bevy_reflect: Default attribute paths

[MrGVSV]

Objective

Followup to #9322 but with a breaking change.

With syn2, it's a whole lot easier to customize attributes. One small improvement we could make would be to change the default attribute to take a type path directly instead of a stringified path.

Solution

Updated the parsing logic to take an ExprPath directly.

---

Changelog

• Default functions passed to #[reflect(default)] no longer need to be string literals (i.e. they can specify the path directly like #[reflect(default = path::to::func)])

Migration Guide

The optional function passed to #[reflect(default)] is no longer a stringified path. It can now be a standard type path:

mod defaults {
  fn foo() -> i32 {
    123
  }
}

// BEFORE
#[derive(Reflect)]
struct Foo {
  #[reflect(default = "defaults::foo")]
  value: i32
}

// AFTER
#[derive(Reflect)]
struct Foo {
  #[reflect(default = defaults::foo)]
  value: i32
}

[killercup]

This should also allow autocomplete to work in these cases, right?

[MrGVSV]

This should also allow autocomplete to work in these cases, right?

Ideally yes, but I can't speak for all editors.