Implement and use BikeshedGuaranteedNoDrop for union/unsafe field validity

compiler/rustc_middle/src/traits/select.rs

@@ -168,6 +168,8 @@ pub enum SelectionCandidate<'tcx> {
     BuiltinObjectCandidate,
 
     BuiltinUnsizeCandidate,
+
+    BikeshedGuaranteedNoDropCandidate,
 }
 
 /// The result of trait evaluation. The order is important

compiler/rustc_middle/src/ty/adt.rs

@@ -217,6 +217,10 @@ impl<'tcx> rustc_type_ir::inherent::AdtDef<TyCtxt<'tcx>> for AdtDef<'tcx> {
         self.is_phantom_data()
     }
 
+    fn is_manually_drop(self) -> bool {
+        self.is_manually_drop()
+    }
+
     fn all_field_tys(
         self,
         tcx: TyCtxt<'tcx>,

compiler/rustc_middle/src/ty/context.rs

@@ -690,6 +690,7 @@ bidirectional_lang_item_map! {
     AsyncFnOnce,
     AsyncFnOnceOutput,
     AsyncIterator,
+    BikeshedGuaranteedNoDrop,
     CallOnceFuture,
     CallRefFuture,
     Clone,