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docs: document trait upcasting rules in Unsize trait #143652

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Jul 10, 2025
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docs: document trait upcasting rules in Unsize trait #143652

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5 changes: 5 additions & 0 deletions library/core/src/marker.rs
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Expand Up @@ -210,6 +210,11 @@ pub trait PointeeSized {
/// - `Trait` is dyn-compatible[^1].
/// - The type is sized.
/// - The type outlives `'a`.
/// - Trait objects `dyn TraitA + AutoA... + 'a` implement `Unsize<dyn TraitB + AutoB... + 'b>`
/// if all of these conditions are met:
/// - `TraitB` is a supertrait of `TraitA`.
/// - `AutoB...` is a subset of `AutoA...`.
/// - `'a` outlives `'b`.
/// - Structs `Foo<..., T1, ..., Tn, ...>` implement `Unsize<Foo<..., U1, ..., Un, ...>>`
/// where any number of (type and const) parameters may be changed if all of these conditions
/// are met:
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