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DynMetadata equality is not documented #99388
Description
Activity
Vtables are likely not unique at the moment. Yes, documenting that explicitly sounds good. Related: #69757
Is it necessary to have
PartialEqimplementaton onDynMetadata?All raw pointers (including to trait objects) implement
PartialEq, and they conceptually do it by comparing(ptr as *const (), metadata(ptr)). For that formulation to work, we’d needDynMetadata: PartialEq. This is currently reflected inPointee::Metadatahaving aPartialEqbound.However the current implementation of raw pointer comparison is just bitwise comparison and does not go through
metadata(), so it would technically be possible to remove this bound and impl. I’m not sure it’s desirable, though.One can compare vtable pointers via wide raw pointer comparison, so for better or worse I think we should also allow it on
DynMetadata. That might be a good place to document the footguns related to comparison of vtables.- added a commit that references this issue
on Feb 12, 2024
Location
https://doc.rust-lang.org/std/ptr/struct.DynMetadata.html
Summary
There is no documentation as far as I should see that defines the behaviour of the
PartialEqimpl forDynMetadata. I think the guarantees that are made or not made should be documented. For instance, are vtables for a given concrete type guaranteed to be unique? People will assume they are, and that==will tell you if the types match. It would not surprise me to learn that this is wrong.