Rollup merge of #63934 - Aaron1011:fix/impl-trait-coherence, r=nikomatsakis

Fix coherence checking for impl trait in type aliases

**UPDATE**: This PR now treats all opaque types as remote. The original description appears below, but is no longer accurate.

Fixes #63677

[RFC 2071](https://github.com/rust-lang/rfcs/pull/2071) (impl-trait-existential-types) does not explicitly state how `type_alias_impl_trait` should interact with coherence. However, there's only one choice which makes sense - coherence should look at the underlying type (i.e. the *"defining"* type of the `impl Trait`) of the type alias, just like we do for non-`impl Trait` type aliases.

Specifically, `impl Trait` type aliases that resolve to a local type should be treated like a local type with respect to coherence (e.g. `impl Trait` type aliases which resolve to a foreign type should be treated as a foreign type, and those that resolve to a local type should be treated as a local type).

Since neither inherent impls nor direct trait impl (i.e. `impl MyType` or `impl MyTrait for MyType`) are allowed for type aliases, this usually does not come up. Before we ever attempt to do coherence checking, we will have errored out if an `impl Trait` type alias was used directly in an `impl` clause.

However, during trait selection, we sometimes need to prove bounds like `T: Sized` for some type `T`. If `T` is an impl trait type alias, this requires to know the coherence behavior for `impl Trait` type aliases when we perform coherence checking.

Note: Since determining the underlying type of an `impl Trait` type alias requires us to perform body type checking, this commit causes us to type check some bodies easier than we otherwise would have. However, since this is done through a query, this shouldn't cause any problems

For completeness, I've added an additional test of the coherence-related behavior of `impl Trait` type aliases.

cc https://github.com/rust-lang/rust/issues/63063

src/librustc/traits/coherence.rs

@@ -432,7 +432,7 @@ fn orphan_check_trait_ref<'tcx>(
 }
 
 fn uncovered_tys<'tcx>(tcx: TyCtxt<'_>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> {
-    if ty_is_local_constructor(ty, in_crate) {
+    if ty_is_local_constructor(tcx, ty, in_crate) {
         vec![]
     } else if fundamental_ty(ty) {
         ty.walk_shallow()
@@ -451,7 +451,7 @@ fn is_possibly_remote_type(ty: Ty<'_>, _in_crate: InCrate) -> bool {
 }
 
 fn ty_is_local(tcx: TyCtxt<'_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
-    ty_is_local_constructor(ty, in_crate) ||
+    ty_is_local_constructor(tcx, ty, in_crate) ||
         fundamental_ty(ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
 }
 
@@ -472,7 +472,7 @@ fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
     }
 }
 
-fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
+fn ty_is_local_constructor(tcx: TyCtxt<'_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
     debug!("ty_is_local_constructor({:?})", ty);
 
     match ty.sty {
@@ -504,6 +504,15 @@ fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
 
         ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
         ty::Foreign(did) => def_id_is_local(did, in_crate),
+        ty::Opaque(did, _) => {
+            // Check the underlying type that this opaque
+            // type resolves to.
+            // This recursion will eventually terminate,
+            // since we've already managed to successfully
+            // resolve all opaque types by this point
+            let real_ty = tcx.type_of(did);
+            ty_is_local_constructor(tcx, real_ty, in_crate)
+        }
 
         ty::Dynamic(ref tt, ..) => {
             if let Some(principal) = tt.principal() {
@@ -518,8 +527,7 @@ fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
         ty::UnnormalizedProjection(..) |
         ty::Closure(..) |
         ty::Generator(..) |
-        ty::GeneratorWitness(..) |
-        ty::Opaque(..) => {
+        ty::GeneratorWitness(..) => {
             bug!("ty_is_local invoked on unexpected type: {:?}", ty)
         }
     }

src/librustc/traits/specialize/specialization_graph.rs

@@ -395,7 +395,7 @@ impl<'tcx> Graph {
     /// The parent of a given impl, which is the `DefId` of the trait when the
     /// impl is a "specialization root".
     pub fn parent(&self, child: DefId) -> DefId {
-        *self.parent.get(&child).unwrap()
+        *self.parent.get(&child).unwrap_or_else(|| panic!("Failed to get parent for {:?}", child))
     }
 }
 

src/test/ui/impl-trait/auto-trait.rs

@@ -0,0 +1,23 @@
+// Tests that type alias impls traits do not leak auto-traits for
+// the purposes of coherence checking
+#![feature(type_alias_impl_trait)]
+
+trait OpaqueTrait { }
+impl<T> OpaqueTrait for T { }
+type OpaqueType = impl OpaqueTrait;
+fn mk_opaque() -> OpaqueType { () }
+
+#[derive(Debug)]
+struct D<T>(T);
+
+trait AnotherTrait { }
+impl<T: Send> AnotherTrait for T { }
+
+// This is in error, because we cannot assume that `OpaqueType: !Send`.
+// (We treat opaque types as "foreign types" that could grow more impls
+// in the future.)
+impl AnotherTrait for D<OpaqueType> {
+    //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
+}
+
+fn main() {}

src/test/ui/impl-trait/auto-trait.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`:
+  --> $DIR/auto-trait.rs:19:1
+   |
+LL | impl<T: Send> AnotherTrait for T { }
+   | -------------------------------- first implementation here
+...
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

src/test/ui/impl-trait/negative-reasoning.rs

@@ -0,0 +1,22 @@
+// Tests that we cannot assume that an opaque type does *not* implement some
+// other trait
+#![feature(type_alias_impl_trait)]
+
+trait OpaqueTrait { }
+impl<T> OpaqueTrait for T { }
+type OpaqueType = impl OpaqueTrait;
+fn mk_opaque() -> OpaqueType { () }
+
+#[derive(Debug)]
+struct D<T>(T);
+
+trait AnotherTrait { }
+impl<T: std::fmt::Debug> AnotherTrait for T { }
+
+
+// This is in error, because we cannot assume that `OpaqueType: !Debug`
+impl AnotherTrait for D<OpaqueType> {
+    //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
+}
+
+fn main() {}

src/test/ui/impl-trait/negative-reasoning.stderr

@@ -0,0 +1,14 @@
+error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`:
+  --> $DIR/negative-reasoning.rs:18:1
+   |
+LL | impl<T: std::fmt::Debug> AnotherTrait for T { }
+   | ------------------------------------------- first implementation here
+...
+LL | impl AnotherTrait for D<OpaqueType> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
+   |
+   = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

src/test/ui/type-alias-impl-trait/auxiliary/foreign-crate.rs

@@ -0,0 +1,2 @@
+pub trait ForeignTrait {}
+pub struct ForeignType<T>(pub T);

src/test/ui/type-alias-impl-trait/coherence.rs

@@ -0,0 +1,17 @@
+// aux-build:foreign-crate.rs
+#![feature(type_alias_impl_trait)]
+
+extern crate foreign_crate;
+
+trait LocalTrait {}
+impl<T> LocalTrait for foreign_crate::ForeignType<T> {}
+
+type AliasOfForeignType<T> = impl LocalTrait;
+fn use_alias<T>(val: T) -> AliasOfForeignType<T> {
+    foreign_crate::ForeignType(val)
+}
+
+impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
+//~^ ERROR the type parameter `T` is not constrained by the impl trait, self type, or predicates
+
+fn main() {}

src/test/ui/type-alias-impl-trait/coherence.stderr

@@ -0,0 +1,9 @@
+error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/coherence.rs:14:6
+   |
+LL | impl<T> foreign_crate::ForeignTrait for AliasOfForeignType<T> {}
+   |      ^ unconstrained type parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0207`.

src/test/ui/type-alias-impl-trait/issue-63677-type-alias-coherence.rs

@@ -0,0 +1,21 @@
+// check-pass
+// Regression test for issue #63677 - ensure that
+// coherence checking can properly handle 'impl trait'
+// in type aliases
+#![feature(type_alias_impl_trait)]
+
+pub trait Trait {}
+pub struct S1<T>(T);
+pub struct S2<T>(T);
+
+pub type T1 = impl Trait;
+pub type T2 = S1<T1>;
+pub type T3 = S2<T2>;
+
+impl<T> Trait for S1<T> {}
+impl<T: Trait> S2<T> {}
+impl T3 {}
+
+pub fn use_t1() -> T1 { S1(()) }
+
+fn main() {}