Limits - The Rust Reference (original) (raw)

The Rust Reference

Limits

The following attributes affect compile-time limits.

The recursion_limit attribute

The recursionlimit 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 MetaNameValueStrsyntax 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-dereference.
(|_: &u8| {})(&&&1);
}

The type_length_limit attribute

The typelengthlimit 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 = "4"]

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

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