mpsc may be crashed as tail is some

[qiangshou007]

rust/src/libstd/sync/mpsc/mpsc_queue.rs

Line 97 in 938d4ff

assert!((*tail).value.is_none());

tail may be some as hold the shadow value when concurrent

[steveklabnik]

Triage: no change

[jyn514]

Should be fixed by #93563.