Compile warning on rustc 1.86.0-nightly: function pointer comparisons do not produce meaningful results

[zirconium-n]

warning: function pointer comparisons do not produce meaningful results since their addresses are not guaranteed to be unique
   --> capnp/src/introspect.rs:275:55
    |
275 |         core::ptr::eq(self.generic, other.generic) && self.field_types == other.field_types
    |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: the address of the same function can vary between different codegen units
    = note: furthermore, different functions could have the same address after being merged together
    = note: for more information visit <https://doc.rust-lang.org/nightly/core/ptr/fn.fn_addr_eq.html>
    = note: `#[warn(unpredictable_function_pointer_comparisons)]` on by default

> rustc --version
rustc 1.86.0-nightly (b3b368a18 2025-01-05)
> git rev-parse HEAD
1ec4709b437ba247bc2bae02000535a03f494a33

[dwrensha]

Thanks for the report. I think this means that we need to remove the PartialEq and Eq impls in introspect.rs. I started a draft PR to do that: #537.

[dwrensha]

Fixed by #537. I ended up keeping the PartialEq impl, but it's now more expensive.

[dwrensha]

Unfortunately, I needed to revert to the old impl in #550. We're now suppressing the warning, and nothing internal to capnp depends on the possible unpredictable behavior. But still downstream users might be unpleasantly surprised if they use equality on types, so I opened #551 to track further progress on this issue.