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Rollup merge of #131864 - lrh2000:upcast_reorder, r=WaffleLapkin

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Never emit `vptr` for empty/auto traits

Emiting `vptr`s for empty/auto traits is unnecessary (#114942) and causes unsoundness in `trait_upcasting` (#131813). This PR should ensure that we never emit vtables for such traits. See the linked issues for more details.

I'm not sure if I can add tests for the vtable layout. So this PR only adds tests for the soundness hole (i.e., the segmentation fault will disappear after this PR).

Fixes #114942
Fixes #131813

Cc #65991 (tracking issue for `trait_upcasting`)

r? `@WaffleLapkin`  (per https://github.com/rust-lang/rust/issues/131813#issuecomment-2419969745)
This commit is contained in:
许杰友 Jieyou Xu (Joe)
2024-10-18 14:52:25 +01:00
committed by GitHub
parent 39af44dae9 781bff0499
commit 765e8c75b0
4 changed files with 36 additions and 14 deletions
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11
compiler/rustc_trait_selection/src/traits/vtable.rs
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@@ -154,18 +154,17 @@ fn prepare_vtable_segments_inner<'tcx, T>(
// emit innermost item, move to next sibling and stop there if possible, otherwise jump to outer level.
while let Some((inner_most_trait_ref, emit_vptr, mut siblings)) = stack.pop() {
let has_entries =
has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id());
segment_visitor(VtblSegment::TraitOwnEntries {
trait_ref: inner_most_trait_ref,
emit_vptr: emit_vptr && !tcx.sess.opts.unstable_opts.no_trait_vptr,
emit_vptr: emit_vptr && has_entries && !tcx.sess.opts.unstable_opts.no_trait_vptr,
})?;
// If we've emitted (fed to `segment_visitor`) a trait that has methods present in the vtable,
// we'll need to emit vptrs from now on.
if !emit_vptr_on_new_entry
&& has_own_existential_vtable_entries(tcx, inner_most_trait_ref.def_id())
{
emit_vptr_on_new_entry = true;
}
emit_vptr_on_new_entry |= has_entries;
if let Some(next_inner_most_trait_ref) =
siblings.find(|&sibling| visited.insert(sibling.upcast(tcx)))