Add trait_upcasting related languages changes

src/expressions/operator-expr.md

@@ -385,7 +385,12 @@ reference types and `mut` or `const` in pointer types.
 | [Function pointer]    | Integer               | Function pointer to address cast |
 | Closure \*\*\*        | Function pointer      | Closure to function pointer cast |
 
-\* or `T` and `V` are compatible unsized types, e.g., both slices, both the same trait object.
+\* or `T` and `V` are unsized types with compatible metadata:
+* both slice metadata (`*[u16]` -> `*[u8]`, `*str` -> `*(u8, [u32])`).
+* both the same trait object metadata, modulo dropping auto traits (`*dyn Debug` -> `*(u16, dyn Debug)`, `*dyn Debug + Send` -> `*dyn Debug`).
+    * **Note**: *adding* auto traits is only allowed if the principal trait has the auto trait as a super trait
+      (given `trait T: Send {}`, `*dyn T` -> `*dyn T + Send` is valid, but `*dyn Debug` -> `*dyn Debug + Send` is not).
+    * **Note**: generics (including lifetimes) must match (`*dyn T<'a, A>` -> `*dyn T<'b, B>` requires `'a = 'b` and `A = B`).
 
 \*\* only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference to
 `const` pointer is allowed.

src/type-coercions.md

@@ -159,10 +159,13 @@ Coercion is allowed between the following types:
 ### Unsized Coercions
 
 The following coercions are called `unsized coercions`, since they
-relate to converting sized types to unsized types, and are permitted in a few
+relate to converting types to unsized types, and are permitted in a few
 cases where other coercions are not, as described above. They can still happen
 anywhere else a coercion can occur.
 
+> Note: "unsizing" is a bit of a misnomer,
+> since this covers unsized->unsized coercions too.
+
 Two traits, [`Unsize`] and [`CoerceUnsized`], are used
 to assist in this process and expose it for library use. The following
 coercions are built-ins and, if `T` can be coerced to `U` with one of them, then
@@ -172,6 +175,11 @@ an implementation of `Unsize<U>` for `T` will be provided:
 
 * `T` to `dyn U`, when `T` implements `U + Sized`, and `U` is [object safe].
 
+* `dyn T` to `dyn U` when `U` is one of `T`'s [supertraits].
+    * This allows dropping auto traits, i.e. `dyn T + Auto` to `dyn U` is allowed.
+    * This allows adding auto traits if the principal trait has the auto trait as a super trait,
+      i.e. given `trait T: U + Send {}`, `dyn T` to `dyn T + Send` or to `dyn U + Send` coercions are allowed.
+
 * `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:
     * `Foo` is a struct.
     * `T` implements `Unsize<U>`.
@@ -274,3 +282,4 @@ precisely.
 [`Unsize`]: std::marker::Unsize
 [`CoerceUnsized`]: std::ops::CoerceUnsized
 [method-call expressions]: expressions/method-call-expr.md
+[supertraits]: items/traits.md#supertraits