Remove a false statement from Unsize docs, add a test

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.
 ///

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() {}