My PR to introduce double borders and curved corners for styling widget blocks in tui-rs was just merged into master. Not sure when a new release is planned though.
Example usage:
Block::default()
.title("With styled and double borders")
.border_style(Style::default().fg(Color::Cyan))
.borders(Borders::LEFT | Borders::RIGHT)
.border_type(BorderType::Double)
and for rounded corners:
Block::default()
.title("With rounded corners")
.border_style(Style::default().fg(Color::Cyan))
.borders(Borders::LEFT | Borders::RIGHT)
.border_type(BorderType::Rounded)
Because there are no unicode characters for double arcs, the doubled border and rounded corners are mutually exclusive.