fix: make unhack_observed type-stable

Author: AayushSabharwal

src/systems/nonlinear/initializesystem.jl

@@ -823,30 +823,19 @@ Counteracts the CSE/array variable hacks in `symbolics_tearing.jl` so it works w
 initialization.
 """
 function unhack_observed(obseqs::Vector{Equation}, eqs::Vector{Equation})
-    subs = Dict()
-    tempvars = Set()
-    rm_idxs = Int[]
+    subs = Dict{SymbolicT, SymbolicT}()
+    mask = trues(length(obseqs))
     for (i, eq) in enumerate(obseqs)
-        iscall(eq.rhs) || continue
-        if operation(eq.rhs) == StructuralTransformations.change_origin
-            push!(rm_idxs, i)
-            continue
-        end
-    end
-
-    for (i, eq) in enumerate(obseqs)
-        if eq.lhs in tempvars
-            subs[eq.lhs] = eq.rhs
-            push!(rm_idxs, i)
-        end
+        mask[i] = !iscall(eq.rhs) || operation(eq.rhs) !== StructuralTransformations.change_origin
     end
 
-    obseqs = obseqs[setdiff(eachindex(obseqs), rm_idxs)]
-    obseqs = map(obseqs) do eq
-        fixpoint_sub(eq.lhs, subs) ~ fixpoint_sub(eq.rhs, subs)
+    obseqs = obseqs[mask]
+    for i in eachindex(obseqs)
+        obseqs[i] = fixpoint_sub(obseqs[i].lhs, subs) ~ fixpoint_sub(obseqs[i], subs)
     end
-    eqs = map(eqs) do eq
-        fixpoint_sub(eq.lhs, subs) ~ fixpoint_sub(eq.rhs, subs)
+    eqs = copy(eqs)
+    for i in eachindex(eqs)
+        eqs[i] = fixpoint_sub(eqs[i].lhs, subs) ~ fixpoint_sub(eqs[i], subs)
     end
     return obseqs, eqs
 end