fix precise live drops

compiler/rustc_const_eval/src/transform/check_consts/post_drop_elaboration.rs

@@ -79,9 +79,9 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
             mir::TerminatorKind::Drop { place: dropped_place, .. } => {
                 let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
                 if !NeedsNonConstDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
-                    bug!(
-                        "Drop elaboration left behind a Drop for a type that does not need dropping"
-                    );
+                    // Instead of throwing a bug, we just return here. This is because we have to
+                    // run custom `const Drop` impls.
+                    return;
                 }
 
                 if dropped_place.is_indirect() {

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

@@ -1,23 +1,23 @@
 error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:28:5
+  --> $DIR/const-drop-fail.rs:30:5
    |
 LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:19:19
+  --> $DIR/const-drop-fail.rs:21:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
-  --> $DIR/const-drop-fail.rs:30:5
+  --> $DIR/const-drop-fail.rs:32:5
    |
 LL |     ConstImplWithDropGlue(NonTrivialDrop),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:19:19
+  --> $DIR/const-drop-fail.rs:21:19
    |
 LL | const fn check<T: ~const Drop>(_: T) {}
    |                   ^^^^^^^^^^^ required by this bound in `check`

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

@@ -1,6 +1,8 @@
+// revisions: stock precise
 #![feature(const_trait_impl)]
 #![feature(const_mut_refs)]
 #![feature(const_fn_trait_bound)]
+#![cfg_attr(precise, feature(const_precise_live_drops))]
 
 struct NonTrivialDrop;
 

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

@@ -0,0 +1,27 @@
+error[E0277]: the trait bound `NonTrivialDrop: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:30:5
+   |
+LL |     NonTrivialDrop,
+   |     ^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `NonTrivialDrop`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:21:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error[E0277]: the trait bound `ConstImplWithDropGlue: Drop` is not satisfied
+  --> $DIR/const-drop-fail.rs:32:5
+   |
+LL |     ConstImplWithDropGlue(NonTrivialDrop),
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Drop` is not implemented for `ConstImplWithDropGlue`
+   |
+note: required by a bound in `check`
+  --> $DIR/const-drop-fail.rs:21:19
+   |
+LL | const fn check<T: ~const Drop>(_: T) {}
+   |                   ^^^^^^^^^^^ required by this bound in `check`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

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

@@ -1,8 +1,10 @@
 // run-pass
+// revisions: stock precise
 #![feature(const_trait_impl)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_mut_refs)]
 #![feature(const_panic)]
+#![cfg_attr(precise, feature(const_precise_live_drops))]
 
 struct S<'a>(&'a mut u8);