Rust Error E0565: Invalid Literal in Built-in Attribute

Understanding Rust Error E0565

Rust Error E0565 occurs when a literal value is used in a built-in attribute that does not support literals. The Rust documentation provides the following erroneous code example:

#[repr("C")]
struct Repr {}

fn main() {}

In this code, the literal value "C" is used as a parameter for the #[repr()] attribute. However, this attribute only accepts identifier (unquoted) values.

Resolving Rust Error E0565

To fix Rust Error E0565, you simply need to replace the literal value with an unquoted identifier value. Here's the corrected code example:

#[repr(C)]
struct Repr {}

fn main() {}

In this code, the #[repr()] attribute now uses the unquoted identifier value 'C'. This corrects the E0565 error, allowing the code to compile and run successfully.

The Ongoing Effort to Support Literals in Attributes

The Rust language is continuously evolving, and efforts are being made to support literals in built-in attributes where appropriate. For now, it's essential to use unquoted identifier values in attributes that do not support literals.

Additional Examples of Rust Error E0565

Here are more examples illustrating Rust Error E0565 and how to fix them:

Example 1: Erroneous code using a literal value in #[inline()] attribute

#[inline("always")]
fn example() {}

fn main() {}

To fix this error, replace the literal value with an unquoted identifier:

#[inline(always)]
fn example() {}

fn main() {}

Example 2: Erroneous code using a literal value in #[non_exhaustive()] attribute

#[non_exhaustive("true")]
pub enum Example {
    A,
    B,
}

fn main() {}

To fix this error, simply remove the literal value:

#[non_exhaustive]
pub enum Example {
    A,
    B,
}

fn main() {}