Method drop

regex/regex-syntax/src/ast/mod.rs:1368–1412

(&mut self)

/// space but heap space proportional to the depth of the `Ast`.
impl Drop for Ast {
    fn drop(&mut self) {
        use std::mem;

        match *self {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_)
            // Classes are recursive, so they get their own Drop impl.
            | Ast::Class(_) => return,
            Ast::Repetition(ref x) if !x.ast.has_subexprs() => return,
            Ast::Group(ref x) if !x.ast.has_subexprs() => return,
            Ast::Alternation(ref x) if x.asts.is_empty() => return,
            Ast::Concat(ref x) if x.asts.is_empty() => return,
            _ => {}
        }

        let empty_span = || Span::splat(Position::new(0, 0, 0));
        let empty_ast = || Ast::Empty(empty_span());
        let mut stack = vec![mem::replace(self, empty_ast())];
        while let Some(mut ast) = stack.pop() {
            match ast {
                Ast::Empty(_)
                | Ast::Flags(_)
                | Ast::Literal(_)
                | Ast::Dot(_)
                | Ast::Assertion(_)
                // Classes are recursive, so they get their own Drop impl.
                | Ast::Class(_) => {}
                Ast::Repetition(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Group(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Alternation(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
                Ast::Concat(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
            }
        }
    }
}

/// A custom `Drop` impl is used for `ClassSet` such that it uses constant