Verify trait impls’ signatures before checking dyn compatibility

[nagisa]

Given a code like this:

mod obj_unsafe {
    pub trait Serialize {
        fn serialize<T>(&self, t: T);
    }
}

mod obj_safe {
    pub trait Serialize {
        fn serialize(&self);
    }
}

trait DoSomething {
    fn foo(&self) -> Box<dyn obj_safe::Serialize>;
}

impl DoSomething for () {
    fn foo(&self) -> Box<dyn obj_unsafe::Serialize> {
        unimplemented!()
    }
}

We emit an error like this:

error[E0038]: the trait `obj_unsafe::Serialize` cannot be made into an object
  --> src/lib.rs:18:5
   |
18 |     fn foo(&self) -> Box<dyn obj_unsafe::Serialize> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `obj_unsafe::Serialize` cannot be made into an object
   |
   = note: method `serialize` has generic type parameters

---

This error is something that was encountered by someone who read documentation and saw they needed to implement a method returning Box<dyn Serialize>, An obvious assumption is that this refers to serde::Serialize, so that’s what they specified in their return type when implementing DoSomething.

The error they got does not point them at the problem at all. Sure, Serialize from serde is not object safe, but that is irrelevant, because they are trying to make an object of an entirely wrong trait in the first place.

Instead we should be emitting error[E0053]: method foo has an incompatible type for trait first.

[nagisa]

At least we should emit the incompatible type error in addition to the object safe error.

[estebank]

Current output:

error[[E0038]](https://doc.rust-lang.org/stable/error_codes/E0038.html): the trait `obj_unsafe::Serialize` cannot be made into an object
  --> src/lib.rs:18:26
   |
18 |     fn foo(&self) -> Box<dyn obj_unsafe::Serialize> {
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^ `obj_unsafe::Serialize` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> src/lib.rs:3:12
   |
2  |     pub trait Serialize {
   |               --------- this trait cannot be made into an object...
3  |         fn serialize<T>(&self, t: T);
   |            ^^^^^^^^^ ...because method `serialize` has generic type parameters
   = help: consider moving `serialize` to another trait