Rollup merge of #103386 - compiler-errors:no-coerceunsized-to-dynstar, r=eholk

Don't allow `CoerceUnsized` into `dyn*` (except for trait upcasting)

This makes sure we don't accidentally allow coercions like `Box<T>` -> `Box<dyn* Trait>`, or in the case of this ICE, `&T` to `&dyn* Trait`. These coercions don't make sense, at least not via the `CoerceUnsized` trait.

Fixes #102172
Fixes #102429

3 files changed, 44 insertions, 0 deletions

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));
+}
diff --git a/src/test/ui/dyn-star/unsize-into-ref-dyn-star.rs b/src/test/ui/dyn-star/unsize-into-ref-dyn-star.rs
new file mode 100644
index 00000000000..1e8cafe1561
--- /dev/null
+++ b/src/test/ui/dyn-star/unsize-into-ref-dyn-star.rs
@@ -0,0 +1,9 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+fn main() {
+    let i = 42 as &dyn* Debug;
+    //~^ ERROR non-primitive cast: `i32` as `&dyn* Debug`
+}
diff --git a/src/test/ui/dyn-star/unsize-into-ref-dyn-star.stderr b/src/test/ui/dyn-star/unsize-into-ref-dyn-star.stderr
new file mode 100644
index 00000000000..f6444a60a46
--- /dev/null
+++ b/src/test/ui/dyn-star/unsize-into-ref-dyn-star.stderr
@@ -0,0 +1,9 @@
+error[E0605]: non-primitive cast: `i32` as `&dyn* Debug`
+  --> $DIR/unsize-into-ref-dyn-star.rs:7:13
+   |
+LL |     let i = 42 as &dyn* Debug;
+   |             ^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0605`.