Wrap promoted generator fields in MaybeUninit

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.

Author: tmandry

Diff for: src/libcore/mem/maybe_uninit.rs

@@ -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> {

Diff for: src/librustc/middle/lang_items.rs

@@ -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.

Diff for: src/librustc/ty/context.rs

@@ -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()
@@ -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))

Diff for: src/librustc/ty/layout.rs

@@ -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);
 

Diff for: src/test/ui/async-await/issues/issue-59972.rs

@@ -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
@@ -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();
 }