From 6e6c49e7ced47224ecf1a9d14f63f9481fc24cce Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Thu, 10 Nov 2022 23:36:20 +0000
Subject: [PATCH] Don't CoerceUnsized dyn* to dyn*

---
 .../src/traits/select/candidate_assembly.rs   |  2 +-
 .../src/traits/select/confirmation.rs         |  2 +-
 .../dyn-star/dont-unsize-coerce-dyn-star.rs   | 26 +++++++++++++++++++
 3 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 src/test/ui/dyn-star/dont-unsize-coerce-dyn-star.rs

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index dd9bb8ca5f9..1f912894ccc 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -779,7 +779,7 @@ fn assemble_candidates_for_unsizing(
 
         match (source.kind(), target.kind()) {
             // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
-            (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
+            (&ty::Dynamic(ref data_a, _, ty::Dyn), &ty::Dynamic(ref data_b, _, ty::Dyn)) => {
                 // Upcast coercions permit several things:
                 //
                 // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 28b4bae7cbe..f132fbc2e40 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -1145,7 +1145,7 @@ fn confirm_builtin_unsize_candidate(
                 }));
             }
 
-            _ => bug!(),
+            _ => bug!("source: {source}, target: {target}"),
         };
 
         Ok(ImplSourceBuiltinData { nested })
diff --git a/src/test/ui/dyn-star/dont-unsize-coerce-dyn-star.rs b/src/test/ui/dyn-star/dont-unsize-coerce-dyn-star.rs
new file mode 100644
index 00000000000..b4ff8a22286
--- /dev/null
+++ b/src/test/ui/dyn-star/dont-unsize-coerce-dyn-star.rs
@@ -0,0 +1,26 @@
+// check-pass
+
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+trait AddOne {
+    fn add1(&mut self) -> usize;
+}
+
+impl AddOne for usize {
+    fn add1(&mut self) -> usize {
+        *self += 1;
+        *self
+    }
+}
+
+fn add_one(i: &mut (dyn* AddOne + '_)) -> usize {
+    i.add1()
+}
+
+fn main() {
+    let mut x = 42usize as dyn* AddOne;
+
+    println!("{}", add_one(&mut x));
+    println!("{}", add_one(&mut x));
+}