src/attributes/limits.md

# Limits

r[attributes.limits]

The following [attributes] affect compile-time limits.

## The `recursion_limit` attribute

r[attributes.limits.recursion_limit]

r[attributes.limits.recursion_limit.intro]
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.

r[attributes.limits.recursion_limit.syntax]
It uses the [_MetaNameValueStr_]
syntax to specify the recursion depth.

> Note: The default in `rustc` is 128.

```rust,compile_fail
#![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!{}
```

```rust,compile_fail
#![recursion_limit = "1"]

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

## The `type_length_limit` attribute

r[attributes.limits.type_length_limit]

> **Note**: This limit is only enforced when the nightly `-Zenforce-type-length-limit` flag is active.
>
> For more information, see <https://github.com/rust-lang/rust/pull/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.

```rust,ignore
#![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));
```

[_MetaNameValueStr_]: ../attributes.md#meta-item-attribute-syntax
[attributes]: ../attributes.md
[crate]: ../crates-and-source-files.md