RS.CLIPPY.LARGE_ENUM_VARIANT

Large size difference between variants on an enum

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: large_enum_variant. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for large size differences between variants on enums.

Why is this bad?

Enum size is bounded by the largest variant. Having one large variant can penalize the memory layout of that enum.

Known problems

This lint obviously cannot take the distribution of variants in your running program into account. It is possible that the smaller variants make up less than 1% of all instances, in which case the overhead is negligible and the boxing is counter-productive. Always measure the change this lint suggests.

For types that implement Copy, the suggestion to Box a variant's data would require removing the trait impl. The types can of course still be Clone, but that is worse ergonomically. Depending on the use case it may be possible to store the large data in an auxiliary structure (e.g. Arena or ECS).

The lint will ignore the impact of generic types to the type layout by assuming every type parameter is zero-sized. Depending on your use case, this may lead to a false positive.

Example

enum Test {
    A(i32),
    B([i32; 8000]),
}

Use instead:

// Possibly better
enum Test2 {
    A(i32),
    B(Box<[i32; 8000]>),
}

Configuration

enum-variant-size-threshold: The maximum size of an enum's variant to avoid box suggestion

(default: 200)