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Autotrait bounds on dyn-safe trait methods

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David Tolnay 2023-01-19 01:24:45 -08:00
parent 9e1c600f74
commit 4501d3abe1
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4 changed files with 110 additions and 10 deletions
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22
compiler/rustc_hir_analysis/src/coherence/orphan.rs
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@ -115,6 +115,28 @@ fn do_orphan_check_impl<'tcx>(
// impl MyAuto for dyn Trait {} // NOT OKAY
// impl<T: ?Sized> MyAuto for T {} // NOT OKAY
//
// With this restriction, it's guaranteed that an auto-trait is
// implemented for a trait object if and only if the auto-trait is one
// of the trait object's trait bounds (or a supertrait of a bound). In
// other words `dyn Trait + AutoTrait` always implements AutoTrait,
// while `dyn Trait` never implements AutoTrait.
//
// This is necessary in order for autotrait bounds on methods of trait
// objects to be sound.
//
// auto trait AutoTrait {}
//
// trait ObjectSafeTrait {
// fn f(&self) where Self: AutoTrait;
// }
//
// We can allow f to be called on `dyn ObjectSafeTrait + AutoTrait`.
//
// If we didn't deny `impl AutoTrait for dyn Trait`, it would be unsound
// for the ObjectSafeTrait shown above to be object safe because someone
// could take some type implementing ObjectSafeTrait but not AutoTrait,
// unsize it to `dyn ObjectSafeTrait`, and call .f() which has no
// concrete implementation (issue #50781).
enum LocalImpl {
Allow,
Disallow { problematic_kind: &'static str },