static Matrix4x4 _viewMat;
static ref readonly Matrix4x4 GetMatrix() => ref _viewMat;
static void Test() {
ref readonly var mat = ref GetMatrix();
var matCopy = mat;
mat.M11 = 3f; // This line won't compile, we can not modify a readonly ref
matCopy.M11 = 3f; // This line is fine, 'matCopy' is a local stack copy of the variable pointed to by 'mat'
}