Limits - The Rust Reference

# Limits

The following attributes affect compile-time limits.

## The `recursion_limit` attribute

The `recursion_limit` attribute may be applied at the crate level to set the maximum depth for potentially infinitely-recursive compile-time operations like macro expansion or auto-dereference. It uses the MetaNameValueStr syntax to specify the recursion depth.

> Note: The default in `rustc` is 128.

    #![allow(unused)]
    #![recursion_limit = "4"]

    fn main() {
    macro_rules! a {
        () => { a!(1) };
        (1) => { a!(2) };
        (2) => { a!(3) };
        (3) => { a!(4) };
        (4) => { };
    }

    // This fails to expand because it requires a recursion depth greater than 4.
    a!{}
    }

    #![allow(unused)]
    #![recursion_limit = "1"]

    fn main() {
    // This fails because it requires two recursive steps to auto-derefence.
    (|_: &u8| {})(&&1);
    }

## The `type_length_limit` attribute

The `type_length_limit` attribute limits the maximum number of type substitutions made when constructing a concrete type during monomorphization. It is applied at the crate level, and uses the MetaNameValueStr syntax to set the limit based on the number of type substitutions.

> Note: The default in `rustc` is 1048576.

    #![type_length_limit = "8"]

    fn f<T>(x: T) {}

    // This fails to compile because monomorphizing to
    // `f::<(i32, i32, i32, i32, i32, i32, i32, i32, i32)>>` requires more
    // than 8 type elements.
    f((1, 2, 3, 4, 5, 6, 7, 8, 9));