From c9ff1609d4f7ccd001d9a1487e644831d0b06947 Mon Sep 17 00:00:00 2001
From: Moulins <arthur.heuillard@orange.fr>
Date: Tue, 8 Jul 2025 21:10:40 +0200
Subject: [PATCH] docs: document trait upcasting rules in `Unsize` trait

---
 library/core/src/marker.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 2aeb0b0c31e46..f092916eb8c1c 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -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: