Code shared from the Rust Playground

playground.rs

#![feature(const_generics)]
#![feature(const_generic_impls_guard)]

trait CsvRecord<const COLS: usize> {
    fn columns() -> [&'static str; COLS];
    fn values(&self) -> [String; COLS];
}

struct Foo {
    id: u32,
    bar: String,
}

impl CsvRecord<2> for Foo {
    fn columns() -> [&'static str; 2] {
        ["id", "bar"]
    }
    fn values(&self) -> [String; 2] {
        [self.id.to_string(), self.bar.clone()]
    }
}

struct Bar {
    id: u32,
    bar: String,
    baz: String,
}

impl CsvRecord<3> for Bar {
    fn columns() -> [&'static str; 3] {
        ["id", "bar", "baz"]
    }
    fn values(&self) -> [String; 3] {
        [self.id.to_string(), self.bar.clone(), self.baz.clone()]
    }
}

fn write<R: CsvRecord<COLS>, const COLS: usize>(record: &R)
where 
    [&'static str; COLS]: std::array::LengthAtMost32,
    [String; COLS]: std::array::LengthAtMost32
{
    println!("{:?} {:?}", R::columns(), record.values());
}

fn main() {
    let foo = Foo {
        id: 21,
        bar: "fds".into(),
    };
    
    write(&foo);
    
    let bar = Bar {
        id: 21,
        bar: "fds".into(),
        baz: "xdr".into(),
    };
    
    write(&bar);
}