[rust-lang/rust] TAIT + next-solver will allow constructing a unit struct without mentioning its type, causing `pinned-init` to be unsound.

### ROOT CAUSE The issue stems from the interaction between the `type_alias_impl_trait` (TAIT) feature and the next-solver. TAIT allows the creation of impl trait types that can be constructed without mentioning their specific type, bypassing the safety guarantees of the `pinned-init` crate. The `pinned-init` crate relies on a type being impossible to construct outside its macro to ensure soundness. With TAIT and the next-solver, this guarantee is broken, leading to unsound code that can cause undefined behavior or crashes. ### CODE FIX ```rust // We need to modify the `pinned_init` crate to use a different mechanism for ensuring type safety. // One approach is to replace the current type-based safety with a compile-time check using const generics or associated types. // However, this requires significant changes to the crate's internals. // Alternatively, we can restrict the use of TAIT in code that relies on `pinned-init` by using const generics or other safe constructs. // Since the issue involves the next-solver and TAIT, a more fundamental fix might involve changes to the trait solver itself to prevent the construction of impl trait types in certain contexts. // This would require a deep dive into the solver's implementation and is beyond the scope of a simple patch. // Due to the complexity and potential impact, we recommend: // 1. Updating the `pinned-init` crate to use a different safety mechanism. // 2. Avoiding the use of TAIT in code that uses `pinned-init`. // 3. Engaging with the Rust team to address this in the next-solver. // Unfortunately, without more context or code, we cannot provide a specific fix. ```