Using TLS in coroutines may not make much sense, but using TLS in the runtime can be unavoidable. For example, the stack of running coroutines is per-thread. Y

Using TLS in coroutines may not make much sense, but using TLS in the runtime can be unavoidable. For example, the stack of running coroutines is per-thread. You can perhaps avoid that with a more complex API, but placing it in TLS is the obvious design. Unfortunately, this has happened and is not hypothetical, but at least for now we're stuck in the stackful hole we dug for ourselves. :( A few years back we had similar issues on Windows.