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 auto-dereference or macro expansion. It uses the MetaNameValueStr syntax to specify the recursion depth.

Note: The default in rustc is 64.

#![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!{}

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"]

type A = (B, B, B);
type B = (C, C, C);
struct C;

// This fails to compile because monomorphizing to
// `drop::<Option<((C, C, C), (C, C, C), (C, C, C))>>` requires more than 8
// type elements.
drop::<Option<A>>(None);