bevy_reflect: TypeInfo casting methods

[MrGVSV]

Objective

There are times when we might know the type of a TypeInfo ahead of time. Or we may have already checked it one way or another.

In such cases, it's a bit cumbersome to have to pattern match every time we want to access the nested info:

if let TypeInfo::List(info) = <Vec<i32>>::type_info() {
  // ...
} else {
  panic!("expected list info");
}

Ideally, there would be a way to simply perform the cast down to ListInfo since we already know it will succeed.

Or even if we don't, perhaps we just want a cleaner way of exiting a function early (i.e. with the ? operator).

Solution

Taking a bit from mirror-mirror, TypeInfo now has methods for attempting a cast into the variant's info type.

let info = <Vec<i32>>::type_info().as_list().unwrap();
// ...

These new conversion methods return a Result where the error type is a new TypeInfoError enum.

A Result was chosen as the return type over Option because if we do choose to unwrap it, the error message will give us some indication of what went wrong. In other words, it can truly replace those instances where we were panicking in the else case.

Open Questions

1. Should the error types instead be a struct? I chose an enum for future-proofing, but right now it only has one error state. Alternatively, we could make it a reflect-wide casting error so it could be used for similar methods on ReflectRef and friends.
2. I was going to do it in a separate PR but should I just go ahead and add similar methods to ReflectRef, ReflectMut, and ReflectOwned? 🤔
3. Should we name these try_as_*** instead of as_*** since they return a Result?

Testing

You can test locally by running:

cargo test --package bevy_reflect

---

Changelog

Added

• TypeInfoError enum
• TypeInfo::kind method
• TypeInfo::as_struct method
• TypeInfo::as_tuple_struct method
• TypeInfo::as_tuple method
• TypeInfo::as_list method
• TypeInfo::as_array method
• TypeInfo::as_map method
• TypeInfo::as_enum method
• TypeInfo::as_value method
• VariantInfoError enum
• VariantInfo::variant_type method
• VariantInfo::as_unit_variant method
• VariantInfo::as_tuple_variant method
• VariantInfo::as_struct_variant method

[mweatherley]

Looks good! I don't have strong opinions about how exactly the errors are organized, but I think that the current scope of this PR is good :)