`struct_field_align_threshold` doesn't apply when using `..Default::default()`

I have been using the struct_field_align_threshold option to align struct fields and it is great, however one issue I have noticed whilst using it is that the formatting isn't applied when using ..Default::default(). For example, formatting the follow struct has no impact:

StandardMaterial {
    base_color_texture: Some(asset_server.load("textures/card_back.png")),
    cull_mode: Some(Face::Front),
    fog_enabled: false,
    unlit: true,
    alpha_mode: AlphaMode::Blend,
    ..Default::default()
}

I would expect:

StandardMaterial {
    base_color_texture: Some(asset_server.load("textures/card_back.png")),
    cull_mode:          Some(Face::Front),
    fog_enabled:        false,
    unlit:              true,
    alpha_mode:         AlphaMode::Blend,
    ..Default::default()
}

If I remove the ..Default::default() line, rustfmt will align the fields as expected:

StandardMaterial {
    base_color_texture: Some(asset_server.load("textures/card_back.png")),
    cull_mode:          Some(Face::Front),
    fog_enabled:        false,
    unlit:              true,
    alpha_mode:         AlphaMode::Blend,
}

Start by reproducing the provided struct formatting example in rustfmt and tracing the struct-field alignment handling for expressions using ..Default::default(). Done means the configured struct_field_align_threshold aligns the explicit fields in this case, with a regression test covering the example.