1 files changed, 23 insertions, 7 deletions
diff --git a/src/test/ui/consts/std/cell.rs b/src/test/ui/consts/std/cell.rs
index cf6c0f2379d..f1ef541319a 100644
--- a/src/test/ui/consts/std/cell.rs
+++ b/src/test/ui/consts/std/cell.rs
@@ -1,18 +1,31 @@
+#![feature(const_refs_to_cell)]
+
 use std::cell::*;
 
-// not ok, because this would create a silent constant with interior mutability.
-// the rules could be relaxed in the future
+// not ok, because this creates a dangling pointer, just like `let x = Cell::new(42).as_ptr()` would
 static FOO: Wrap<*mut u32> = Wrap(Cell::new(42).as_ptr());
-//~^ ERROR cannot borrow a constant which may contain interior mutability
+//~^ ERROR encountered dangling pointer
+const FOO_CONST: Wrap<*mut u32> = Wrap(Cell::new(42).as_ptr());
+//~^ ERROR encountered dangling pointer
 
+// Ok, these are just base values and it is the `Wrap` author's job to uphold `Send` and `Sync`
+// invariants, since they used `unsafe impl`.
 static FOO3: Wrap<Cell<u32>> = Wrap(Cell::new(42));
-// ok
+const FOO3_CONST: Wrap<Cell<u32>> = Wrap(Cell::new(42));
+
+// ok, we are referring to the memory of another static item.
 static FOO4: Wrap<*mut u32> = Wrap(FOO3.0.as_ptr());
 
-// not ok, because the `as_ptr` call takes a reference to a type with interior mutability
-// which is not allowed in constants
+// not ok, the use of a constant here is equivalent to an inline declaration of the value, so
+// its memory will get freed before the constant is finished evaluating, thus creating a dangling
+// pointer. This would happen exactly the same at runtime.
+const FOO4_CONST: Wrap<*mut u32> = Wrap(FOO3_CONST.0.as_ptr());
+//~^ ERROR encountered dangling pointer
+
+// not ok, because the `as_ptr` call takes a reference to a temporary that will get freed
+// before the constant is finished evaluating.
 const FOO2: *mut u32 = Cell::new(42).as_ptr();
-//~^ ERROR cannot borrow a constant which may contain interior mutability
+//~^ ERROR encountered dangling pointer
 
 struct IMSafeTrustMe(UnsafeCell<u32>);
 unsafe impl Send for IMSafeTrustMe {}
@@ -21,10 +34,13 @@ unsafe impl Sync for IMSafeTrustMe {}
 static BAR: IMSafeTrustMe = IMSafeTrustMe(UnsafeCell::new(5));
 
 
+
 struct Wrap<T>(T);
 unsafe impl<T> Send for Wrap<T> {}
 unsafe impl<T> Sync for Wrap<T> {}
 
 static BAR_PTR: Wrap<*mut u32> = Wrap(BAR.0.get());
 
+const fn fst_ref<T, U>(x: &(T, U)) -> &T { &x.0 }
+
 fn main() {}