add test for may_dangle and ManuallyDrop

Author: RalfJung

tests/ui/drop/dropck-eyepatch-manuallydrop.rs

@@ -0,0 +1,22 @@
+// check-pass
+//! This test checks that dropck knows that ManuallyDrop does not drop its field.
+#![feature(dropck_eyepatch)]
+
+use std::mem::ManuallyDrop;
+
+struct S<T>(ManuallyDrop<T>);
+
+unsafe impl<#[may_dangle] T> Drop for S<T> {
+    fn drop(&mut self) {}
+}
+
+struct NonTrivialDrop<'a>(&'a str);
+impl<'a> Drop for NonTrivialDrop<'a> {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let s = String::from("string");
+    let _t = S(ManuallyDrop::new(NonTrivialDrop(&s)));
+    drop(s);
+}