refactor: update tearing_substitute_expr, full_equations to use observed

Author: AayushSabharwal

src/structural_transformation/symbolics_tearing.jl

@@ -107,8 +107,7 @@ end
 function tearing_substitute_expr(sys::AbstractSystem, expr; simplify = false)
     empty_substitutions(sys) && return expr
     substitutions = get_substitutions(sys)
-    @unpack subs = substitutions
-    solved = Dict(eq.lhs => eq.rhs for eq in subs)
+    solved = Dict(eq.lhs => eq.rhs for eq in substitutions)
     return tearing_sub(expr, solved, simplify)
 end
 
@@ -121,20 +120,17 @@ These equations matches generated numerical code.
 See also [`equations`](@ref) and [`ModelingToolkit.get_eqs`](@ref).
 """
 function full_equations(sys::AbstractSystem; simplify = false)
-    empty_substitutions(sys) && return equations(sys)
-    substitutions = get_substitutions(sys)
-    substitutions.subed_eqs === nothing || return substitutions.subed_eqs
-    @unpack subs = substitutions
-    solved = Dict(eq.lhs => eq.rhs for eq in subs)
+    isempty(observed(sys)) && return equations(sys)
+    subs = Dict([eq.lhs => eq.rhs for eq in observed(sys)])
     neweqs = map(equations(sys)) do eq
         if iscall(eq.lhs) && operation(eq.lhs) isa Union{Shift, Differential}
-            return tearing_sub(eq.lhs, solved, simplify) ~ tearing_sub(eq.rhs, solved,
+            return tearing_sub(eq.lhs, subs, simplify) ~ tearing_sub(eq.rhs, subs,
                 simplify)
         else
             if !(eq.lhs isa Number && eq.lhs == 0)
                 eq = 0 ~ eq.rhs - eq.lhs
             end
-            rhs = tearing_sub(eq.rhs, solved, simplify)
+            rhs = tearing_sub(eq.rhs, subs, simplify)
             if rhs isa Symbolic
                 return 0 ~ rhs
             else # a number
@@ -143,7 +139,6 @@ function full_equations(sys::AbstractSystem; simplify = false)
         end
         eq
     end
-    substitutions.subed_eqs = neweqs
     return neweqs
 end