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Fixes rust-lang#154619. If `DerefPure` were dyn-compatible, a trait object of a subtrait of `DerefPure` could be created by unsize-coercing an existing type that implements `DerefPure`. But then the trait object could have its own non-pure impl of `Deref`/`DerefMut`, which is unsound, since the trait object would implement `DerefPure`. Thus, we make `DerefPure` dyn-incompatible.
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Apr 2, 2026
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Fixes #154619.
If
DerefPurewere dyn-compatible, a trait object of a subtrait ofDerefPurecould be created by unsize-coercing an existing type that implementsDerefPure. But then the trait object could have its own non-pure impl ofDeref/DerefMut, which is unsound, since the trait object would implementDerefPure. Thus, we makeDerefPuredyn-incompatible.r? types