How do I convert a string to hex in Rust?

Thanks to the user jey in the ##rust irc channel in freenode. You can just use the hex representation fmt provides,

>> let mut s = String::new();
>> use std::fmt::Write as FmtWrite; // renaming import to avoid collision
>> for b in "hello world".as_bytes() { write!(s, "{:02x}", b); }
()
>> s
"68656c6c6f20776f726c64"
>> 

or a bit silly one,

>> "hello world".as_bytes().iter().map(|x| format!("{:02x}", x)).collect::<String>()
"68656c6c6f20776f726c64"

A hexadecimal representation can be generated with a function like this:

pub fn hex_push(buf: &mut String, blob: &[u8]) {
    for ch in blob {
        fn hex_from_digit(num: u8) -> char {
            if num < 10 {
                (b'0' + num) as char
            } else {
                (b'A' + num - 10) as char
            }
        }
        buf.push(hex_from_digit(ch / 16));
        buf.push(hex_from_digit(ch % 16));
    }
}

This is a tad more efficient than the generic radix formatting implemented currently in the language.

Here's a benchmark:

test bench_specialized_hex_push   ... bench:          12 ns/iter (+/- 0) = 250 MB/s
test bench_specialized_fomat      ... bench:          42 ns/iter (+/- 12) = 71 MB/s
test bench_specialized_format     ... bench:          47 ns/iter (+/- 2) = 63 MB/s
test bench_specialized_hex_string ... bench:          76 ns/iter (+/- 9) = 39 MB/s
test bench_to_hex                 ... bench:          82 ns/iter (+/- 12) = 36 MB/s
test bench_format                 ... bench:          97 ns/iter (+/- 8) = 30 MB/s

I will go out on a limb here, and suggest that the solution is for hash to be of type Vec<u8>.

---

The issue is that while you can indeed convert a String to a &[u8] using as_bytes and then use to_hex, you first need to have a valid String object to start with.

While any String object can be converted to a &[u8], the reverse is not true. A String object is solely meant to hold a valid UTF-8 encoded Unicode string: not all bytes pattern qualify.

Therefore, it is incorrect for gen_sha256 to produce a String. A more correct type would be Vec<u8> which can, indeed, accept any bytes pattern. And from then on, invoking to_hex is easy enough:

hash.as_slice().to_hex()

It appears the source for ToHex has the solution I'm looking for. It contains a test:

#[test]
pub fn test_to_hex() {
    assert_eq!("foobar".as_bytes().to_hex(), "666f6f626172");
}

My revised code is:

let hash = gen_sha256("example");

hash.as_bytes().to_hex()

This appears to work. I will take some time before I accept this solution if anyone has an alternative answer.