Version Used:

c# 7.3

Steps to Reproduce:

  public ref struct Matrix<T> where T: unmanaged
   {
       private readonly int w;
       private readonly int h;
       private readonly Span<T> span;

       public Matrix(int w, int h)
       {
           this.w = w;
           this.h = h;
           this.span = stackalloc T[w*h];
       }

results in error
CS8353: A result of a stackalloc expression of type 'Span<T>' cannot be used in this context because it may be exposed outside of the containing method

I think I understand the error message and why it is necessary to prevent it.
But in this specific case the this.span is a field on a ref struct itself, so the stack will be maintained and the matrix itself cannot "be exposed outside the context".