Function apply_fields 

fn apply_fields(
    content: RoomMemberEventContent,
    profile_values: &[(ProfileFieldName, Option<Value>)],
    unchanged: bool,
    current_displayname: Option<&str>,
    current_avatar_url: Option<&MxcUri>,
) -> Option<RoomMemberEventContent>

Lays a profile write over a room’s member content.

Returns the content only when a field differs from what the room holds, so a write restoring the stored value emits no member event. Under Propagation::Unchanged a room whose value departs from the user’s prior global value keeps its override.