Address nits, add test for implicit dyn-star coercion without feature gate

compiler/rustc_codegen_ssa/src/base.rs

@@ -270,8 +270,11 @@ pub fn cast_to_dyn_star<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     dst_ty: Ty<'tcx>,
     old_info: Option<Bx::Value>,
 ) -> (Bx::Value, Bx::Value) {
-    debug!("unsize_ptr: {:?} => {:?}", src_ty_and_layout.ty, dst_ty);
-    assert!(matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)));
+    debug!("cast_to_dyn_star: {:?} => {:?}", src_ty_and_layout.ty, dst_ty);
+    assert!(
+        matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
+        "destination type must be a dyn*"
+    );
     // FIXME(dyn-star): this is probably not the best way to check if this is
     // a pointer, and really we should ensure that the value is a suitable
     // pointer earlier in the compilation process.

compiler/rustc_hir_analysis/src/check/coercion.rs

@@ -777,6 +777,8 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
         }
 
+        // Check the obligations of the cast -- for example, when casting
+        // `usize` to `dyn* Clone + 'static`:
         let obligations = predicates
             .iter()
             .map(|predicate| {
@@ -787,7 +789,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 let predicate = predicate.with_self_ty(self.tcx, a);
                 Obligation::new(self.cause.clone(), self.param_env, predicate)
             })
-            // Enforce the region bound `'static` (e.g., `usize: 'static`, in our example).
+            // Enforce the region bound (e.g., `usize: 'static`, in our example).
             .chain([Obligation::new(
                 self.cause.clone(),
                 self.param_env,

src/test/ui/dyn-star/auxiliary/dyn-star-foreign.rs

@@ -0,0 +1,10 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Display;
+
+pub fn require_dyn_star_display(_: dyn* Display) {}
+
+fn works_locally() {
+    require_dyn_star_display(1usize);
+}

src/test/ui/dyn-star/make-dyn-star.rs

@@ -8,6 +8,11 @@ fn make_dyn_star(i: usize) {
     let _dyn_i: dyn* Debug = i;
 }
 
+fn make_dyn_star_explicit(i: usize) {
+    let _dyn_i: dyn* Debug = i as dyn* Debug;
+}
+
 fn main() {
     make_dyn_star(42);
+    make_dyn_star_explicit(42);
 }

src/test/ui/dyn-star/no-implicit-dyn-star.rs

@@ -0,0 +1,8 @@
+// aux-build:dyn-star-foreign.rs
+
+extern crate dyn_star_foreign;
+
+fn main() {
+    dyn_star_foreign::require_dyn_star_display(1usize);
+    //~^ ERROR mismatched types
+}

src/test/ui/dyn-star/no-implicit-dyn-star.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/no-implicit-dyn-star.rs:6:48
+   |
+LL |     dyn_star_foreign::require_dyn_star_display(1usize);
+   |     ------------------------------------------ ^^^^^^ expected trait object `dyn std::fmt::Display`, found `usize`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected trait object `(dyn* std::fmt::Display + 'static)`
+                      found type `usize`
+note: function defined here
+  --> $DIR/auxiliary/dyn-star-foreign.rs:6:8
+   |
+LL | pub fn require_dyn_star_display(_: dyn* Display) {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.