Module syn::visit_mut

source ·
Available on crate feature visit-mut only.
Expand description

Syntax tree traversal to mutate an exclusive borrow of a syntax tree in place.

Each method of the VisitMut trait is a hook that can be overridden to customize the behavior when mutating the corresponding type of node. By default, every method recursively visits the substructure of the input by invoking the right visitor method of each of its fields.

pub trait VisitMut {
    /* ... */

    fn visit_expr_binary_mut(&mut self, node: &mut ExprBinary) {
        visit_expr_binary_mut(self, node);

    /* ... */

pub fn visit_expr_binary_mut<V>(v: &mut V, node: &mut ExprBinary)
    V: VisitMut + ?Sized,
    for attr in &mut node.attrs {
    v.visit_expr_mut(&mut *node.left);
    v.visit_bin_op_mut(&mut node.op);
    v.visit_expr_mut(&mut *node.right);

/* ... */

This module is available only if Syn is built with the "visit-mut" feature.


This mut visitor replace occurrences of u256 suffixed integer literals like 999u256 with a macro invocation bigint::u256!(999).

// [dependencies]
// quote = "1.0"
// syn = { version = "1.0", features = ["full", "visit-mut"] }

use quote::quote;
use syn::visit_mut::{self, VisitMut};
use syn::{parse_quote, Expr, File, Lit, LitInt};

struct BigintReplace;

impl VisitMut for BigintReplace {
    fn visit_expr_mut(&mut self, node: &mut Expr) {
        if let Expr::Lit(expr) = &node {
            if let Lit::Int(int) = &expr.lit {
                if int.suffix() == "u256" {
                    let digits = int.base10_digits();
                    let unsuffixed: LitInt = syn::parse_str(digits).unwrap();
                    *node = parse_quote!(bigint::u256!(#unsuffixed));

        // Delegate to the default impl to visit nested expressions.
        visit_mut::visit_expr_mut(self, node);

fn main() {
    let code = quote! {
        fn main() {
            let _ = 999u256;

    let mut syntax_tree: File = syn::parse2(code).unwrap();
    BigintReplace.visit_file_mut(&mut syntax_tree);
    println!("{}", quote!(#syntax_tree));


  • Syntax tree traversal to mutate an exclusive borrow of a syntax tree in place.