2021-Oct: Lang team update

Summary

• Impl is largely ready
• Blocked on deciding the question of upcast safety

Goals for this month

• Hold design meeting about upcast safety and reach a conclusion

Deref coercion

It was discovered in #89190 that extending "unsizing coercions" to include upcasts can cause deref coercions not to trigger, if those deref coercions "deref" to a dyn type. Current plan is to issue future compatibility warnings in #89461, since the only known use of this is emulating the upcasting coercion (and hence the code will continue to work afterwards, but the Deref impl is not invoked).