Comment by grantjpowell

~I love when I see programming languages who's first advertised features are implementable in 8 lines of rust~

Edit: ^ the above had the wrong tone. Thanks to dang for pointing it out. What I meant to express was that it's possible to accomplish a similar safety/ergonomics at the library level in rust in not too many SLOC. My personal preference is towards Rust's approach because the type system gives really powerful composable primitives which makes it possible to have the compiler check a wide range of invariants, instead of just the ones that are common/special enough to go into the language itself

(Example edited after comments from mumblemumble)

    // The struct is public, but the contents are private, meaning you can't directly access the secret once it's inside the struct
    pub struct Secret<T>(T);

    impl<T> Secret<T> {
        // The only public way to access the secret, returns a new secret
        pub fn map<U>(&self, func: impl FnOnce(&T) -> U) -> Secret<U> {
            Secret(func(&self.0))
        }
    }
    
    impl<T: AsRef<[u8]>> PartialEq<&[u8]> for Secret<T> {
        // == does the correct thing (and only works for types that would make sense (`AsRef<[u8]>`)
        fn eq(&self, other: &&[u8]) -> bool {
            constant_time_eq(self.0.as_ref(), other)
        }
    }

    /* Some other file */

    use secret::Secret;
    
    // Translated from the example
    fn check_mac<T: AsRef<[u8]>>(mac_secret: Secret<T>, message: &[u8], mac: &[u8]) -> bool {
        // This returns a new Secret<[u8; 32]>
        let computed_mac = mac_secret.map(|secret| hmac_sha_256(secret.as_ref(), message));

        // This uses the `constant_time_eq` impl from above
        computed_mac == mac
    }

Edit: It looks like you can implment SOA as a macro too https://github.com/lumol-org/soa-derive

Edit: mumblemumble helpfully points out I demonstrated this poorly, so I tried to better demostrate what I was going for in this comment https://news.ycombinator.com/item?id=33764037