Rollup merge of #77663 - HeroicKatora:regression-tests-27675-object-safe, r=Aaron1011

Add compile fail test for issue 27675

A recently merged PR (#73905) strengthened the checks on bounds of associated items. This rejects the attack path of #27675 which consisted of constructing a `dyn Trait<Item=T>` where `T` would not fulfill the bounds required on `Item` of the `Trait` behind the dyn object.

This regression test, extracted from [the weaponized instance](https://github.com/rust-lang/rust/issues/27675#issuecomment-696956878), checks that this is rejected.

src/test/compile-fail/issue-27675-unchecked-bounds.rs

@@ -0,0 +1,19 @@
+/// The compiler previously did not properly check the bound of `From` when it was used from type
+/// of the dyn trait object (use in `copy_any` below). Since the associated type is under user
+/// control in this usage, the compiler could be tricked to believe any type implemented any trait.
+/// This would ICE, except for pure marker traits like `Copy`. It did not require providing an
+/// instance of the dyn trait type, only name said type.
+trait Setup {
+    type From: Copy;
+}
+
+fn copy<U: Setup + ?Sized>(from: &U::From) -> U::From {
+    *from
+}
+
+pub fn copy_any<T>(t: &T) -> T {
+    copy::<dyn Setup<From=T>>(t)
+    //~^ ERROR the trait bound `T: Copy` is not satisfied
+}
+
+fn main() {}