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Rollup merge of #135383 - BoxyUwU:cov_tag_ptr, r=compiler-errors

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De-abstract tagged ptr and make it covariant

In #135272 I needed to use a tagged ptr in `hir::TyKind` in order to not regress hir type sizes. Unfortunately the existing `CopyTaggedPtr` abstraction is insufficient as it makes the `'hir` lifetime invariant.

I spent some time trying to keep existing functionality while making it covariant but in the end I realised that actually we dont use *any* of this code *anywhere* in rustc, so I've just removed everything and replaced it with a much less general abstraction that is suitable for what I need in #135272.

Idk if anyone has a preference for just keeping all the abstractions here in case anyone needs them in the future 🤷‍♀️
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Guillaume Gomez 2025-01-12 23:08:58 +01:00 • committed by GitHub
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compiler/rustc_middle/src/ty/list.rs
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@ -21,7 +21,7 @@ use crate::arena::Arena;
/// pointer.
/// - Because of this, you cannot get a `List<T>` that is a sub-list of another
/// `List<T>`. You can get a sub-slice `&[T]`, however.
/// - `List<T>` can be used with `CopyTaggedPtr`, which is useful within
/// - `List<T>` can be used with `TaggedRef`, which is useful within
/// structs whose size must be minimized.
/// - Because of the uniqueness assumption, we can use the address of a
/// `List<T>` for faster equality comparisons and hashing.