limits.md

r[attributes.limits]

Limits

The following attributes affect compile-time limits.

r[attributes.limits.recursion_limit]

The recursion_limit attribute

r[attributes.limits.recursion_limit.intro] The recursion_limit attribute sets the maximum depth for potentially infinitely-recursive compile-time operations like macro expansion or auto-dereference.

[!EXAMPLE]

#![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!{}

[!EXAMPLE]

#![recursion_limit = "1"]

// This fails because it requires two recursive steps to auto-dereference.
(|_: &u8| {})(&&&1);

Note

The default recursion limit in rustc is 128.

r[attributes.limits.recursion_limit.syntax] The recursion_limit attribute uses the [MetaNameValueStr] syntax to specify the recursion depth. The value in the string must be a non-negative integer.

r[attributes.limits.recursion_limit.allowed-positions] The recursion_limit attribute may only be applied to the crate root.

Note

rustc currently warns in other positions, but this may be rejected in the future.

r[attributes.limits.recursion_limit.duplicates] Only the first instance of recursion_limit on an item is honored. Subsequent recursion_limit attributes are ignored.

Note

rustc currently warns on following duplicate recursion_limit attributes. This may become an error in the future.

r[attributes.limits.type_length_limit]

The type_length_limit attribute

Note

This limit is only enforced when the nightly -Zenforce-type-length-limit flag is active.

For more information, see rust-lang/rust#127670.

r[attributes.limits.type_length_limit.intro] The type_length_limit attribute limits the maximum number of type substitutions made when constructing a concrete type during monomorphization.

r[attributes.limits.type_length_limit.syntax] 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));