bevy_reflect: streamline generated FromReflect::from_reflect

[nnethercote]

Objective

Generated from_reflect methods use closures in a weird way, e.g.:

    x: (|| {
        <f32 as ::bevy::reflect::FromReflect>::from_reflect(
            ::bevy::reflect::Struct::field(__ref_struct, "x")?,
        )   
    })()?,

The reason for this is because when #[reflect(Default)] is used, you instead get stuff like this:

    if let ::core::option::Option::Some(__field) = (|| {
        <f32 as ::bevy::reflect::FromReflect>::from_reflect(
            ::bevy::reflect::Struct::field(__ref_struct, "x")?,
        )   
    })() {
        __this.x = __field;
    } 

and the closure is necessary to contain the scope of the ?. But the first case is more common.

Helps with #19873.

Solution

Avoid the closure in the common case.

Testing

I used cargo expand to confirm the closures are no longer produced in the common case.

-Zmacro-stats output tells me this reduces the size of the Reflect code produced for bevy_ui by 0.5%.