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Rollup merge of #120248 - WaffleLapkin:bonk-ptr-object-casts, r=compiler-errors,oli-obk,lnicola

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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`
This commit is contained in:
Matthias Krüger 2024-07-08 16:28:15 +02:00 • committed by GitHub
commit c4ee2df539
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31 changed files with 758 additions and 144 deletions
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14
compiler/rustc_errors/src/diagnostic_impls.rs
View file

@ -298,15 +298,21 @@ impl IntoDiagArg for hir::def::Namespace {
}
#[derive(Clone)]
pub struct DiagSymbolList(Vec<Symbol>);
pub struct DiagSymbolList<S = Symbol>(Vec<S>);
impl From<Vec<Symbol>> for DiagSymbolList {
fn from(v: Vec<Symbol>) -> Self {
impl<S> From<Vec<S>> for DiagSymbolList<S> {
fn from(v: Vec<S>) -> Self {
DiagSymbolList(v)
}
}
impl IntoDiagArg for DiagSymbolList {
impl<S> FromIterator<S> for DiagSymbolList<S> {
fn from_iter<T: IntoIterator<Item = S>>(iter: T) -> Self {
iter.into_iter().collect::<Vec<_>>().into()
}
}
impl<S: std::fmt::Display> IntoDiagArg for DiagSymbolList<S> {
fn into_diag_arg(self) -> DiagArgValue {
DiagArgValue::StrListSepByAnd(
self.0.into_iter().map(|sym| Cow::Owned(format!("`{sym}`"))).collect(),