From 6fdc133daad5ea1a2fccd460fd3b65a715f0f272 Mon Sep 17 00:00:00 2001 From: Michael Goulet Date: Mon, 23 Sep 2024 13:52:02 -0400 Subject: [PATCH] Add a debug assertion in codegen that unsize casts of the same principal trait def id are truly NOPs --- src/unsize.rs | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/src/unsize.rs b/src/unsize.rs index 8cfe93b4d9c9..339628053a95 100644 --- a/src/unsize.rs +++ b/src/unsize.rs @@ -34,7 +34,22 @@ pub(crate) fn unsized_info<'tcx>( let old_info = old_info.expect("unsized_info: missing old info for trait upcasting coercion"); if data_a.principal_def_id() == data_b.principal_def_id() { - // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables. + // Codegen takes advantage of the additional assumption, where if the + // principal trait def id of what's being casted doesn't change, + // then we don't need to adjust the vtable at all. This + // corresponds to the fact that `dyn Tr: Unsize>` + // requires that `A = B`; we don't allow *upcasting* objects + // between the same trait with different args. If we, for + // some reason, were to relax the `Unsize` trait, it could become + // unsound, so let's assert here that the trait refs are *equal*. + // + // We can use `assert_eq` because the binders should have been anonymized, + // and because higher-ranked equality now requires the binders are equal. + debug_assert_eq!( + data_a.principal(), + data_b.principal(), + "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}" + ); return old_info; }