QueueWriteBufferView is strange.

[botahamec]

According to the documentation for Queue::write_buffer_with:

The returned value can be dereferenced to a &mut [u8]; dereferencing it to a &[u8] panics!

According to the documentation for Deref:

this trait should never fail. Failure during dereferencing can be extremely confusing when Deref is invoked implicitly.

[teoxoy]

Sadly there is no nice way to express this in rust since it's always valid to reborrow a &mut T as a & T.

[botahamec]

@teoxoy There's no reason why it needs to implement DerefMut. Preferably there would be a safer interface that replicates only the features of an array which make sense to use.

[kpreid]

Whatever interface is used, it would have to return an &mut [u8] eventually, because the point of this function is to allow ordinary Rust bulk-data operations to write into the GPU buffer rather than requiring a copy from elsewhere.

[botahamec]

@kpreid Is the idea that there might be a method that modifies a &mut [u8] but doesn't read from it that someone might want to use for writing to the buffer? The better interface there would probably just to use a method that returns a &mut [u8]. But I must say that this seems like a weird thing to need.

[teoxoy]

Is the idea that there might be a method that modifies a &mut [u8] but doesn't read from it that someone might want to use for writing to the buffer?

Yes.

The better interface there would probably just to use a method that returns a &mut [u8].

It might be but there was precedent to go this route.

It was done to discourage reading since it will be slow to do so.

Thread on the PR where this was discussed previously: #2777 (comment)

[botahamec]

What was the precedent?

I get that it's slow, but is it really worth it to crash on an implicit dereference? Especially when the docs for Deref say that it "must not fail", in bold. It's still possible to read from it anyway. If you really want to prevent reading, we could get something similar by just implementing AsMut<[u8]> but not AsRef<[u8]>.

[teoxoy]

What was the precedent?

BufferViewMut

I get that it's slow, but is it really worth it to crash on an implicit dereference? Especially when the docs for Deref say that it "must not fail", in bold. It's still possible to read from it anyway. If you really want to prevent reading, we could get something similar by just implementing AsMut<[u8]> but not AsRef<[u8]>.

That's fair, I just think we should also change BufferViewMut and maybe BufferView as well if we go this route.

[botahamec]

I could probably implement that, at least for the QueueWriteBufferView.

BufferViewMut might need some more brainstorming, as it is sometimes readable, but it shouldn't be too bad. The docs for AsRef have a pretty clear recommendation for what to do if it can fail, which is to not implement it, and instead just make your own as_ref which returns an Option or a Result.

BufferView already just implements Deref and AsRef, so as far as I can tell, it should already be fine.