, index) = nothing
+
+@inline function foreach_enumerate(func, collection, index)
+    element = first(collection)
+    remaining_collection = Base.tail(collection)
+
+    func((index, element))
+
+    # Process remaining collection
+    foreach_enumerate(func, remaining_collection, index + 1)
+end
+
 # Recursive call of the RHS for the semidiscretizations using Lispy tuple programming
 # to avoid type instability.
 @inline function rhs!(u_ode, du_ode, t, semi, i, semi_, semi_tuple...)