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Update recursion_limit to use the attribute template #1916

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Update recursion_limit to use the attribute template #1916

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2ad74b6
Unwrap recursion_limit
ehuss Jun 23, 2025
12ad17a
Move recursion_limit examples to intro and example block
ehuss Jun 23, 2025
8157052
Move recursion_limit note to the intro
ehuss Jun 23, 2025
4b528d2
Update recursion_limit to use the attribute template
ehuss Jun 23, 2025
7d0c3c8
Minor update of `recursion_limit`
ehuss Sep 22, 2025
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60 changes: 36 additions & 24 deletions src/attributes/limits.md
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Expand Up @@ -3,42 +3,54 @@ r[attributes.limits]

The following [attributes] affect compile-time limits.

<!-- template:attributes -->
r[attributes.limits.recursion_limit]
## The `recursion_limit` attribute

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.
The *`recursion_limit` [attribute][attributes]* sets 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.
> [!EXAMPLE]
> ```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!{}
> ```

> [!EXAMPLE]
> ```rust,compile_fail
> #![recursion_limit = "1"]
>
> // This fails because it requires two recursive steps to auto-dereference.
> (|_: &u8| {})(&&&1);
> ```

> [!NOTE]
> The default in `rustc` is 128.
> The default recursion limit in `rustc` is 128.

```rust,compile_fail
#![recursion_limit = "4"]
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.

macro_rules! a {
() => { a!(1); };
(1) => { a!(2); };
(2) => { a!(3); };
(3) => { a!(4); };
(4) => { };
}
r[attributes.limits.recursion_limit.allowed-positions]
The `recursion_limit` attribute may only be applied to the crate root.

// This fails to expand because it requires a recursion depth greater than 4.
a!{}
```
> [!NOTE]
> `rustc` ignores use in other positions but lints against it. This may become an error in the future.

```rust,compile_fail
#![recursion_limit = "1"]
r[attributes.limits.recursion_limit.duplicates]
Only the first use of `recursion_limit` on an item has effect.

// This fails because it requires two recursive steps to auto-dereference.
(|_: &u8| {})(&&&1);
```
> [!NOTE]
> `rustc` lints against any use following the first. This may become an error in the future.

<!-- template:attributes -->
r[attributes.limits.type_length_limit]
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