Add note when inherent impl for a alias type defined outside of the crate

[xizheyin]

Fixes #141679

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[BoxyUwU]

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[xizheyin]

@rustbot ready

[xizheyin]

It seems difficult to apply structured suggestions here, because not only do we have to implement a new type, we also need to implement it with impl. Maybe we can use a clearer explanation:

help: consider defining a trait and implementing it for the type or using a newtype wrapper like `struct MyType(ExternalType);` and implement it.
Note: For more details about orphan rules, see <https://doc.rust-lang.org/reference/items/implementations.html?highlight=orphan#orphan-rules>

[fee1-dead]

do you not get TyKind::Alias when you use type_of on the impl? coherence is probably fine to use middle queries

[xizheyin]

I've tried using type_of before, but I don't seem to be getting the information I'm looking for. When I try this code

            let ty = self.tcx.type_of(impl_def_id).skip_binder();
            if let ty::Adt(def, _) = ty.kind() {
                let def_id = def.did();
                let ty_name = self.tcx.def_path_str(def_id);
                let alias_ty_name = self.tcx.type_of(def_id).skip_binder().to_string();
                println!("ty_name: {:?}", ty_name);
                println!("alias_ty_name: {:?}", alias_ty_name);
            }

it prints:

ty_name: "Rc"
alias_ty_name: "Rc<T, A>"

It should be:

ty_name: "Function"
alias_ty_name: "Rc<Foo>"

Local variable ty is a adt of type Rc. Maybe the information we need only exists in HIR? Or am I using it wrong?

[xizheyin]

I merged the two ifs.

[compiler-errors]

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