Fix soundness issue in Yoke::attach_to_cart() around implied bounds

utils/yoke/derive/src/lib.rs

@@ -81,8 +81,6 @@ fn yokeable_derive_impl(input: &DeriveInput) -> TokenStream2 {
                     f(self)
                 }
             }
-            // This is safe because there are no lifetime parameters.
-            unsafe impl<'a, #(#tybounds),*> yoke::IsCovariant<'a> for #name<#(#typarams),*> where #(#static_bounds),* {}
         }
     } else {
         if lts != 1 {
@@ -250,9 +248,6 @@ fn yokeable_derive_impl(input: &DeriveInput) -> TokenStream2 {
                     unsafe { f(core::mem::transmute::<&'a mut Self, &'a mut Self::Output>(self)) }
                 }
             }
-            // This is safe because it is in the same block as the above impl, which only compiles
-            // if 'a is a covariant lifetime.
-            unsafe impl<'a, #(#tybounds),*> yoke::IsCovariant<'a> for #name<'a, #(#typarams),*> where #(#static_bounds),* {}
         }
     }
 }

utils/yoke/src/lib.rs

@@ -48,7 +48,6 @@ extern crate alloc;
 pub mod either;
 #[cfg(feature = "alloc")]
 pub mod erased;
-mod is_covariant;
 mod macro_impls;
 pub mod trait_hack;
 mod yoke;
@@ -59,7 +58,6 @@ mod zero_from;
 #[cfg(feature = "derive")]
 pub use yoke_derive::Yokeable;
 
-pub use crate::is_covariant::IsCovariant;
 pub use crate::yoke::{CloneableCart, Yoke};
 pub use crate::yokeable::Yokeable;
 

utils/yoke/src/macro_impls.rs

@@ -6,7 +6,7 @@
 // than using pointer casts
 #![allow(clippy::transmute_ptr_to_ptr)]
 
-use crate::{IsCovariant, Yokeable};
+use crate::Yokeable;
 use core::{mem, ptr};
 
 macro_rules! copy_yoke_impl {
@@ -38,7 +38,6 @@ macro_rules! impl_copy_type {
             type Output = Self;
             copy_yoke_impl!();
         }
-        unsafe impl<'a> IsCovariant<'a> for $ty {}
     };
 }