Use explicit drop impl

Author: bjorn3

compiler/rustc_interface/src/passes.rs

@@ -93,17 +93,22 @@ mod boxed_resolver {
 
     pub struct BoxedResolver(Pin<Box<BoxedResolverInner>>);
 
-    // Note: Drop order is important to prevent dangling references. Resolver must be dropped first,
-    // then resolver_arenas and finally session.
-    // The drop order is defined to be from top to bottom in RFC1857, so there is no need for
-    // ManuallyDrop for as long as the fields are not reordered.
     struct BoxedResolverInner {
-        resolver: Option<Resolver<'static>>,
-        resolver_arenas: ResolverArenas<'static>,
         session: Lrc<Session>,
+        resolver_arenas: Option<ResolverArenas<'static>>,
+        resolver: Option<Resolver<'static>>,
         _pin: PhantomPinned,
     }
 
+    // Note: Drop order is important to prevent dangling references. Resolver must be dropped first,
+    // then resolver_arenas and finally session.
+    impl Drop for BoxedResolverInner {
+        fn drop(&mut self) {
+            self.resolver.take();
+            self.resolver_arenas.take();
+        }
+    }
+
     impl BoxedResolver {
         pub(super) fn new<F>(session: Lrc<Session>, make_resolver: F) -> Result<(ast::Crate, Self)>
         where
@@ -114,15 +119,15 @@ mod boxed_resolver {
         {
             let mut boxed_resolver = Box::new(BoxedResolverInner {
                 session,
-                resolver_arenas: Resolver::arenas(),
+                resolver_arenas: Some(Resolver::arenas()),
                 resolver: None,
                 _pin: PhantomPinned,
             });
             unsafe {
                 let (crate_, resolver) = make_resolver(
                     std::mem::transmute::<&Session, &Session>(&boxed_resolver.session),
                     std::mem::transmute::<&ResolverArenas<'_>, &ResolverArenas<'_>>(
-                        &boxed_resolver.resolver_arenas,
+                        boxed_resolver.resolver_arenas.as_ref().unwrap(),
                     ),
                 )?;
                 boxed_resolver.resolver =