tokio-stream 0.1.18 added `Stream::size_hint` for
`ReceiverStream` and `UnboundedReceiverStream` (PR #7492),
which calls `Receiver::is_closed()` and `Receiver::len()`
(added in tokio 1.37.0) and `Receiver::capacity()` and
`Receiver::max_capacity()` (added in tokio 1.38.0).
The declared minimum of tokio 1.15.0 is no longer sufficient,
causing compilation failures when resolved via
`-Z direct-minimal-versions`.
Refs: #7492
