Repr attributes on ADTs do not have restrictions.

According to rust, the valid repr attributes are Rust (default), C, align, packed, transparent, simd, i8, u8, i16, u16, i32, u32, i64, u64, i128, u128, isize and usize.
But gccrs allows us to use anything as the attribute and this code compiles without any error.

#[repr(this_repr_doesnot_exist)]
enum Starter{
    Bulbasaur,
    Charmander,
    Squirtle,
}

#[repr(!)]
enum Type{
    Grass,
    Fire,
    Water,
}

#[repr(hmmmmmmm)]
struct Stats {
    base_attack: u32,
    base_defense: u32,
    base_speed: u32,
}

fn main(){}

Godbolt: https://godbolt.org/z/f8rhET9ch

Start with the Rust repr attribute rules listed in the issue and reproduce the examples using the linked Godbolt case. Trace gccrs handling of repr attributes for enums and structs; done means unsupported forms such as this_repr_doesnot_exist and ! are rejected while the listed valid forms remain accepted.