limits.md

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.

#![recursion_limit = "4"]

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!{}

#![recursion_limit = "1"]

// 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));