Unable to implement a trait in terms of more than one other trait

[pkgw]

I'd like to implement a trait in terms of a few other ones, but the compiler tells me that I have conflicting trait implementations. The most succinct example is

trait Generic { }
trait SpecificOne { }
trait SpecificTwo { }
impl<T: SpecificOne> T: Generic { }
impl<T: SpecificTwo> T: Generic { }

Now, if there existed a type implementing both SpecificOne and SpecificTwo, that would be a problem, but there isn't.

I'm pretty sure the compiler should be able to determine whether the implementation is legal or not. (It's not clear to me whether you have to worry about something introducing ambiguities by linking to your crate later.) And unless I'm overlooking something (not to imply that this is unlikely), it seems to me that this is an important thing to be able to do.

My specific use case is for SHA1 hashing of things. I want to do something along the lines of:

trait Hashable { fn hash (&self, Sha1 sha); }

impl<T: to_bytes::IterBytes> T: Hashable { /* send the bytes to the sha1 */ }

impl<T: io::Reader> T: Hashable { /* send the stream to the sha1 */ }

Readers can't be IterBytes because the bytes iterator function is defined as being pure.

[pkgw]

Never mind, I'm an idiot.

impl SpecificOne: Generic { }
impl SpecificTwo: Generic { }

I had been reading the tutorial and didn't catch anything saying that you can provide an impl for a bare trait in this way, and the people that I briefly consulted on IRC didn't immediately come up with this, so maybe that's something that should be added to, or made more prominent in, the tutorial.