Rollup merge of #100386 - compiler-errors:sized-coinductive-redux, r=lcnr

Make `Sized` coinductive, again

A revival of #83647

---

What exactly makes co-induction sound? Better question: are there any unsoundness risks from this? `Sized` can't be implemented by custom `impl` blocks, nor can it be conditionally implemented based on anything other than child fields being `Sized`, right?

r? `@nikomatsakis` for whenever he gets back from vacation

Author: Manishearth

compiler/rustc_middle/src/ty/mod.rs

@@ -2506,6 +2506,10 @@ impl<'tcx> TyCtxt<'tcx> {
         self.trait_def(trait_def_id).has_auto_impl
     }
 
+    pub fn trait_is_coinductive(self, trait_def_id: DefId) -> bool {
+        self.trait_is_auto(trait_def_id) || self.lang_items().sized_trait() == Some(trait_def_id)
+    }
+
     /// Returns layout of a generator. Layout might be unavailable if the
     /// generator is tainted by errors.
     pub fn generator_layout(self, def_id: DefId) -> Option<&'tcx GeneratorLayout<'tcx>> {

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -959,7 +959,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
         let result = match predicate.kind().skip_binder() {
-            ty::PredicateKind::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()),
+            ty::PredicateKind::Trait(ref data) => self.tcx().trait_is_coinductive(data.def_id()),
             ty::PredicateKind::WellFormed(_) => true,
             _ => false,
         };

src/test/ui/generic-associated-types/bugs/issue-80626.rs

@@ -1,15 +1,12 @@
-// check-fail
-// known-bug: #80626
-
-// This should pass, but it requires `Sized` to be coinductive.
+// check-pass
 
 trait Allocator {
     type Allocated<T>;
 }
 
 enum LinkedList<A: Allocator> {
     Head,
-    Next(A::Allocated<Self>)
+    Next(A::Allocated<Self>),
 }
 
 fn main() {}

src/test/ui/generic-associated-types/bugs/issue-80626.stderr

@@ -1,3 +1,5 @@
+// check-pass
+
 trait PointerFamily {
     type Pointer<T>;
 }
@@ -10,11 +12,13 @@ impl PointerFamily for RcFamily {
 }
 
 #[allow(dead_code)]
-enum Node<T, P: PointerFamily> where P::Pointer<Node<T, P>>: Sized {
+enum Node<T, P: PointerFamily>
+where
+    P::Pointer<Node<T, P>>: Sized,
+{
     Cons(P::Pointer<Node<T, P>>),
 }
 
 fn main() {
     let _list: <RcFamily as PointerFamily>::Pointer<Node<i32, RcFamily>>;
-    //~^ ERROR overflow evaluating the requirement `Node<i32, RcFamily>: Sized`
 }

src/test/ui/generic-associated-types/issue-87750.rs

@@ -21,6 +21,7 @@ impl<T> Foo for Number<T> {
     // ```
     // which it is :)
     type Item = [T] where [T]: Sized;
+    //~^ ERROR overflow evaluating the requirement `<Number<T> as Foo>::Item == _`
 }
 
 struct OnlySized<T> where T: Sized { f: T }
@@ -40,7 +41,6 @@ impl<T> Bar for T where T: Foo {
     // can use the bound on `Foo::Item` for this, but that requires
     // `wf(<T as Foo>::Item)`, which is an invalid cycle.
     type Assoc = OnlySized<<T as Foo>::Item>;
-    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: Sized`
 }
 
 fn foo<T: Print>() {

src/test/ui/generic-associated-types/issue-87750.stderr

@@ -1,14 +1,8 @@
-error[E0275]: overflow evaluating the requirement `<T as Foo>::Item: Sized`
-  --> $DIR/projection-bound-cycle-generic.rs:42:18
+error[E0275]: overflow evaluating the requirement `<Number<T> as Foo>::Item == _`
+  --> $DIR/projection-bound-cycle-generic.rs:23:5
    |
-LL |     type Assoc = OnlySized<<T as Foo>::Item>;
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `OnlySized`
-  --> $DIR/projection-bound-cycle-generic.rs:26:18
-   |
-LL | struct OnlySized<T> where T: Sized { f: T }
-   |                  ^ required by this bound in `OnlySized`
+LL |     type Item = [T] where [T]: Sized;
+   |     ^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/generic-associated-types/projection-bound-cycle-generic.rs

@@ -24,6 +24,7 @@ impl Foo for Number {
     // ```
     // which it is :)
     type Item = str where str: Sized;
+    //~^ ERROR overflow evaluating the requirement `<Number as Foo>::Item == _`
 }
 
 struct OnlySized<T> where T: Sized { f: T }
@@ -43,7 +44,6 @@ impl<T> Bar for T where T: Foo {
     // can use the bound on `Foo::Item` for this, but that requires
     // `wf(<T as Foo>::Item)`, which is an invalid cycle.
     type Assoc = OnlySized<<T as Foo>::Item>;
-    //~^ ERROR overflow evaluating the requirement `<T as Foo>::Item: Sized`
 }
 
 fn foo<T: Print>() {

src/test/ui/generic-associated-types/projection-bound-cycle-generic.stderr

@@ -1,14 +1,8 @@
-error[E0275]: overflow evaluating the requirement `<T as Foo>::Item: Sized`
-  --> $DIR/projection-bound-cycle.rs:45:18
+error[E0275]: overflow evaluating the requirement `<Number as Foo>::Item == _`
+  --> $DIR/projection-bound-cycle.rs:26:5
    |
-LL |     type Assoc = OnlySized<<T as Foo>::Item>;
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: required by a bound in `OnlySized`
-  --> $DIR/projection-bound-cycle.rs:29:18
-   |
-LL | struct OnlySized<T> where T: Sized { f: T }
-   |                  ^ required by this bound in `OnlySized`
+LL |     type Item = str where str: Sized;
+   |     ^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/generic-associated-types/projection-bound-cycle.rs

@@ -0,0 +1,14 @@
+// check-pass
+struct Node<C: Trait>(C::Assoc::<Self>);
+
+trait Trait {
+    type Assoc<T>;
+}
+
+impl Trait for Vec<()> {
+    type Assoc<T> = Vec<T>;
+}
+
+fn main() {
+    let _ = Node::<Vec<()>>(Vec::new());
+}

src/test/ui/generic-associated-types/projection-bound-cycle.stderr

@@ -0,0 +1,14 @@
+// check-pass
+struct Node<C: Trait<Self>>(C::Assoc);
+
+trait Trait<T> {
+    type Assoc;
+}
+
+impl<T> Trait<T> for Vec<()> {
+    type Assoc = Vec<T>;
+}
+
+fn main() {
+    let _ = Node::<Vec<()>>(Vec::new());
+}

src/test/ui/sized/coinductive-1-gat.rs

@@ -0,0 +1,28 @@
+// run-pass
+struct Node<C: CollectionFactory<Self>> {
+    _children: C::Collection,
+}
+
+trait CollectionFactory<T> {
+    type Collection;
+}
+
+impl<T> CollectionFactory<T> for Vec<()> {
+    type Collection = Vec<T>;
+}
+
+trait Collection<T>: Sized {
+    fn push(&mut self, v: T);
+}
+
+impl<T> Collection<T> for Vec<T> {
+    fn push(&mut self, v: T) {
+        self.push(v)
+    }
+}
+
+fn main() {
+    let _ = Node::<Vec<()>> {
+        _children: Vec::new(),
+    };
+}

src/test/ui/sized/coinductive-1.rs

@@ -0,0 +1,10 @@
+// check-pass
+trait A { type Assoc; }
+
+impl A for () {
+    // FIXME: it would be nice for this to at least cause a warning.
+    type Assoc = Foo<()>;
+}
+struct Foo<T: A>(T::Assoc);
+
+fn main() {}

src/test/ui/sized/coinductive-2.rs

@@ -0,0 +1,13 @@
+// build-fail
+//~^ ERROR cycle detected when computing layout of `Foo<()>`
+
+trait A { type Assoc: ?Sized; }
+
+impl A for () {
+    type Assoc = Foo<()>;
+}
+struct Foo<T: A>(T::Assoc);
+
+fn main() {
+    let x: Foo<()>;
+}

src/test/ui/sized/recursive-type-1.rs

@@ -0,0 +1,13 @@
+error[E0391]: cycle detected when computing layout of `Foo<()>`
+   |
+   = note: ...which requires computing layout of `<() as A>::Assoc`...
+   = note: ...which again requires computing layout of `Foo<()>`, completing the cycle
+note: cycle used when elaborating drops for `main`
+  --> $DIR/recursive-type-2.rs:11:1
+   |
+LL | fn main() {
+   | ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

src/test/ui/sized/recursive-type-2.rs

@@ -1,10 +1,12 @@
+// check-pass
+
 trait A<Y, N> {
     type B;
 }
 
 type MaybeBox<T> = <T as A<T, Box<T>>>::B;
 struct P {
-    t: MaybeBox<P>, //~ ERROR: overflow evaluating the requirement `P: Sized`
+    t: MaybeBox<P>,
 }
 
 impl<Y, N> A<Y, N> for P {