Add lint for possibly incorrect type_id usage in reflection

[MrGVSV]

Problem

Reflection relies on TypeId for a lot of things. And to get this TypeId it seems sensible to do something like value.type_id(). However, in a majority of cases within reflection code, this is not what you want.

A lot of times, value isn't a concrete type, but a type-erased dyn Reflect trait object. This means the TypeId returned by value.type_id() is actually the TypeId of the trait object, not the underlying type.

Instead, what you really want is to get the TypeId from the associated TypeInfo. This can be done either by calling and unwrapping value.get_represented_type_info() or, in 0.15, calling value.reflect_type_info() (assuming value is a dyn Reflect and not a dyn PartialReflect). Then with the TypeInfo you can call info.type_id().

In other words, this is wrong:

let value: Box<dyn Reflect> = Box::new(123u32);
let type_id = value.type_id();
assert_eq!(type_id, TypeId::of::<u32>());

This is right:

let value: Box<dyn Reflect> = Box::new(123u32);
let type_id = value.get_represented_type_info().unwrap().type_id();
assert_eq!(type_id, TypeId::of::<u32>());

Solution

We should add a lint to warn against possibly incorrect usages of std::any::Any::type_id when the type of the receiver is dyn Reflect (or dyn PartialReflect starting in 0.15).

It should point out the potential footgun with using std::any::Any::type_id and point the user to an appropriate alternative: Reflect::get_represented_type_info (or Reflect::reflect_type_info/PartialReflect::get_represented_type_info for 0.15).

Possible name: reflect_object_type_id?

Possible group: SUSPICIOUS