better test

src/test/ui/rfc-2632-const-trait-impl/const-drop.rs

@@ -1,23 +1,28 @@
+// run-pass
 #![feature(const_trait_impl)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_mut_refs)]
 #![feature(const_panic)]
 
-struct S;
+struct S<'a>(&'a mut u8);
 
-impl const Drop for S {
+impl<'a> const Drop for S<'a> {
     fn drop(&mut self) {
-        // NB: There is no way to tell that a const destructor is ran,
+        *self.0 += 1;
-        // because even if we can operate on mutable variables, it will
-        // not be reflected because everything is `const`. So we panic
-        // here, attempting to make the CTFE engine error.
-        panic!("much const drop")
-        //~^ ERROR evaluation of constant value failed
     }
 }
 
 const fn a<T: ~const Drop>(_: T) {}
 
-const _: () = a(S);
+const fn b() -> u8 {
+    let mut c = 0;
+    let _ = S(&mut c);
+    a(S(&mut c));
+    c
+}
 
-fn main() {}
+const C: u8 = b();
+
+fn main() {
+    assert_eq!(C, 2);
+}

src/test/ui/rfc-2632-const-trait-impl/const-drop.stderr

@@ -1,25 +0,0 @@
-error[E0080]: evaluation of constant value failed
-  --> $DIR/const-drop.rs:14:9
-   |
-LL |         panic!("much const drop")
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
-   |         |
-   |         the evaluated program panicked at 'much const drop', $DIR/const-drop.rs:14:9
-   |         inside `<S as Drop>::drop` at $SRC_DIR/std/src/panic.rs:LL:COL
-...
-LL | const fn a<T: ~const Drop>(_: T) {}
-   |                                   - inside `a::<S>` at $DIR/const-drop.rs:19:35
-LL | 
-LL | const _: () = a(S);
-   |               ---- inside `_` at $DIR/const-drop.rs:21:15
-   |
-  ::: $SRC_DIR/core/src/ptr/mod.rs:LL:COL
-   |
-LL | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
-   | ------------------------------------------------------- inside `std::ptr::drop_in_place::<S> - shim(Some(S))` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
-   |
-   = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0080`.