Understanding Rust Error E0517

Overview of Rust Error E0517

Rust Error E0517 occurs when a #[repr(..)] attribute is misused i.e., the attribute is applied to an item that does not support it. This error usually arises in three different scenarios:

1. #[repr(C)] is applied to items other than enums and structs. 2. #[repr(packed)] or #[repr(simd)] is applied to items other than structs. 3. #[repr(u8)], #[repr(i16)], and so on, are applied to items other than enums.

To fix Rust Error E0517, it's essential to know about the supported attributes for different Rust items and how they work.

Understanding #[repr(..)] attributes

The #[repr(..)] attributes provide representation hints to the Rust compiler on how you want a particular item to be represented in memory. They can either control the memory layout or specify the size of a data type. Among the many #[repr(..)] attributes, three are commonly used:

1. #[repr(C)] 2. #[repr(packed)] 3. #[repr(u8)] and similar numeric attributes

Using #[repr(C)] on structs and enums

The #[repr(C)] attribute is allowed on both structs and enums. As this attribute enforces a C-compatible memory layout, it is primarily used during FFI (Foreign Function Interface) or when interacting with low-level code.

Using #[repr(packed)] and #[repr(simd)] on structs

The #[repr(packed)] attribute is used on structs to minimize padding between the fields. It reduces the size of the structure, but accessing fields may be slower due to alignment restrictions. The #[repr(simd)] attribute, on the other hand, enables SIMD (Single Instruction Multiple Data) when applied to structs that consist of homogeneous series of machine types like u8 and i32.

Using numeric #[repr(..)] attributes on enums

Numeric representation attributes like #[repr(u8)] and #[repr(i16)] are applied to enums to define discriminant size. These attributes determine the size of the enum by considering the provided type and make it possible to cast the enum to a value of that type.

Solving Rust Error E0517: Examples

The following code snippets showcase erroneous ways to use #[repr(..)] attributes:

type Foo = u8;

enum Foo {Bar, Baz}

struct Foo {bar: bool, baz: bool}

impl Foo {
    // ...

To fix the error in these examples, apply the #[repr(..)] attributes to their respective supported items:

Correct usage examples for #[repr(..)] attributes

1. For #[repr(C)] on enums and structs:

Correct #[repr(C)] usage:

struct Foo {bar: bool, baz: bool}

enum Color {Red, Green, Blue}

2. For #[repr(packed)] on structs:

Correct #[repr(packed)] usage:

struct Foo {bar: bool, baz: bool}

3. For numeric attributes on enums:

Correct numeric attribute usage:

enum Color {Red, Green, Blue}


Rust Error E0517 occurs when an unsupported Rust item is assigned a #[repr(..)] attribute. To fix the error, apply these attributes to their respective, supported items. To learn more about using and understanding Rust attributes and representation hints, follow the resources available in the recommended reading section.

Recommended Reading