More 'lowering' Rc <dyn Trait> problems

[Finn]

This seems to be a desirable feature and has indeed been discussed before, see:

Upcasting a trait behind a smart pointer (Rc, etc)

A subtle fact about CoerceUnsized coercions is that they cannot upcast something that is already a trait object. use std::any::Any; struct Foo; trait Trait: Any { } impl Trait for Foo { } let _: &Any = &Foo; // ok let x: &Trait = &Foo; let _: &Any = x; // ERROR: mismatched types This StackOverflow post has both an explanation and a solution, but the solution only goes halfway: The explanation is that the vtable for Derived doesn't actually necessarily contain anywhere inside of it a fu…

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