type_id is not sufficiently collision-resistant

[apiraino]

Meeting proposal info

• Title: type_id is not sufficiently collision-resistant
• Type: technical

Summary

See opening comment in rust#129014:

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:

1. switch to a stronger hash function, or
2. switch to a different scheme that doesn't rely on collision-resistance of the hash function.

Discussed on Zulip during triage meeting.

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[RalfJung]

Cc rust-lang/rust#129030