add a test for pointer casts involving un/re/wrapping trait objects

the errors should not be there, this is a bug/missing feature.

Author: WaffleLapkin

tests/ui/cast/ptr-to-trait-obj-wrap.rs

@@ -0,0 +1,32 @@
+#![allow(unused)]
+
+trait A {}
+
+struct W<T: ?Sized>(T);
+struct X<T: ?Sized>(T);
+
+fn unwrap(a: *const W<dyn A>) -> *const dyn A {
+    a as _
+    //~^ error
+    //~| error
+}
+
+fn unwrap_nested(a: *const W<W<dyn A>>) -> *const W<dyn A> {
+    a as _
+    //~^ error
+    //~| error
+}
+
+fn rewrap(a: *const W<dyn A>) -> *const X<dyn A> {
+    a as _
+}
+
+fn rewrap_nested(a: *const W<W<dyn A>>) -> *const W<X<dyn A>> {
+    a as _
+}
+
+fn wrap(a: *const dyn A) -> *const W<dyn A> {
+    a as _
+}
+
+fn main() {}

tests/ui/cast/ptr-to-trait-obj-wrap.stderr

@@ -0,0 +1,57 @@
+error[E0277]: the trait bound `W<(dyn A + 'static)>: A` is not satisfied
+  --> $DIR/ptr-to-trait-obj-wrap.rs:9:5
+   |
+LL |     a as _
+   |     ^ the trait `A` is not implemented for `W<(dyn A + 'static)>`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/ptr-to-trait-obj-wrap.rs:3:1
+   |
+LL | trait A {}
+   | ^^^^^^^
+   = note: required for the cast from `*const W<(dyn A + 'static)>` to `*const dyn A`
+
+error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
+  --> $DIR/ptr-to-trait-obj-wrap.rs:9:5
+   |
+LL |     a as _
+   |     ^ doesn't have a size known at compile-time
+   |
+   = help: within `W<(dyn A + 'static)>`, the trait `Sized` is not implemented for `(dyn A + 'static)`
+note: required because it appears within the type `W<(dyn A + 'static)>`
+  --> $DIR/ptr-to-trait-obj-wrap.rs:5:8
+   |
+LL | struct W<T: ?Sized>(T);
+   |        ^
+   = note: required for the cast from `*const W<(dyn A + 'static)>` to `*const dyn A`
+
+error[E0277]: the trait bound `W<(dyn A + 'static)>: A` is not satisfied
+  --> $DIR/ptr-to-trait-obj-wrap.rs:15:5
+   |
+LL |     a as _
+   |     ^ the trait `A` is not implemented for `W<(dyn A + 'static)>`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/ptr-to-trait-obj-wrap.rs:3:1
+   |
+LL | trait A {}
+   | ^^^^^^^
+   = note: required for the cast from `*const W<W<(dyn A + 'static)>>` to `*const W<dyn A>`
+
+error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
+  --> $DIR/ptr-to-trait-obj-wrap.rs:15:5
+   |
+LL |     a as _
+   |     ^ doesn't have a size known at compile-time
+   |
+   = help: within `W<(dyn A + 'static)>`, the trait `Sized` is not implemented for `(dyn A + 'static)`
+note: required because it appears within the type `W<(dyn A + 'static)>`
+  --> $DIR/ptr-to-trait-obj-wrap.rs:5:8
+   |
+LL | struct W<T: ?Sized>(T);
+   |        ^
+   = note: required for the cast from `*const W<W<(dyn A + 'static)>>` to `*const W<dyn A>`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0277`.