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Wrap promoted generator fields in MaybeUninit
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This prevents uninhabited fields from "infecting" the abi and
largest_niche of the generator layout.

This fixes a latent bug, where an uninhabited field could be promoted to
the generator prefix and cause the entire generator to become
uninhabited.
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tmandry committed Jul 29, 2019
1 parent c43753f commit 6fae7f8
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Showing 5 changed files with 37 additions and 10 deletions.
2 changes: 2 additions & 0 deletions src/libcore/mem/maybe_uninit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -208,6 +208,8 @@ use crate::mem::ManuallyDrop;
/// guarantee may evolve.
#[allow(missing_debug_implementations)]
#[stable(feature = "maybe_uninit", since = "1.36.0")]
// Lang item so we can wrap other types in it. This is useful for generators.
#[cfg_attr(not(bootstrap), lang = "maybe_uninit")]
#[derive(Copy)]
#[repr(transparent)]
pub union MaybeUninit<T> {
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2 changes: 2 additions & 0 deletions src/librustc/middle/lang_items.rs
Original file line number Diff line number Diff line change
Expand Up @@ -365,6 +365,8 @@ language_item_table! {

ManuallyDropItem, "manually_drop", manually_drop, Target::Struct;

MaybeUninitLangItem, "maybe_uninit", maybe_uninit, Target::Union;

DebugTraitLangItem, "debug_trait", debug_trait, Target::Trait;

// Align offset for stride != 1, must not panic.
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21 changes: 16 additions & 5 deletions src/librustc/ty/context.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2347,18 +2347,17 @@ impl<'tcx> TyCtxt<'tcx> {
self.mk_ty(Foreign(def_id))
}

pub fn mk_box(self, ty: Ty<'tcx>) -> Ty<'tcx> {
let def_id = self.require_lang_item(lang_items::OwnedBoxLangItem);
let adt_def = self.adt_def(def_id);
let substs = InternalSubsts::for_item(self, def_id, |param, substs| {
fn mk_generic_adt(self, wrapper_def_id: DefId, ty_param: Ty<'tcx>) -> Ty<'tcx> {
let adt_def = self.adt_def(wrapper_def_id);
let substs = InternalSubsts::for_item(self, wrapper_def_id, |param, substs| {
match param.kind {
GenericParamDefKind::Lifetime |
GenericParamDefKind::Const => {
bug!()
}
GenericParamDefKind::Type { has_default, .. } => {
if param.index == 0 {
ty.into()
ty_param.into()
} else {
assert!(has_default);
self.type_of(param.def_id).subst(self, substs).into()
Expand All @@ -2369,6 +2368,18 @@ impl<'tcx> TyCtxt<'tcx> {
self.mk_ty(Adt(adt_def, substs))
}

#[inline]
pub fn mk_box(self, ty: Ty<'tcx>) -> Ty<'tcx> {
let def_id = self.require_lang_item(lang_items::OwnedBoxLangItem);
self.mk_generic_adt(def_id, ty)
}

#[inline]
pub fn mk_maybe_uninit(self, ty: Ty<'tcx>) -> Ty<'tcx> {
let def_id = self.require_lang_item(lang_items::MaybeUninitLangItem);
self.mk_generic_adt(def_id, ty)
}

#[inline]
pub fn mk_ptr(self, tm: TypeAndMut<'tcx>) -> Ty<'tcx> {
self.mk_ty(RawPtr(tm))
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7 changes: 2 additions & 5 deletions src/librustc/ty/layout.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1406,24 +1406,21 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
Abi::Scalar(s) => s.clone(),
_ => bug!(),
};
// FIXME(eddyb) wrap each promoted type in `MaybeUninit` so that they
// don't poison the `largest_niche` or `abi` fields of `prefix`.
let promoted_layouts = ineligible_locals.iter()
.map(|local| subst_field(info.field_tys[local]))
.map(|ty| tcx.mk_maybe_uninit(ty))
.map(|ty| self.layout_of(ty));
let prefix_layouts = substs.prefix_tys(def_id, tcx)
.map(|ty| self.layout_of(ty))
.chain(iter::once(Ok(discr_layout)))
.chain(promoted_layouts)
.collect::<Result<Vec<_>, _>>()?;
let mut prefix = self.univariant_uninterned(
let prefix = self.univariant_uninterned(
ty,
&prefix_layouts,
&ReprOptions::default(),
StructKind::AlwaysSized,
)?;
// FIXME(eddyb) need `MaybeUninit` around promoted types (see above).
prefix.largest_niche = None;

let (prefix_size, prefix_align) = (prefix.size, prefix.align);

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15 changes: 15 additions & 0 deletions src/test/ui/async-await/issues/issue-59972.rs
Original file line number Diff line number Diff line change
@@ -1,3 +1,7 @@
// Incorrect handling of uninhabited types could cause us to mark generator
// types as entirely uninhabited, when they were in fact constructible. This
// caused us to hit "unreachable" code (illegal instruction on x86).

// run-pass

// compile-flags: --edition=2018
Expand All @@ -19,7 +23,18 @@ async fn contains_never() {
let error2 = error;
}

#[allow(unused)]
async fn overlap_never() {
let error1 = uninhabited_async();
noop().await;
let error2 = uninhabited_async();
drop(error1);
noop().await;
drop(error2);
}

#[allow(unused_must_use)]
fn main() {
contains_never();
overlap_never();
}

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