doc: document `from_std` functions panic (#2056)

Document that conversion from `std` types must be done from within
the Tokio runtime context.

1 files changed, 8 insertions, 0 deletions

diff --git a/tokio/src/net/udp/socket.rs b/tokio/src/net/udp/socket.rs
index c22ebd92..909ef760 100644
--- a/tokio/src/net/udp/socket.rs
+++ b/tokio/src/net/udp/socket.rs
@@ -57,6 +57,14 @@ impl UdpSocket {
     /// This can be used in conjunction with net2's `UdpBuilder` interface to
     /// configure a socket before it's handed off, such as setting options like
     /// `reuse_address` or binding to multiple addresses.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if thread-local runtime is not set.
+    ///
+    /// The runtime is usually set implicitly when this function is called
+    /// from a future driven by a tokio runtime, otherwise runtime can be set
+    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.
     pub fn from_std(socket: net::UdpSocket) -> io::Result<UdpSocket> {
         let io = mio::net::UdpSocket::from_socket(socket)?;
         let io = PollEvented::new(io)?;