Error code E0565

A literal was used in a built-in attribute that doesn’t support literals.

Erroneous code example:

#[repr("C")] // error: meta item in `repr` must be an identifier
struct Repr {}

fn main() {}

Not all attributes support literals in their input, and in some cases they expect an identifier instead. That would be the solution in the case of repr:

#[repr(C)] // ok!
struct Repr {}

fn main() {}