Implement Borrow and BorrowMut for Matrix

[AtheMathmo]

This is a mirror to rusty-machine/109.

[AtheMathmo]

Some issues with implementing AsRef/Borrow:

impl<'a, T> AsRef<MatrixSlice<'a, T>> for Matrix<T> {
    fn as_ref(&self) -> &MatrixSlice<'a, T> {
        &self.as_slice()
    }
}

impl<'a, T> AsMut<MatrixSliceMut<'a, T>> for Matrix<T> {
    fn as_mut(&mut self) -> &mut MatrixSliceMut<'a, T> {
        &self.as_mut_slice()
    }
}

This code fails to compile with the following errors:

src/convert.rs:33:15: 33:23 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements [E0495]
src/convert.rs:33         &self.as_slice()
                                ^~~~~~~~
src/convert.rs:32:5: 34:6 help: consider using an explicit lifetime parameter as shown: fn as_ref(&'a self) -> &MatrixSlice<'a, T>
src/convert.rs:32     fn as_ref(&self) -> &MatrixSlice<'a, T> {
                      ^
src/convert.rs:39:19: 39:31 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements [E0495]
src/convert.rs:39         &mut self.as_mut_slice()
                                    ^~~~~~~~~~~~
src/convert.rs:38:5: 40:6 help: consider using an explicit lifetime parameter as shown: fn as_mut(&'a mut self) -> &mut MatrixSliceMut<'a, T>
src/convert.rs:38     fn as_mut(&mut self) -> &mut MatrixSliceMut<'a, T> {
                      ^
error: aborting due to 2 previous errors
error: Could not compile `rulinalg`.

But we cannot use an explicit lifetime as it goes against the trait:

error: method `as_ref` has an incompatible type for trait:
 expected bound lifetime parameter ,
    found concrete lifetime [E0053]

Do you see anything obviously wrong with this approach @tafia ? If not I'd propose we make our own internal traits: AsSliceand AsMutSlice. We can implement these for Matrix, MatrixSlice and MatrixSliceMut as appropriate - and use these over our generic functions. It should look something like this:

pub trait AsSlice<T> where T: ?Sized {
    fn as_slice(&self) -> T;
}

(I haven't tested this yet).

[tafia]

Sorry for the late answer. I think a more aggressive change of MatrixSlice might work.
I'll try and see if I get something:

• have it use a [T] instead of a *const T (it becomes !Size)
• remove the PhantomData
• make sure all impl work on &MatrixSlice (Sized)
• implement AsRef and perhaps Deref as well ( not sure)

[AtheMathmo]

I'm not sure that I entirely follow how this will solve the problem - but I look forward to seeing if you can pull it off!

Let me know if I can (try to) help at all.

[tafia]

It doesn't worked as well as I was expecting.
I'm trying another approach (#18) to clean up a bit and hopefully it'll be simpler then.

[AtheMathmo]

I'm closing this issue. We wont be able to implement this in the way we want until Custom DSTs land. In which case we will not be implementing Borrow so much as Deref.