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Update type_length_limit to use the attribute template #1917

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Update type_length_limit to use the attribute template #1917

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09ebb2e
Unwrap type_length_limit
ehuss Jun 23, 2025
b83596e
Move type_length_limit example to the intro
ehuss Jun 23, 2025
051af05
Move type_length_limit notes to the intro
ehuss Jun 23, 2025
ee2bd23
Update type_length_limit to use the attribute template
ehuss Jun 23, 2025
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42 changes: 27 additions & 15 deletions src/attributes/limits.md
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Expand Up @@ -43,31 +43,43 @@ a!{}
r[attributes.limits.type_length_limit]
## The `type_length_limit` attribute

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

> [!NOTE]
> This limit is only enforced when the nightly `-Zenforce-type-length-limit` flag is active.
> In `rustc` 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.
> [!EXAMPLE]
> <!-- ignore: not enforced without nightly flag -->
> ```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));
> ```

> [!NOTE]
> The default in `rustc` is 1048576.

```rust,ignore
#![type_length_limit = "4"]
r[attributes.limits.type_length_limit.syntax]
The `type_length_limit` attribute uses the [MetaNameValueStr] syntax to set the limit. The value in the string must be a non-negative number.

fn f<T>(x: T) {}
r[attributes.limits.type_length_limit.allowed-positions]
The `type_length_limit` attribute may only be applied to the crate root.

// This fails to compile because monomorphizing to
// `f::<((((i32,), i32), i32), i32)>` requires more than 4 type elements.
f(((((1,), 2), 3), 4));
```
> [!NOTE]
> `rustc` currently warns in other positions, but this may be rejected in the future.

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

> [!NOTE]
> `rustc` currently warns on following duplicate `type_length_limit` attributes. This may become an error in the future.

[attributes]: ../attributes.md
[crate]: ../crates-and-source-files.md