Auto merge of #120251 - matthiaskrgr:rollup-gttrw68, r=matthiaskrgr

Rollup of 8 pull requests

Successful merges:

 - #119664 (Fix tty detection for msys2's `/dev/ptmx`)
 - #120104 (never_patterns: Count `!` bindings as diverging)
 - #120109 (Move cmath into `sys`)
 - #120143 (Consolidate logic around resolving built-in coroutine trait impls)
 - #120159 (Track `verbose` and `verbose_internals`)
 - #120216 (Fix a `trimmed_def_paths` assertion failure.)
 - #120220 (Document `Token{Stream,Tree}::Display` more thoroughly.)
 - #120233 (Revert stabilization of trait_upcasting feature)

r? `@ghost`
`@rustbot` modify labels: rollup

1 files changed, 27 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/language-features/trait-upcasting.md b/src/doc/unstable-book/src/language-features/trait-upcasting.md
new file mode 100644
index 00000000000..3697ae38f9d
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/trait-upcasting.md
@@ -0,0 +1,27 @@
+# `trait_upcasting`
+
+The tracking issue for this feature is: [#65991]
+
+[#65991]: https://github.com/rust-lang/rust/issues/65991
+
+------------------------
+
+The `trait_upcasting` feature adds support for trait upcasting coercion. This allows a
+trait object of type `dyn Bar` to be cast to a trait object of type `dyn Foo`
+so long as `Bar: Foo`.
+
+```rust,edition2018
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo {}
+
+trait Bar: Foo {}
+
+impl Foo for i32 {}
+
+impl<T: Foo + ?Sized> Bar for T {}
+
+let bar: &dyn Bar = &123;
+let foo: &dyn Foo = bar;
+```