Auto merge of #1602 - RalfJung:box, r=RalfJung

test Box::into_raw aliasing

Directly test aliasing problems caused by `Box::into_raw` issues (like we have them again right now due to rust-lang/rust#77187, but the pinned rustc is older than that so this should still be able to land).

tests/run-pass/box.rs

@@ -0,0 +1,60 @@
+#![feature(ptr_internals)]
+
+fn main() {
+    into_raw();
+    into_unique();
+    boxed_pair_to_vec();
+}
+
+fn into_raw() { unsafe {
+    let b = Box::new(4i32);
+    let r = Box::into_raw(b);
+
+    // "lose the tag"
+    let r2 = ((r as usize)+0) as *mut i32;
+    *(&mut *r2) = 7;
+
+    // Use original ptr again
+    *(&mut *r) = 17;
+    drop(Box::from_raw(r));
+}}
+
+fn into_unique() { unsafe {
+    let b = Box::new(4i32);
+    let u = Box::into_unique(b);
+
+    // "lose the tag"
+    let r = ((u.as_ptr() as usize)+0) as *mut i32;
+    *(&mut *r) = 7;
+
+    // Use original ptr again.
+    drop(Box::from_raw(u.as_ptr()));
+}}
+
+fn boxed_pair_to_vec() {
+    #[repr(C)]
+    #[derive(Debug)]
+    struct PairFoo {
+        fst: Foo,
+        snd: Foo,
+    }
+
+    #[derive(Debug)]
+    struct Foo(u64);
+    fn reinterstruct(box_pair: Box<PairFoo>) -> Vec<Foo> {
+        let ref_pair = Box::leak(box_pair) as *mut PairFoo;
+        let ptr_foo = unsafe { &mut (*ref_pair).fst as *mut Foo };
+        unsafe {
+            Vec::from_raw_parts(ptr_foo, 2, 2)
+        }
+    }
+
+    let pair_foo = Box::new(PairFoo {
+        fst: Foo(42),
+        snd: Foo(1337),
+    });
+    println!("pair_foo = {:?}", pair_foo);
+    for (n, foo) in reinterstruct(pair_foo).into_iter().enumerate() {
+        println!("foo #{} = {:?}", n, foo);
+    }
+}