The soundness of functions like downcast relies on the type_id of two different types never being equal. Currently, the type_id is a 128-bit hash of the full type identity, computed specifically via SipHash-1-3 with an all-zero key. This is not a strong enough hash function for this purpose.
and:
We should do one of the following:
switch to a stronger hash function, or
switch to a different scheme that doesn't rely on collision-resistance of the hash function.
This issue corresponds to a meeting proposal for the compiler team
steering meeting. It corresponds to a possible topic of
discussion. You can read more about the steering meeting procedure
here.
Comment policy
These issues are meant to be used as an "announcements channel"
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RalfJung
commented
3 months ago 
apiraino
Meeting proposal info
Summary
See opening comment in rust#129014:
and:
Discussed on Zulip during triage meeting.
About this issue
This issue corresponds to a meeting proposal for the compiler team steering meeting. It corresponds to a possible topic of discussion. You can read more about the steering meeting procedure here.
Comment policy
These issues are meant to be used as an "announcements channel" regarding the proposal, and not as a place to discuss the technical details. Feel free to subscribe to updates. We'll post comments when reviewing the proposal in meetings or making a scheduling decision. In the meantime, if you have questions or ideas, ping the proposers on Zulip (or elsewhere).