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
rustcis 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
Note
This limit is only enforced when the nightly
-Zenforce-type-length-limitflag is active.For more information, see https://github.com/rust-lang/rust/pull/127670.
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
rustcis 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));