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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
commit 765e8c75b0
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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)))

4
tests/ui/traits/object/print_vtable_sizes.stdout
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@ -1,8 +1,8 @@
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "E", "entries": "6", "entries_ignoring_upcasting": "4", "entries_for_upcasting": "2", "upcasting_cost_percent": "50" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "G", "entries": "14", "entries_ignoring_upcasting": "11", "entries_for_upcasting": "3", "upcasting_cost_percent": "27.27272727272727" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "A", "entries": "6", "entries_ignoring_upcasting": "5", "entries_for_upcasting": "1", "upcasting_cost_percent": "20" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "G", "entries": "13", "entries_ignoring_upcasting": "11", "entries_for_upcasting": "2", "upcasting_cost_percent": "18.181818181818183" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "B", "entries": "4", "entries_ignoring_upcasting": "4", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "D", "entries": "4", "entries_ignoring_upcasting": "4", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "E", "entries": "4", "entries_ignoring_upcasting": "4", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "F", "entries": "6", "entries_ignoring_upcasting": "6", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "_::S", "entries": "3", "entries_ignoring_upcasting": "3", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }
print-vtable-sizes { "crate_name": "print_vtable_sizes", "trait_name": "_::S", "entries": "3", "entries_ignoring_upcasting": "3", "entries_for_upcasting": "0", "upcasting_cost_percent": "0" }

29
tests/ui/traits/upcast_reorder.rs Normal file
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@ -0,0 +1,29 @@
//@ run-pass
//
// issue: <https://github.com/rust-lang/rust/issues/131813>
#![feature(trait_upcasting)]
trait Pollable {
#[allow(unused)]
fn poll(&self) {}
}
trait FileIo: Pollable + Send + Sync {
fn read(&self) {}
}
trait Terminal: Send + Sync + FileIo {}
struct A;
impl Pollable for A {}
impl FileIo for A {}
impl Terminal for A {}
fn main() {
let a = A;
let b = &a as &dyn Terminal;
let c = b as &dyn FileIo;
c.read();
}

6
tests/ui/traits/vtable/multiple-markers.stderr
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@ -14,7 +14,6 @@ error: vtable entries for `<S as B>`: [
MetadataSize,
MetadataAlign,
Method(<S as T>::method),
TraitVPtr(<S as M2>),
]
--> $DIR/multiple-markers.rs:24:1
|
@ -26,8 +25,6 @@ error: vtable entries for `<S as C>`: [
MetadataSize,
MetadataAlign,
Method(<S as T>::method),
TraitVPtr(<S as M1>),
TraitVPtr(<S as M2>),
]
--> $DIR/multiple-markers.rs:27:1
|
@ -39,9 +36,6 @@ error: vtable entries for `<S as D>`: [
MetadataSize,
MetadataAlign,
Method(<S as T>::method),
TraitVPtr(<S as M0>),
TraitVPtr(<S as M1>),
TraitVPtr(<S as M2>),
]
--> $DIR/multiple-markers.rs:30:1
|