# 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 64.

```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-derefence.
(|_: &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.

```rust,compile_fail
#![type_length_limit = "8"]

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

// This fails to compile because monomorphizing to
// `f::<(i32, i32, i32, i32, i32, i32, i32, i32, i32)>>` requires more
// than 8 type elements.
f(((1, 2, 3, 4, 5, 6, 7, 8, 9));
```

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