adt_destructor: sanity-check returned item

compiler/rustc_middle/src/ty/util.rs

@@ -414,6 +414,11 @@ impl<'tcx> TyCtxt<'tcx> {
                 continue;
             };
 
+            if self.def_kind(item_id) != DefKind::AssocFn {
+                self.dcx().span_delayed_bug(self.def_span(item_id), "drop is not a function");
+                continue;
+            }
+
             if let Some(old_item_id) = dtor_candidate {
                 self.dcx()
                     .struct_span_err(self.def_span(item_id), "multiple drop impls found")

tests/ui/drop/nonsense-drop-impl-issue-139278.rs

@@ -0,0 +1,10 @@
+//@ check-fail
+struct Foo;
+
+impl Drop for Foo { //~ ERROR: not all trait items implemented
+    const SPLOK: u32 = 0; //~ ERROR: not a member of trait
+}
+
+const X: Foo = Foo;
+
+fn main() {}

tests/ui/drop/nonsense-drop-impl-issue-139278.stderr

@@ -0,0 +1,18 @@
+error[E0438]: const `SPLOK` is not a member of trait `Drop`
+  --> $DIR/nonsense-drop-impl-issue-139278.rs:5:5
+   |
+LL |     const SPLOK: u32 = 0;
+   |     ^^^^^^^^^^^^^^^^^^^^^ not a member of trait `Drop`
+
+error[E0046]: not all trait items implemented, missing: `drop`
+  --> $DIR/nonsense-drop-impl-issue-139278.rs:4:1
+   |
+LL | impl Drop for Foo {
+   | ^^^^^^^^^^^^^^^^^ missing `drop` in implementation
+   |
+   = help: implement the missing item: `fn drop(&mut self) { todo!() }`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0046, E0438.
+For more information about an error, try `rustc --explain E0046`.