Template trait

Description

Currently TemplateOnce::render_once method consumes the object itself and not useful if you want to re-use the struct.

Template trait helps those situation. Template trait has render() method, which does not consume the object itself.

Like TemplateOnce, Template trait can be implemented using derive macro.

pub trait Template {
    fn render(&self) -> RenderResult;
}

Since RenderError can be converted into fmt::Error, we can now implement Display trait for those structs.

impl<T: Template> Display for T {
    ...
}

If you derive this trait, you cannot move out the struct fields. For example, the following template

<% for msg in messages { %>
<div><%\html msg %></div>
<% } %>

will be transformed into the Rust code like

for msg in self.messages {
    render_text!(_ctx, "<div>");
    render!(_ctx, msg);
    render_text!(_ctx, "</div>");
}

which causes an compilation error because self.messages cannot be moved.