Switch can_eq and can_sub to DefineOpaqueTypes::Yes

They are mostly used in diagnostics anyway

compiler/rustc_infer/src/infer/mod.rs

@@ -843,7 +843,9 @@ pub fn can_sub<T>(&self, param_env: ty::ParamEnv<'tcx>, expected: T, actual: T)
     {
         let origin = &ObligationCause::dummy();
         self.probe(|_| {
-            self.at(origin, param_env).sub(DefineOpaqueTypes::No, expected, actual).is_ok()
+            // We're only answering whether there could be a subtyping relation, and with
+            // opaque types, "there could be one", via registering a hidden type.
+            self.at(origin, param_env).sub(DefineOpaqueTypes::Yes, expected, actual).is_ok()
         })
     }
 
@@ -852,7 +854,9 @@ pub fn can_eq<T>(&self, param_env: ty::ParamEnv<'tcx>, a: T, b: T) -> bool
         T: at::ToTrace<'tcx>,
     {
         let origin = &ObligationCause::dummy();
-        self.probe(|_| self.at(origin, param_env).eq(DefineOpaqueTypes::No, a, b).is_ok())
+        // We're only answering whether the types could be the same, and with
+        // opaque types, "they can be the same", via registering a hidden type.
+        self.probe(|_| self.at(origin, param_env).eq(DefineOpaqueTypes::Yes, a, b).is_ok())
     }
 
     #[instrument(skip(self), level = "debug")]

tests/ui/impl-trait/nested_impl_trait.stderr

@@ -42,20 +42,20 @@ LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |
    = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
+error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfied
   --> $DIR/nested_impl_trait.rs:6:46
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
    |
    = help: the trait `Into<U>` is implemented for `T`
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
 
-error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
+error[E0277]: the trait bound `impl Into<u32>: Into<impl Debug>` is not satisfied
   --> $DIR/nested_impl_trait.rs:19:34
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
-   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`, which is required by `impl Into<u32>: Into<impl Debug>`
+   |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Into<u32>`, which is required by `impl Into<u32>: Into<impl Debug>`
    |
    = help: the trait `Into<U>` is implemented for `T`
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`

tests/ui/self/arbitrary-self-opaque.rs

@@ -0,0 +1,12 @@
+#![feature(type_alias_impl_trait)]
+struct Foo;
+
+type Bar = impl Sized;
+//~^ ERROR unconstrained opaque type
+
+impl Foo {
+    fn foo(self: Bar) {}
+    //~^ ERROR: invalid `self` parameter type: Bar
+}
+
+fn main() {}

tests/ui/self/arbitrary-self-opaque.stderr

@@ -0,0 +1,20 @@
+error: unconstrained opaque type
+  --> $DIR/arbitrary-self-opaque.rs:4:12
+   |
+LL | type Bar = impl Sized;
+   |            ^^^^^^^^^^
+   |
+   = note: `Bar` must be used in combination with a concrete type within the same module
+
+error[E0307]: invalid `self` parameter type: Bar
+  --> $DIR/arbitrary-self-opaque.rs:8:18
+   |
+LL |     fn foo(self: Bar) {}
+   |                  ^^^
+   |
+   = note: type of `self` must be `Self` or a type that dereferences to it
+   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0307`.