What restrictions does the C# compiler enforce on Span<T> because it's a 'ref struct'?

Because Span<T> can point directly at stack memory, the compiler restricts it to prevent that memory from outliving its valid scope: it cannot be boxed, cannot be a field of a non-ref-struct class, cannot be used in async methods or iterators (yield return), and cannot be captured by a lambda or local function that might outlive the current stack frame.

// class Container { Span<int> data; } // Error: cannot use Span<T> as a field in a regular class

async Task Bad() {
  Span<int> span = stackalloc int[10];
  await Task.Delay(1); // Error: cannot use Span<T> across an await
}