mapper 1.0.1

derive(Mapper)
Documentation 
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struct Args {
    vars: HashSet<Ident>,
}

impl Args {
    fn should_print_expr(&self, e: &Expr) -> bool {
        if let Expr::Path(ref e) = e {
            if e.path.leading_colon.is_some() {
                false
            } else if e.path.segments.len() != 1 {
                false
            } else {
                let first = e.path.segments.first().unwrap();
                self.vars.contains(&first.ident) && first.arguments.is_empty()
            }
        } else {
            false
        }
    }

    fn should_print_pat(&self, p: &Pat) -> bool {
        if let Pat::Ident(ref p) = p {
            self.vars.contains(&p.ident)
        } else {
            false
        }
    }

    fn assign_and_print(&mut self, left: Expr, op: &dyn ToTokens, right: Expr) -> Expr {
        let right = fold::fold_expr(self, right);
        parse_quote!({
            println!(concat!(stringify!(#left), " = {:?}"), #left);
        })
    }

    fn let_and_print(&mut self, local: Local) -> Stmt {
        let Local { pat, init, .. } = local;
        let init = self.fold_expr(*init.unwrap().1);
        // get the variable name of assigned variable
        let ident = match pat {
            Pat::Ident(ref p) => &p.ident,
            _ => unreachable!(),
        };
        // new sub tree
        parse_quote! {
            let #pat = {
                #[allow(unused_mut)]
                let #pat = #init;
                println!(concat!(stringify!(#ident), " = {:?}"), #ident);
                #ident
            };
        }
    }
}

impl Fold for Args {
    fn fold_expr(&mut self, e: Expr) -> Expr {
        match e {
            Expr::Assign(e) => {
                if self.should_print_expr(&e.left) {
                    self.assign_and_print(*e.left, &e.eq_token, *e.right)
                } else {
                    Expr::Assign(fold::fold_expr_assign(self, e))
                }
            }
            Expr::AssignOp(e) => {
                if self.should_print_expr(&e.left) {
                    self.assign_and_print(*e.left, &e.op, *e.right)
                } else {
                    Expr::AssignOp(fold::fold_expr_assign_op(self, e))
                }
            }
            _ => fold::fold_expr(self, e),
        }
    }

    fn fold_stmt(&mut self, s: Stmt) -> Stmt {
        match s {
            Stmt::Local(s) => {
                if s.init.is_some() && self.should_print_pat(&s.pat) {
                    self.let_and_print( s)
                } else {
                    Stmt::Local(fold::fold_local(self, s))
                }
            }
            _ => fold::fold_stmt(self, s)
        }
    }
}
impl Parse for Args {
    fn parse(input: ParseStream) -> Result<Self> {
        // parses a,b,c, or a,b,c where a,b and c are Indent
        let vars = Punctuated::<Ident, Token![,]>::parse_terminated(input)?;
        Ok(Args {
            vars: vars.into_iter().collect(),
        })
    }
}

#[proc_macro_attribute]
pub fn trace_vars(metadata: TokenStream, input: TokenStream) -> TokenStream {
    // parsing rust function to easy to use struct
    let input_fn = parse_macro_input!(input as ItemFn);
    let mut args = parse_macro_input!(metadata as Args);
    let output = args.fold_item_fn(input_fn);
    TokenStream::from(quote! {#output})
}