Associated type constraints on super traits allowing for unsound transmutation to trait objects · Issue #114389 · rust-lang/rust (original) (raw)

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Description

This code allows for unsound transmutation to trait objects of trait with impossible constraints.

trait TypeEq { type This: ?Sized; }

impl<T: ?Sized> TypeEq for T { type This = T; }

fn identity<T: ?Sized>(t: &::This) -> &T { t // T::This is deduced to be T }

trait Cat { fn meow(&self) { println!("meow"); } }

struct SomeType; impl Cat for SomeType {}

// dyn Dog isn't a valid type, but we are still allowed to create a trait object of this trait Dog: TypeEq<This = dyn Cat> { fn bark(&self) { println!("bark"); } }

pub fn main() { let normal_coercion: &dyn Cat = &SomeType;

// `T` is inferred to be `dyn Dog`, `T::This` is inferred as `dyn Cat`, which is different from how the compiler
// inferred these types in the function definition.
let oh_no_my_vtable: &dyn Dog = identity(normal_coercion);

oh_no_my_vtable.bark(); // meow :3

}

I'm not sure if this is a special case of one of the numerous similar known bugs, so I thought I should share it just in case. Simpler cases where the types' memory layouts do not match cause the compiler to panic or llvm to segfault during optimization.