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b3ab80c119
Enable coinduction support for Safe Transmute This patch adds the `#[rustc_coinductive]` annotation to `BikeshedIntrinsicFrom`, so that it's possible to compute transmutability for recursive types. ## Motivation Safe Transmute currently already supports references (#110662). However, if a type is implemented recursively, it leads to an infinite loop when we try to check if transmutation is safe. A couple simple examples that one might want to write, that are currently not possible to check transmutability for: ```rs #[repr(C)] struct A(&'static B); #[repr(C)] struct B(&'static A); ``` ```rs #[repr(C)] enum IList<'a> { Nil, Cons(isize, &'a IList<'a>) } #[repr(C)] enum UList<'a> { Nil, Cons(usize, &'a UList<'a>) } ``` Previously, `@jswrenn` was considering writing a co-inductive solver from scratch, just for the `rustc_tranmsute` crate. Later on as I started working on Safe Transmute myself, I came across the `#[rustc_coinductive]` annotation, which is currently only being used for the `Sized` trait. Leveraging this trait actually solved the problem entirely, and it saves a lot of duplicate work that would have had to happen in `rustc_transmute`. |
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