From 6af30ec720c3a8f074b61c9003b7e63f78f148a0 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 1 Nov 2023 20:16:11 +0000
Subject: [PATCH 1/2] Remove a false statement from Unsize docs, add a test

---
 library/core/src/marker.rs                    |  1 -
 .../unsize-coerce-multiple-adt-params.rs      | 29 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/unsized/unsize-coerce-multiple-adt-params.rs

diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index d396a707b55da..39e4abb10e926 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -158,7 +158,6 @@ pub trait Sized {
 /// - Types implementing a trait `Trait` also implement `Unsize<dyn Trait>`.
 /// - Structs `Foo<..., T, ...>` implement `Unsize<Foo<..., U, ...>>` if all of these conditions
 ///   are met:
-///   - `T: Unsize<U>`.
 ///   - Only the last field of `Foo` has a type involving `T`.
 ///   - `Bar<T>: Unsize<Bar<U>>`, where `Bar<T>` stands for the actual type of that last field.
 ///
diff --git a/tests/ui/unsized/unsize-coerce-multiple-adt-params.rs b/tests/ui/unsized/unsize-coerce-multiple-adt-params.rs
new file mode 100644
index 0000000000000..eba341ff2847f
--- /dev/null
+++ b/tests/ui/unsized/unsize-coerce-multiple-adt-params.rs
@@ -0,0 +1,29 @@
+// check-pass
+
+struct Foo<T, U>
+where
+    (T, U): Trait,
+{
+    f: <(T, U) as Trait>::Assoc,
+}
+
+trait Trait {
+    type Assoc: ?Sized;
+}
+
+struct Count<const N: usize>;
+
+impl<const N: usize> Trait for (i32, Count<N>) {
+    type Assoc = [(); N];
+}
+
+impl<'a> Trait for (u32, ()) {
+    type Assoc = [()];
+}
+
+// Test that we can unsize several trait params in creative ways.
+fn unsize<const N: usize>(x: &Foo<i32, Count<N>>) -> &Foo<u32, ()> {
+    x
+}
+
+fn main() {}

From 1221b7b652a9859340fad0cd6db78c7493f3847f Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 1 Nov 2023 20:16:24 +0000
Subject: [PATCH 2/2] Rework unsize documentation

---
 library/core/src/marker.rs | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 39e4abb10e926..13437d8f96120 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -155,11 +155,18 @@ pub trait Sized {
 /// Those implementations are:
 ///
 /// - Arrays `[T; N]` implement `Unsize<[T]>`.
-/// - Types implementing a trait `Trait` also implement `Unsize<dyn Trait>`.
-/// - Structs `Foo<..., T, ...>` implement `Unsize<Foo<..., U, ...>>` if all of these conditions
-///   are met:
-///   - Only the last field of `Foo` has a type involving `T`.
-///   - `Bar<T>: Unsize<Bar<U>>`, where `Bar<T>` stands for the actual type of that last field.
+/// - A type implements `Unsize<dyn Trait + 'a>` if all of these conditions are met:
+///   - The type implements `Trait`.
+///   - `Trait` is object safe.
+///   - The type is sized.
+///   - The type outlives `'a`.
+/// - 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:
+///   - Only the last field of `Foo` has a type involving the parameters `T1`, ..., `Tn`.
+///   - All other parameters of the struct are equal.
+///   - `Field<T1, ..., Tn>: Unsize<Field<U1, ..., Un>>`, where `Field<...>` stands for the actual
+///     type of the struct's last field.
 ///
 /// `Unsize` is used along with [`ops::CoerceUnsized`] to allow
 /// "user-defined" containers such as [`Rc`] to contain dynamically-sized