Auto merge of #44295 - plietar:extern-types, r=arielb1

Implement RFC 1861: Extern types

A few notes :

- Type parameters are not supported. This was an unresolved question from the RFC. It is not clear how useful this feature is, and how variance should be treated. This can be added in a future PR.

- `size_of_val` / `align_of_val` can be called with extern types, and respectively return 0 and 1. This differs from the RFC, which specified that they should panic, but after discussion with @eddyb on IRC this seems like a better solution.
If/when a `DynSized` trait is added, this will be disallowed statically.

- Auto traits are not implemented by default, since the contents of extern types is unknown. This means extern types are `!Sync`, `!Send` and `!Freeze`. This seems like the correct behaviour to me.
Manual `unsafe impl Sync for Foo` is still possible.

- This PR allows extern type to be used as the tail of a struct, as described by the RFC :
```rust
extern {
    type OpaqueTail;
}

#[repr(C)]
struct FfiStruct {
    data: u8,
    more_data: u32,
    tail: OpaqueTail,
}
```

However this is undesirable, as the alignment of `tail` is unknown (the current PR assumes an alignment of 1). Unfortunately we can't prevent it in the general case as the tail could be a type parameter :
```rust
#[repr(C)]
struct FfiStruct<T: ?Sized> {
    data: u8,
    more_data: u32,
    tail: T,
}
```

Adding a `DynSized` trait would solve this as well, by requiring tail fields to be bound by it.

- Despite being unsized, pointers to extern types are thin and can be casted from/to integers. However it is not possible to write a `null<T>() -> *const T` function which works with extern types, as I've explained here : #43467 (comment)

- Trait objects cannot be built from extern types. I intend to support it eventually, although how this interacts with `DynSized`/`size_of_val` is still unclear.

- The definition of `c_void` is unmodified

src/librustc/hir/def.rs

@@ -35,6 +35,7 @@ pub enum Def {
     Variant(DefId),
     Trait(DefId),
     TyAlias(DefId),
+    TyForeign(DefId),
     AssociatedTy(DefId),
     PrimTy(hir::PrimTy),
     TyParam(DefId),
@@ -152,7 +153,7 @@ impl Def {
             Def::AssociatedTy(id) | Def::TyParam(id) | Def::Struct(id) | Def::StructCtor(id, ..) |
             Def::Union(id) | Def::Trait(id) | Def::Method(id) | Def::Const(id) |
             Def::AssociatedConst(id) | Def::Macro(id, ..) |
-            Def::GlobalAsm(id) => {
+            Def::GlobalAsm(id) | Def::TyForeign(id) => {
                 id
             }
 
@@ -186,6 +187,7 @@ impl Def {
             Def::StructCtor(.., CtorKind::Fictive) => bug!("impossible struct constructor"),
             Def::Union(..) => "union",
             Def::Trait(..) => "trait",
+            Def::TyForeign(..) => "foreign type",
             Def::Method(..) => "method",
             Def::Const(..) => "constant",
             Def::AssociatedConst(..) => "associated constant",

src/librustc/hir/intravisit.rs

@@ -704,6 +704,7 @@ pub fn walk_foreign_item<'v, V: Visitor<'v>>(visitor: &mut V, foreign_item: &'v
             }
         }
         ForeignItemStatic(ref typ, _) => visitor.visit_ty(typ),
+        ForeignItemType => (),
     }
 
     walk_list!(visitor, visit_attribute, &foreign_item.attrs);

src/librustc/hir/lowering.rs

@@ -1722,6 +1722,9 @@ impl<'a> LoweringContext<'a> {
                     ForeignItemKind::Static(ref t, m) => {
                         hir::ForeignItemStatic(this.lower_ty(t), m)
                     }
+                    ForeignItemKind::Ty => {
+                        hir::ForeignItemType
+                    }
                 },
                 vis: this.lower_visibility(&i.vis, None),
                 span: i.span,

src/librustc/hir/mod.rs

@@ -1912,13 +1912,16 @@ pub enum ForeignItem_ {
     /// A foreign static item (`static ext: u8`), with optional mutability
     /// (the boolean is true when mutable)
     ForeignItemStatic(P<Ty>, bool),
+    /// A foreign type
+    ForeignItemType,
 }
 
 impl ForeignItem_ {
     pub fn descriptive_variant(&self) -> &str {
         match *self {
             ForeignItemFn(..) => "foreign function",
             ForeignItemStatic(..) => "foreign static item",
+            ForeignItemType => "foreign type",
         }
     }
 }

src/librustc/hir/print.rs

@@ -478,6 +478,13 @@ impl<'a> State<'a> {
                 self.end()?; // end the head-ibox
                 self.end() // end the outer cbox
             }
+            hir::ForeignItemType => {
+                self.head(&visibility_qualified(&item.vis, "type"))?;
+                self.print_name(item.name)?;
+                self.s.word(";")?;
+                self.end()?; // end the head-ibox
+                self.end() // end the outer cbox
+            }
         }
     }
 

src/librustc/ich/impls_hir.rs

@@ -977,7 +977,8 @@ impl_stable_hash_for!(struct hir::ForeignItem {
 
 impl_stable_hash_for!(enum hir::ForeignItem_ {
     ForeignItemFn(fn_decl, arg_names, generics),
-    ForeignItemStatic(ty, is_mutbl)
+    ForeignItemStatic(ty, is_mutbl),
+    ForeignItemType
 });
 
 impl_stable_hash_for!(enum hir::Stmt_ {
@@ -1086,6 +1087,7 @@ impl_stable_hash_for!(enum hir::def::Def {
     PrimTy(prim_ty),
     TyParam(def_id),
     SelfTy(trait_def_id, impl_def_id),
+    TyForeign(def_id),
     Fn(def_id),
     Const(def_id),
     Static(def_id, is_mutbl),

src/librustc/ich/impls_ty.rs

@@ -610,8 +610,7 @@ for ty::TypeVariants<'gcx>
                 def_id.hash_stable(hcx, hasher);
                 closure_substs.hash_stable(hcx, hasher);
             }
-            TyGenerator(def_id, closure_substs, interior)
-             => {
+            TyGenerator(def_id, closure_substs, interior) => {
                 def_id.hash_stable(hcx, hasher);
                 closure_substs.hash_stable(hcx, hasher);
                 interior.hash_stable(hcx, hasher);
@@ -630,6 +629,9 @@ for ty::TypeVariants<'gcx>
             TyParam(param_ty) => {
                 param_ty.hash_stable(hcx, hasher);
             }
+            TyForeign(def_id) => {
+                def_id.hash_stable(hcx, hasher);
+            }
             TyInfer(..) => {
                 bug!("ty::TypeVariants::hash_stable() - Unexpected variant {:?}.", *self)
             }

src/librustc/infer/freshen.rs

@@ -312,6 +312,7 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
             ty::TyNever |
             ty::TyTuple(..) |
             ty::TyProjection(..) |
+            ty::TyForeign(..) |
             ty::TyParam(..) |
             ty::TyAnon(..) => {
                 t.super_fold_with(self)

src/librustc/middle/resolve_lifetime.rs

@@ -365,6 +365,9 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             hir::ForeignItemStatic(..) => {
                 intravisit::walk_foreign_item(self, item);
             }
+            hir::ForeignItemType => {
+                intravisit::walk_foreign_item(self, item);
+            }
         }
     }
 

src/librustc/traits/coherence.rs

@@ -304,6 +304,10 @@ fn ty_is_local_constructor(ty: Ty, infer_is_local: InferIsLocal)-> bool {
             def.did.is_local()
         }
 
+        ty::TyForeign(did) => {
+            did.is_local()
+        }
+
         ty::TyDynamic(ref tt, ..) => {
             tt.principal().map_or(false, |p| p.def_id().is_local())
         }

src/librustc/traits/error_reporting.rs

@@ -255,6 +255,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     AdtKind::Enum => Some(17),
                 },
                 ty::TyGenerator(..) => Some(18),
+                ty::TyForeign(..) => Some(19),
                 ty::TyInfer(..) | ty::TyError => None
             }
         }

src/librustc/traits/select.rs

@@ -1705,6 +1705,12 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                     // say nothing; a candidate may be added by
                     // `assemble_candidates_from_object_ty`.
                 }
+                ty::TyForeign(..) => {
+                    // Since the contents of foreign types is unknown,
+                    // we don't add any `..` impl. Default traits could
+                    // still be provided by a manual implementation for
+                    // this trait and type.
+                }
                 ty::TyParam(..) |
                 ty::TyProjection(..) => {
                     // In these cases, we don't know what the actual
@@ -2022,7 +2028,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 Where(ty::Binder(Vec::new()))
             }
 
-            ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) => Never,
+            ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => Never,
 
             ty::TyTuple(tys, _) => {
                 Where(ty::Binder(tys.last().into_iter().cloned().collect()))
@@ -2066,7 +2072,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 Where(ty::Binder(Vec::new()))
             }
 
-            ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) | ty::TyGenerator(..) |
+            ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
+            ty::TyGenerator(..) | ty::TyForeign(..) |
             ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
                 Never
             }
@@ -2148,6 +2155,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
 
             ty::TyDynamic(..) |
             ty::TyParam(..) |
+            ty::TyForeign(..) |
             ty::TyProjection(..) |
             ty::TyInfer(ty::TyVar(_)) |
             ty::TyInfer(ty::FreshTy(_)) |

src/librustc/ty/context.rs

@@ -1610,7 +1610,7 @@ impl<'a, 'tcx> TyCtxt<'a, 'tcx, 'tcx> {
     pub fn print_debug_stats(self) {
         sty_debug_print!(
             self,
-            TyAdt, TyArray, TySlice, TyRawPtr, TyRef, TyFnDef, TyFnPtr, TyGenerator,
+            TyAdt, TyArray, TySlice, TyRawPtr, TyRef, TyFnDef, TyFnPtr, TyGenerator, TyForeign,
             TyDynamic, TyClosure, TyTuple, TyParam, TyInfer, TyProjection, TyAnon);
 
         println!("Substs interner: #{}", self.interners.substs.borrow().len());
@@ -1861,6 +1861,10 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.mk_ty(TyAdt(def, substs))
     }
 
+    pub fn mk_foreign(self, def_id: DefId) -> Ty<'tcx> {
+        self.mk_ty(TyForeign(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);

src/librustc/ty/error.rs

@@ -182,6 +182,7 @@ impl<'a, 'gcx, 'lcx, 'tcx> ty::TyS<'tcx> {
             ty::TyTuple(ref tys, _) if tys.is_empty() => self.to_string(),
 
             ty::TyAdt(def, _) => format!("{} `{}`", def.descr(), tcx.item_path_str(def.did)),
+            ty::TyForeign(def_id) => format!("extern type `{}`", tcx.item_path_str(def_id)),
             ty::TyArray(_, n) => {
                 if let ConstVal::Integral(ConstInt::Usize(n)) = n.val {
                     format!("array of {} elements", n)

src/librustc/ty/fast_reject.rs

@@ -49,6 +49,7 @@ pub enum SimplifiedTypeGen<D>
     AnonSimplifiedType(D),
     FunctionSimplifiedType(usize),
     ParameterSimplifiedType,
+    ForeignSimplifiedType(DefId),
 }
 
 /// Tries to simplify a type by dropping type parameters, deref'ing away any reference types, etc.
@@ -113,6 +114,9 @@ pub fn simplify_type<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
         ty::TyAnon(def_id, _) => {
             Some(AnonSimplifiedType(def_id))
         }
+        ty::TyForeign(def_id) => {
+            Some(ForeignSimplifiedType(def_id))
+        }
         ty::TyInfer(_) | ty::TyError => None,
     }
 }
@@ -140,6 +144,7 @@ impl<D: Copy + Debug + Ord + Eq + Hash> SimplifiedTypeGen<D> {
             AnonSimplifiedType(d) => AnonSimplifiedType(map(d)),
             FunctionSimplifiedType(n) => FunctionSimplifiedType(n),
             ParameterSimplifiedType => ParameterSimplifiedType,
+            ForeignSimplifiedType(d) => ForeignSimplifiedType(d),
         }
     }
 }
@@ -172,6 +177,7 @@ impl<'gcx, D> HashStable<StableHashingContext<'gcx>> for SimplifiedTypeGen<D>
             GeneratorSimplifiedType(d) => d.hash_stable(hcx, hasher),
             AnonSimplifiedType(d) => d.hash_stable(hcx, hasher),
             FunctionSimplifiedType(n) => n.hash_stable(hcx, hasher),
+            ForeignSimplifiedType(d) => d.hash_stable(hcx, hasher),
         }
     }
 }

src/librustc/ty/flags.rs

@@ -63,7 +63,8 @@ impl FlagComputation {
             &ty::TyFloat(_) |
             &ty::TyUint(_) |
             &ty::TyNever |
-            &ty::TyStr => {
+            &ty::TyStr |
+            &ty::TyForeign(..) => {
             }
 
             // You might think that we could just return TyError for

src/librustc/ty/item_path.rs

@@ -281,6 +281,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 }
             }
 
+            ty::TyForeign(did) => self.push_item_path(buffer, did),
+
             ty::TyBool |
             ty::TyChar |
             ty::TyInt(_) |
@@ -344,8 +346,9 @@ pub fn characteristic_def_id_of_type(ty: Ty) -> Option<DefId> {
                                       .next(),
 
         ty::TyFnDef(def_id, _) |
-        ty::TyClosure(def_id, _) => Some(def_id),
-        ty::TyGenerator(def_id, _, _) => Some(def_id),
+        ty::TyClosure(def_id, _) |
+        ty::TyGenerator(def_id, _, _) |
+        ty::TyForeign(def_id) => Some(def_id),
 
         ty::TyBool |
         ty::TyChar |

src/librustc/ty/layout.rs

@@ -1141,14 +1141,15 @@ impl<'a, 'tcx> Layout {
                 Ok(Scalar { value: Pointer, non_zero: non_zero })
             } else {
                 let unsized_part = tcx.struct_tail(pointee);
-                let meta = match unsized_part.sty {
-                    ty::TySlice(_) | ty::TyStr => {
-                        Int(dl.ptr_sized_integer())
-                    }
-                    ty::TyDynamic(..) => Pointer,
-                    _ => return Err(LayoutError::Unknown(unsized_part))
-                };
-                Ok(FatPointer { metadata: meta, non_zero: non_zero })
+                match unsized_part.sty {
+                    ty::TySlice(_) | ty::TyStr => Ok(FatPointer {
+                        metadata: Int(dl.ptr_sized_integer()),
+                        non_zero: non_zero
+                    }),
+                    ty::TyDynamic(..) => Ok(FatPointer { metadata: Pointer, non_zero: non_zero }),
+                    ty::TyForeign(..) => Ok(Scalar { value: Pointer, non_zero: non_zero }),
+                    _ => Err(LayoutError::Unknown(unsized_part)),
+                }
             }
         };
 
@@ -1239,7 +1240,7 @@ impl<'a, 'tcx> Layout {
                     non_zero: false
                 }
             }
-            ty::TyDynamic(..) => {
+            ty::TyDynamic(..) | ty::TyForeign(..) => {
                 let mut unit = Struct::new(dl, &vec![], &ReprOptions::default(),
                   StructKind::AlwaysSizedUnivariant, ty)?;
                 unit.sized = false;
@@ -2252,7 +2253,8 @@ impl<'a, 'tcx> TyLayout<'tcx> {
             ty::TyFnPtr(_) |
             ty::TyNever |
             ty::TyFnDef(..) |
-            ty::TyDynamic(..) => {
+            ty::TyDynamic(..) |
+            ty::TyForeign(..) => {
                 bug!("TyLayout::field_type({:?}): not applicable", self)
             }
 

src/librustc/ty/mod.rs

@@ -1784,7 +1784,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
                 vec![]
             }
 
-            TyStr | TyDynamic(..) | TySlice(_) | TyError => {
+            TyStr | TyDynamic(..) | TySlice(_) | TyForeign(..) | TyError => {
                 // these are never sized - return the target type
                 vec![ty]
             }

src/librustc/ty/outlives.rs

@@ -142,6 +142,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             ty::TyNever |           // ...
             ty::TyAdt(..) |         // OutlivesNominalType
             ty::TyAnon(..) |        // OutlivesNominalType (ish)
+            ty::TyForeign(..) |     // OutlivesNominalType
             ty::TyStr |             // OutlivesScalar (ish)
             ty::TyArray(..) |       // ...
             ty::TySlice(..) |       // ...

src/librustc/ty/relate.rs

@@ -381,6 +381,12 @@ pub fn super_relate_tys<'a, 'gcx, 'tcx, R>(relation: &mut R,
             Ok(tcx.mk_adt(a_def, substs))
         }
 
+        (&ty::TyForeign(a_id), &ty::TyForeign(b_id))
+            if a_id == b_id =>
+        {
+            Ok(tcx.mk_foreign(a_id))
+        }
+
         (&ty::TyDynamic(ref a_obj, ref a_region), &ty::TyDynamic(ref b_obj, ref b_region)) => {
             let region_bound = relation.with_cause(Cause::ExistentialRegionBound,
                                                        |relation| {

src/librustc/ty/structural_impls.rs

@@ -676,7 +676,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyAnon(did, substs) => ty::TyAnon(did, substs.fold_with(folder)),
             ty::TyBool | ty::TyChar | ty::TyStr | ty::TyInt(_) |
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyError | ty::TyInfer(_) |
-            ty::TyParam(..) | ty::TyNever => return self
+            ty::TyParam(..) | ty::TyNever | ty::TyForeign(..) => return self
         };
 
         if self.sty == sty {
@@ -710,7 +710,7 @@ impl<'tcx> TypeFoldable<'tcx> for Ty<'tcx> {
             ty::TyAnon(_, ref substs) => substs.visit_with(visitor),
             ty::TyBool | ty::TyChar | ty::TyStr | ty::TyInt(_) |
             ty::TyUint(_) | ty::TyFloat(_) | ty::TyError | ty::TyInfer(_) |
-            ty::TyParam(..) | ty::TyNever => false,
+            ty::TyParam(..) | ty::TyNever | ty::TyForeign(..) => false,
         }
     }