Rollup merge of rust-lang#106532 - compiler-errors:dyn-star-to-dyn, r=jackh726

Allow codegen to unsize `dyn*` to `dyn`

`dyn* Trait` is just another type that implements `Trait`, so we should be able to unsize `&dyn* Trait` into `&dyn Trait` perfectly fine, same for `Box` and other unsizeable types.

Fixes rust-lang#106488

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/base.rs

@@ -153,9 +153,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         (
             &ty::Dynamic(ref data_a, _, src_dyn_kind),
             &ty::Dynamic(ref data_b, _, target_dyn_kind),
-        ) => {
-            assert_eq!(src_dyn_kind, target_dyn_kind);
-
+        ) if src_dyn_kind == target_dyn_kind => {
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
             if data_a.principal_def_id() == data_b.principal_def_id() {

tests/ui/dyn-star/dyn-star-to-dyn.rs

@@ -0,0 +1,9 @@
+// build-pass
+
+#![feature(dyn_star)]
+//~^ WARN the feature `dyn_star` is incomplete and may not be safe to use and/or cause compiler crashes
+
+fn main() {
+    let x: dyn* Send = &();
+    let x = Box::new(x) as Box<dyn Send>;
+}

tests/ui/dyn-star/dyn-star-to-dyn.stderr

@@ -0,0 +1,11 @@
+warning: the feature `dyn_star` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/dyn-star-to-dyn.rs:3:12
+   |
+LL | #![feature(dyn_star)]
+   |            ^^^^^^^^
+   |
+   = note: see issue #102425 <https://github.com/rust-lang/rust/issues/102425> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+