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4bd78b0a8d
Make casts of pointers to trait objects stricter This is an attempt to `fix` https://github.com/rust-lang/rust/issues/120222 and https://github.com/rust-lang/rust/issues/120217. This is done by adding restrictions on casting pointers to trait objects. Before this PR the rules were as follows: > When casting `*const X<dyn A>` -> `*const Y<dyn B>`, principal traits in `A` and `B` must refer to the same trait definition (or no trait). With this PR the rules are changed to > When casting `*const X<dyn Src>` -> `*const Y<dyn Dst>` > - if `Dst` has a principal trait `DstP`, > - `Src` must have a principal trait `SrcP` > - `dyn SrcP` and `dyn DstP` must be the same type (modulo the trait object lifetime, `dyn T+'a` -> `dyn T+'b` is allowed) > - Auto traits in `Dst` must be a subset of auto traits in `Src` > - Not adhering to this is currently a FCW (warn-by-default + `FutureReleaseErrorReportInDeps`), instead of an error > - if `Src` has a principal trait `Dst` must as well > - this restriction will be removed in a follow up PR This ensures that 1. Principal trait's generic arguments match (no `*const dyn Tr<A>` -> `*const dyn Tr<B>` casts, which are a problem for [#120222](https://github.com/rust-lang/rust/issues/120222)) 2. Principal trait's lifetime arguments match (no `*const dyn Tr<'a>` -> `*const dyn Tr<'b>` casts, which are a problem for [#120217](https://github.com/rust-lang/rust/issues/120217)) 3. No auto traits can be _added_ (this is a problem for arbitrary self types, see [this comment](https://github.com/rust-lang/rust/pull/120248#discussion_r1463835350)) Some notes: - We only care about the metadata/last field, so you can still cast `*const dyn T` to `*const WithHeader<dyn T>`, etc - The lifetime of the trait object itself (`dyn A + 'lt`) is not checked, so you can still cast `*mut FnOnce() + '_` to `*mut FnOnce() + 'static`, etc - This feels fishy, but I couldn't come up with a reason it must be checked The diagnostics are currently not great, to say the least, but as far as I can tell this correctly fixes the issues. cc `@oli-obk` `@compiler-errors` `@lcnr` |
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