Function insert_toml_item 

fn insert_toml_item(
    parent: &mut dyn TableLike,
    is_inline: bool,
    key: &str,
    item: Item,
)

Inserts a new item at key into parent.

Inline tables can only hold values. There, the new value takes over the whitespace or comment that followed the previous last value, so the inline table keeps its layout.