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-dereference. (|_: &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.

#![allow(unused)] #![type_length_limit = "4"] fn main() { 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)); }