Auto merge of #1766 - RalfJung:dropped-temporary, r=RalfJung

add test to detect dropped temporary

Let's make sure we catch this kind of error.

tests/compile-fail/dangling_pointers/stack_temporary.rs

@@ -0,0 +1,14 @@
+// This should fail even without validation, but some MIR opts mask the error
+// compile-flags: -Zmiri-disable-validation -Zmir-opt-level=0
+
+unsafe fn make_ref<'a>(x: *mut i32) -> &'a mut i32 {
+    &mut *x
+}
+
+fn main() {
+    unsafe {
+        let x = make_ref(&mut 0); // The temporary storing "0" is deallocated at the ";"!
+        let val = *x; //~ ERROR dereferenced after this allocation got freed
+        println!("{}", val);
+    }
+}