prevent opaque types from appearing in impl headers

compiler/rustc_typeck/src/coherence/orphan.rs

@@ -7,6 +7,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_infer::infer::TyCtxtInferExt;
+use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
 use rustc_middle::ty::{self, ImplPolarity, Ty, TyCtxt, TypeFoldable, TypeVisitor};
 use rustc_session::lint;
@@ -141,13 +142,56 @@ fn orphan_check_impl(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGua
         }
     }
 
-    if let ty::Opaque(def_id, _) = *trait_ref.self_ty().kind() {
-        let reported = tcx
-            .sess
-            .struct_span_err(sp, "cannot implement trait on type alias impl trait")
-            .span_note(tcx.def_span(def_id), "type alias impl trait defined here")
-            .emit();
-        return Err(reported);
+    // Ensure no opaque types are present in this impl header. See issues #76202 and #86411 for examples,
+    // and #84660 where it would otherwise allow unsoundness.
+    if trait_ref.has_opaque_types() {
+        trace!("{:#?}", item);
+        // First we find the opaque type in question.
+        for ty in trait_ref.substs {
+            for ty in ty.walk() {
+                let ty::subst::GenericArgKind::Type(ty) = ty.unpack() else { continue };
+                let ty::Opaque(def_id, _) = *ty.kind() else { continue };
+                trace!(?def_id);
+
+                // Then we search for mentions of the opaque type's type alias in the HIR
+                struct SpanFinder<'tcx> {
+                    sp: Span,
+                    def_id: DefId,
+                    tcx: TyCtxt<'tcx>,
+                }
+                impl<'v, 'tcx> hir::intravisit::Visitor<'v> for SpanFinder<'tcx> {
+                    #[instrument(level = "trace", skip(self, _id))]
+                    fn visit_path(&mut self, path: &'v hir::Path<'v>, _id: hir::HirId) {
+                        // You can't mention an opaque type directly, so we look for type aliases
+                        if let hir::def::Res::Def(hir::def::DefKind::TyAlias, def_id) = path.res {
+                            // And check if that type alias's type contains the opaque type we're looking for
+                            for arg in self.tcx.type_of(def_id).walk() {
+                                if let GenericArgKind::Type(ty) = arg.unpack() {
+                                    if let ty::Opaque(def_id, _) = *ty.kind() {
+                                        if def_id == self.def_id {
+                                            // Finally we update the span to the mention of the type alias
+                                            self.sp = path.span;
+                                            return;
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                        hir::intravisit::walk_path(self, path)
+                    }
+                }
+
+                let mut visitor = SpanFinder { sp, def_id, tcx };
+                hir::intravisit::walk_item(&mut visitor, item);
+                let reported = tcx
+                    .sess
+                    .struct_span_err(visitor.sp, "cannot implement trait on type alias impl trait")
+                    .span_note(tcx.def_span(def_id), "type alias impl trait defined here")
+                    .emit();
+                return Err(reported);
+            }
+        }
+        span_bug!(sp, "opque type not found, but `has_opaque_types` is set")
     }
 
     Ok(())

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

@@ -20,6 +20,7 @@ impl<T: Send> AnotherTrait for T {}
 // in the future.)
 impl AnotherTrait for D<OpaqueType> {
     //~^ ERROR conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
+    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}

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

@@ -1,3 +1,15 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/auto-trait.rs:21:25
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   |                         ^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/auto-trait.rs:7:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
 error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
   --> $DIR/auto-trait.rs:21:1
    |
@@ -7,6 +19,6 @@ LL | impl<T: Send> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.

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

@@ -18,6 +18,7 @@ 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>`
+    //~| ERROR cannot implement trait on type alias impl trait
 }
 
 fn main() {}

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

@@ -1,3 +1,15 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/negative-reasoning.rs:19:25
+   |
+LL | impl AnotherTrait for D<OpaqueType> {
+   |                         ^^^^^^^^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/negative-reasoning.rs:7:19
+   |
+LL | type OpaqueType = impl OpaqueTrait;
+   |                   ^^^^^^^^^^^^^^^^
+
 error[E0119]: conflicting implementations of trait `AnotherTrait` for type `D<OpaqueType>`
   --> $DIR/negative-reasoning.rs:19:1
    |
@@ -9,6 +21,6 @@ LL | impl AnotherTrait 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
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs

@@ -5,13 +5,14 @@ type Foo = impl PartialEq<(Foo, i32)>;
 struct Bar;
 
 impl PartialEq<(Foo, i32)> for Bar {
+//~^ ERROR cannot implement trait on type alias impl trait
     fn eq(&self, _other: &(Foo, i32)) -> bool {
         true
     }
 }
 
 fn foo() -> Foo {
-    Bar //~ ERROR can't compare `Bar` with `(Bar, i32)`
+    Bar
 }
 
 fn main() {}

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.stderr

@@ -1,12 +1,14 @@
-error[E0277]: can't compare `Bar` with `(Bar, i32)`
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:14:5
+error: cannot implement trait on type alias impl trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:7:17
    |
-LL |     Bar
-   |     ^^^ no implementation for `Bar == (Bar, i32)`
+LL | impl PartialEq<(Foo, i32)> for Bar {
+   |                 ^^^
    |
-   = help: the trait `PartialEq<(Bar, i32)>` is not implemented for `Bar`
-   = help: the trait `PartialEq<(Foo, i32)>` is implemented for `Bar`
+note: type alias impl trait defined here
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs:3:12
+   |
+LL | type Foo = impl PartialEq<(Foo, i32)>;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.rs

@@ -2,7 +2,6 @@
 
 mod a {
     type Foo = impl PartialEq<(Foo, i32)>;
-    //~^ ERROR unconstrained opaque type
 
     struct Bar;
 
@@ -15,13 +14,12 @@ mod a {
 
 mod b {
     type Foo = impl PartialEq<(Foo, i32)>;
-    //~^ ERROR unconstrained opaque type
 
     struct Bar;
 
     impl PartialEq<(Foo, i32)> for Bar {
+        //~^ ERROR cannot implement trait on type alias impl trait
         fn eq(&self, _other: &(Bar, i32)) -> bool {
-            //~^ ERROR impl has stricter requirements than trait
             true
         }
     }

src/test/ui/impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle.stderr

@@ -1,25 +1,14 @@
-error: unconstrained opaque type
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:4:16
+error: cannot implement trait on type alias impl trait
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:20:21
    |
-LL |     type Foo = impl PartialEq<(Foo, i32)>;
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     impl PartialEq<(Foo, i32)> for Bar {
+   |                     ^^^
    |
-   = note: `Foo` must be used in combination with a concrete type within the same module
-
-error: unconstrained opaque type
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:17:16
+note: type alias impl trait defined here
+  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:16:16
    |
 LL |     type Foo = impl PartialEq<(Foo, i32)>;
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: `Foo` must be used in combination with a concrete type within the same module
-
-error[E0276]: impl has stricter requirements than trait
-  --> $DIR/recursive-type-alias-impl-trait-declaration-too-subtle.rs:23:9
-   |
-LL |         fn eq(&self, _other: &(Bar, i32)) -> bool {
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `b::Bar: PartialEq<(b::Bar, i32)>`
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0276`.

src/test/ui/traits/alias/issue-83613.stderr

@@ -1,8 +1,8 @@
 error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-83613.rs:10:1
+  --> $DIR/issue-83613.rs:10:23
    |
 LL | impl AnotherTrait for OpaqueType {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                       ^^^^^^^^^^
    |
 note: type alias impl trait defined here
   --> $DIR/issue-83613.rs:4:19

src/test/ui/type-alias-impl-trait/issue-65384.stderr

@@ -1,8 +1,8 @@
 error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-65384.rs:10:1
+  --> $DIR/issue-65384.rs:10:18
    |
 LL | impl MyTrait for Bar {}
-   | ^^^^^^^^^^^^^^^^^^^^
+   |                  ^^^
    |
 note: type alias impl trait defined here
   --> $DIR/issue-65384.rs:8:12

src/test/ui/type-alias-impl-trait/issue-76202-trait-impl-for-tait.stderr

@@ -1,8 +1,8 @@
 error: cannot implement trait on type alias impl trait
-  --> $DIR/issue-76202-trait-impl-for-tait.rs:16:1
+  --> $DIR/issue-76202-trait-impl-for-tait.rs:16:15
    |
 LL | impl Test for F {
-   | ^^^^^^^^^^^^^^^
+   |               ^
    |
 note: type alias impl trait defined here
   --> $DIR/issue-76202-trait-impl-for-tait.rs:9:10

src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.rs

@@ -0,0 +1,23 @@
+// Regression test for issues #84660 and #86411: both are variations on #76202.
+// Tests that we don't ICE when we have an opaque type appearing anywhere in an impl header.
+
+#![feature(type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+type Bar = impl Foo;
+fn _defining_use() -> Bar {}
+
+trait TraitArg<T> {
+    fn f();
+}
+
+impl TraitArg<Bar> for () { //~ ERROR cannot implement trait
+    fn f() {
+        println!("ho");
+    }
+}
+
+fn main() {
+    <() as TraitArg<Bar>>::f();
+}

src/test/ui/type-alias-impl-trait/issue-84660-trait-impl-for-tait.stderr

@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/issue-84660-trait-impl-for-tait.rs:15:15
+   |
+LL | impl TraitArg<Bar> for () {
+   |               ^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/issue-84660-trait-impl-for-tait.rs:8:12
+   |
+LL | type Bar = impl Foo;
+   |            ^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.rs

@@ -0,0 +1,41 @@
+// Another example from issue #84660, this time weaponized as a safe transmut: an opaque type in an
+// impl header being accepted was used to create unsoundness.
+
+#![feature(type_alias_impl_trait)]
+
+trait Foo {}
+impl Foo for () {}
+type Bar = impl Foo;
+fn _defining_use() -> Bar {}
+
+trait Trait<T, In> {
+    type Out;
+    fn convert(i: In) -> Self::Out;
+}
+
+impl<In, Out> Trait<Bar, In> for Out { //~ ERROR cannot implement trait
+    type Out = Out;
+    fn convert(_i: In) -> Self::Out {
+        unreachable!();
+    }
+}
+
+impl<In, Out> Trait<(), In> for Out {
+    type Out = In;
+    fn convert(i: In) -> Self::Out {
+        i
+    }
+}
+
+fn transmute<In, Out>(i: In) -> Out {
+    <Out as Trait<Bar, In>>::convert(i)
+}
+
+fn main() {
+    let d;
+    {
+        let x = "Hello World".to_string();
+        d = transmute::<&String, &String>(&x);
+    }
+    println!("{}", d);
+}

src/test/ui/type-alias-impl-trait/issue-84660-unsoundness.stderr

@@ -0,0 +1,14 @@
+error: cannot implement trait on type alias impl trait
+  --> $DIR/issue-84660-unsoundness.rs:16:21
+   |
+LL | impl<In, Out> Trait<Bar, In> for Out {
+   |                     ^^^
+   |
+note: type alias impl trait defined here
+  --> $DIR/issue-84660-unsoundness.rs:8:12
+   |
+LL | type Bar = impl Foo;
+   |            ^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/type-alias-impl-trait/nested-tait-inference3.rs

@@ -4,11 +4,11 @@
 use std::fmt::Debug;
 
 type FooX = impl Debug;
-//~^ unconstrained opaque type
 
 trait Foo<A> { }
 
 impl Foo<FooX> for () { }
+//~^ cannot implement trait on type alias impl trait
 
 fn foo() -> impl Foo<FooX> {
     ()

src/test/ui/type-alias-impl-trait/nested-tait-inference3.stderr

@@ -1,10 +1,14 @@
-error: unconstrained opaque type
+error: cannot implement trait on type alias impl trait
+  --> $DIR/nested-tait-inference3.rs:10:10
+   |
+LL | impl Foo<FooX> for () { }
+   |          ^^^^
+   |
+note: type alias impl trait defined here
   --> $DIR/nested-tait-inference3.rs:6:13
    |
 LL | type FooX = impl Debug;
    |             ^^^^^^^^^^
-   |
-   = note: `FooX` must be used in combination with a concrete type within the same module
 
 error: aborting due to previous error