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Matthias Krüger c4ee2df539
Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola
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`
2024-07-08 16:28:15 +02:00
.github Stop pinning XCode 14 2024-06-29 15:08:04 +00:00
.reuse Rollup merge of #126876 - WaffleLapkin:unignoreconfigtoml, r=Mark-Simulacrum 2024-06-30 10:39:47 +02:00
compiler Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola 2024-07-08 16:28:15 +02:00
library Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola 2024-07-08 16:28:15 +02:00
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src Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola 2024-07-08 16:28:15 +02:00
tests Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola 2024-07-08 16:28:15 +02:00
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