Fix mutable_copy(::BigFloat) in Julia v1.12

src/implementations/BigFloat.jl

@@ -9,12 +9,18 @@
 
 mutability(::Type{BigFloat}) = IsMutable()
 
-# Copied from `deepcopy_internal` implementation in Julia:
-# https://github.com/JuliaLang/julia/blob/7d41d1eb610cad490cbaece8887f9bbd2a775021/base/mpfr.jl#L1041-L1050
-function mutable_copy(x::BigFloat)
-    d = x._d
-    d′ = GC.@preserve d unsafe_string(pointer(d), sizeof(d)) # creates a definitely-new String
-    return Base.MPFR._BigFloat(x.prec, x.sign, x.exp, d′)
+@static if VERSION >= v"1.12"
+    function mutable_copy(x::BigFloat)
+        return Base.MPFR._BigFloat(copy(getfield(x, :d)))
+    end
+else
+    # Copied from `deepcopy_internal` implementation in Julia:
+    # https://github.com/JuliaLang/julia/blob/7d41d1eb610cad490cbaece8887f9bbd2a775021/base/mpfr.jl#L1041-L1050
+    function mutable_copy(x::BigFloat)
+        d = x._d
+        d′ = GC.@preserve d unsafe_string(pointer(d), sizeof(d)) # creates a definitely-new String
+        return Base.MPFR._BigFloat(x.prec, x.sign, x.exp, d′)
+    end
 end
 
 const _MPFRRoundingMode = Base.MPFR.MPFRRoundingMode