refactor: make @constants create non-tunable parameters

Author: AayushSabharwal

src/constants.jl

@@ -1,13 +1,9 @@
-import SymbolicUtils: symtype, term, hasmetadata, issym
-struct MTKConstantCtx end
-
-isconstant(x::Num) = isconstant(unwrap(x))
 """
 Test whether `x` is a constant-type Sym.
 """
 function isconstant(x)
     x = unwrap(x)
-    x isa Symbolic && getmetadata(x, MTKConstantCtx, false)
+    x isa Symbolic && !getmetadata(x, VariableTunable, true)
 end
 
 """
@@ -16,12 +12,11 @@ end
 Maps the parameter to a constant. The parameter must have a default.
 """
 function toconstant(s)
-    hasmetadata(s, Symbolics.VariableDefaultValue) ||
-        throw(ArgumentError("Constant `$(s)` must be assigned a default value."))
-    setmetadata(s, MTKConstantCtx, true)
+    s = toparam(s)
+    setmetadata(s, VariableTunable, false)
 end
 
-toconstant(s::Num) = wrap(toconstant(value(s)))
+toconstant(s::Union{Num, Symbolics.Arr}) = wrap(toconstant(value(s)))
 
 """
 $(SIGNATURES)
@@ -36,15 +31,3 @@ macro constants(xs...)
         xs,
         toconstant) |> esc
 end
-
-"""
-Substitute all `@constants` in the given expression
-"""
-function subs_constants(eqs)
-    consts = collect_constants(eqs)
-    if !isempty(consts)
-        csubs = Dict(c => getdefault(c) for c in consts)
-        eqs = substitute(eqs, csubs)
-    end
-    return eqs
-end