Function walk_expr_mut 

pub fn walk_expr_mut<V: ExprVisitorMut>(
    visitor: &mut V,
    expr: &mut Expr,
) -> Result<bool, V::Error>

A util function that traverses an AST Expr mutably in depth-first order. Returns Ok(true) if all nodes were visited, and Ok(false) if any call to pre_visit or post_visit returned Ok(false) and may have cut short the recursion.