Bug in generic trait implementation?

[tav]

There is a possible bug when you define a generic trait implementation within a different module but within the same file, e.g. given:

type byte = u8;

mod crypto {

// Hash is the trait implemented by all cryptographic hash functions.
pub trait Hash {
    fn block_size() -> int;
    fn digest() -> ~[byte];
    fn digest_size() -> int;
    fn reset();
    fn update(msg: &[byte]);
}

// HashUtil is a utility trait that is auto-implemented for all Hash
// implementations.
pub trait HashUtil {
    fn hexdigest() -> ~str;
    fn update(msg: &str);
}

impl <A: Hash> A: HashUtil {

    fn hexdigest() -> ~str {
        let mut d = ~"";
        for vec::each(self.digest()) |b| {
            d += fmt!("%02x", *b as uint)
        }
        return d;
    }

    fn update(msg: &str) {
        self.update(str::to_bytes(msg));
    }

}

}

mod sha256 {

use crypto::Hash;

const block_size: int = 64;
const digest_size: int = 32;

const i0: u32 = 0x6a09e667;
const i1: u32 = 0xbb67ae85;
const i2: u32 = 0x3c6ef372;
const i3: u32 = 0xa54ff53a;
const i4: u32 = 0x510e527f;
const i5: u32 = 0x9b05688c;
const i6: u32 = 0x1f83d9ab;
const i7: u32 = 0x5be0cd19;

const k: [u32 * 64] =
    [0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1,
     0x923f82a4, 0xab1c5ed5, 0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3,
     0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174, 0xe49b69c1, 0xefbe4786,
     0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
     0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147,
     0x06ca6351, 0x14292967, 0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13,
     0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85, 0xa2bfe8a1, 0xa81a664b,
     0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
     0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a,
     0x5b9cca4f, 0x682e6ff3, 0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208,
     0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2];

struct sha256 {
    b: [mut byte * 64],
    h: [mut u32 * 8],
    mut l: u64,
    mut n: uint,
}

impl sha256 {

    #[inline(always)]
    fn compute_block() {
        let mut h0 = self.h[0];
        let mut h1 = self.h[1];
        let mut h2 = self.h[2];
        let mut h3 = self.h[3];
        let mut h4 = self.h[4];
        let mut h5 = self.h[5];
        let mut h6 = self.h[6];
        let mut h7 = self.h[7];
        self.h[0] = h0;
        self.h[1] = h1;
        self.h[2] = h2;
        self.h[3] = h3;
        self.h[4] = h4;
        self.h[5] = h5;
        self.h[6] = h6;
        self.h[7] = h7;
    }

}

impl sha256: Hash {

    fn block_size() -> int { block_size }

    fn digest() -> ~[byte] {
        ~[1, 2, 37]
    }

    fn digest_size() -> int { digest_size }

    fn reset() {
        self.h[0] = i0;
        self.h[1] = i1;
        self.h[2] = i2;
        self.h[3] = i3;
        self.h[4] = i4;
        self.h[5] = i5;
        self.h[6] = i6;
        self.h[7] = i7;
        self.l = 0;
        self.n = 0;
    }

    fn update(msg: &[byte]) {
        let mut l = msg.len();
        self.l += l as u64;
        if self.n > 0 {
            if l > 64 - self.n {
                l = 64 - self.n;
            }
        }
    }

}

pub fn new() -> sha256 {
    sha256{
        b: [mut 0, ..64],
        h: [mut i0, i1, i2, i3, i4, i5, i6, i7],
        l: 0,
        n: 0
    }
}

}

fn main() {
    io::println(fmt!("val: %?", sha256::new().block_size()));
    io::println(fmt!("val: %?", sha256::new().hexdigest()));
}

The compiler errors with:

error: attempted access of field `hexdigest` on type `sha256::sha256`, but no field or method with that name was found

If, however, you put a use crypto::*; statement before the main(), then everything seems to work. This behaviour feels like a bug since block_size() works and hexdigest() is defined within the same module...

[catamorphism]

Not sure what the bug is here? block_size works because you imported its containing trait, Hash, explicitly. hexdigest doesn't because you didn't import its containing trait, HashUtil. You could remove use crypto::*; and say use crypto::HashUtil to also get a program that compiles.

But maybe I'm misunderstanding?

[tav]

The Hash trait is only imported in the sha256 module. Yet it works without a use crypto::Hash; before the main().

In contrast, it requires an explicit use crypto::HashUtil; before the main(). Surely this isn't the intended behaviour?

[catamorphism]

Sorry, I didn't read your code correctly at first. Are you saying that if you comment out the line in main with the call to hexdigest, it works without inserting a use crypto::*;?

[tav]

Yes :)

[catamorphism]

Okay, weird; I'll try to reproduce when I get a moment.

[catamorphism]

This actually works as intended; you just need to use crypto::HashUtil in the scope where main is declared. But why don't you need to use crypto::Hash explicitly? Because there's an impl of Hash for sha256. Since the impl of HashUtil is generic, you have to use it explicitly.

It still seems a little weird to me, though. Maybe @pcwalton could comment?

[catamorphism]

I'm just going to close this since I think it's intended behavior. If you think it should change, feel free to file an RFC bug :-)

[nikomatsakis]

I actually didn't think that was the intended behavior. I thought you had to import traits to invoke trait methods, and I didn't think it was relevant where the impl for the trait appeared. @pcwalton is this incorrect?

[nikomatsakis]

Seems like this is expected. Closing.