Interesting error message with custom hashers

[alexcrichton]

#[feature(default_type_params)];

extern crate collections;

use std::hash::{Hash, Hasher};

type IntSet = collections::HashSet<int, IntHasher>;

struct IntHasher;

impl Hasher<int> for IntHasher {
    fn hash<T: Hash<int>>(&self, t: &T) -> u64 {
        let mut state = 0;
        t.hash(&mut state);
        state as u64
    }
}

fn main() {
    let mut s: IntSet = collections::HashSet::with_hasher(IntHasher);
    s.insert(3);
    println!("{}", s.contains(&3));
}

$ rustc foo.rs
foo.rs:21:5: 21:16 error: failed to find an implementation of trait std::io::Writer for int
foo.rs:21     s.insert(3);
              ^~~~~~~~~~~

I'm not sure why it's saying that there's no implementation of Writer for int. While true, it shouldn't be looking for that. I think this has to do with default type parameters on Hash.

Note that I forgot to provide impl Hash<int> for int, so it's legitimate that I receive an error, I don't believe I should receive the above error, however. I've also found out that it's illegal anyway to say impl Hash<int> for int, but seems like the message should change?

cc @eddyb

[zkamsler]

It looks like predefined Hash impls for primitives require a Writer bound on the stream.

macro_rules! impl_hash(
    ( $( $ty:ty => $method:ident;)* ) => (
        $(
            impl<S: Writer> Hash<S> for $ty {
                #[inline]
                fn hash(&self, state: &mut S) {
                    state.$method(*self);
                }
            }
        )*
    )
)

[alexcrichton]

Hm, while opaque, it seems accurate.