Function filter_newlines 

pub fn filter_newlines(tokens: Vec<Token>) -> Vec<Token>

Drop the line breaks that don’t end a statement.

A line break is kept only where a statement could end. It is dropped:

• inside ( ) and [ ] (but not inside a { } block nested in them);
• after a token that can’t end an expression, such as a binary operator, ,, => or an opening bracket;
• before a line starting with . (method chains);
• when it repeats a previous line break, or starts the file.