Random musing on the library-level UB casting problem

[traviscross]

Over in:

• arbitrary_self_types + derive_coerce_pointee allows calling methods whose where clauses are violated #136702
• Forbid freely casting lifetime bounds of dyn-types #136776

We're talking about how safe lifetime casts can allow library-level UB. It's interesting to think about other things that fit this pattern that also might be said to cause library-level UB under certain expectations.

steffahn described in #136702 (comment) a plan for using dyn Trait and nightly features for triggering library-level UB. It looks like this:

Playground link

Translating this to a non-dyn context, we get this, which has a similar shape to it:

Playground link

@rustbot labels +T-lang +T-types +C-discussion

[traviscross]

We can demonstrate this also using types rather than lifetimes. If we provide a way to get a W<B> but no safe way to get a W<A>, we can still safely call a function that has a bound requiring W<A>.

Playground link

[Darksonn]

You can call it like this:

fn main() {
    ub::<W<'static>>();
}

When it's a generic parameter, you can call it even if you can't construct it.

[traviscross]

Yes, of course. It is interesting though how much this shares the shape of the other case about which we are worried.