c#: Use Spans instead of e.g. byte[]

[jsturtevant]

Use Spans in wit-bindgen-generated code to minimize copies (example while using streams)

[jsturtevant]

#1138 address using span/Memory for parameters where they are explicitly definded (such as in the WriteAsync) but the link above also points to a different type of API where the paramater isn't a list but a value of the number of items to fill.

// TODO: should we add a special case to the bindings generator
                    // to allow passing a buffer to IStreams.InputStream.Read and
                    // avoid the extra copy?
                    var result = input.Read(16 * 1024);

I think the intent of the comment is that we would have a functions like:

input.Read(buffer.AsSpan());
// use buffer that was filed

The streams wit looks like

@since(version = 0.2.0)
        read: func(
            /// The maximum number of bytes to read
            len: u64
        ) -> result<list<u8>, stream-error>;

We could return a Span here but I don't know how we detect that we should re-write the input.Read to special case it for taking a span and filling it.

[pavelsavara]

I think the intent is that we will use buffer provided by the caller.

Bring You Own Buffer

And that the generated C# proxy would allow that to maximal possible extent. Perhaps even passing that buffer to the host where appropriate.

[jsturtevant]

The hang up for me here is how do translate the WIT:

read: func(len: u64) -> result<list<u8>, stream-error>;

Into a function, that we could pass a buffer so that it can be filled, ie input.Read(buffer.AsSpan());.

Other than the specific wasi interface name there aren't any real heuristics to know that a function signature of read: func(len: u64) -> result<list<u8>, stream-error>; should be translated to input.Read(buffer.AsSpan()). But maybe I am missing something?

I am going to propose to not worry about his for now, since the API will change with WASI 0.3 and there is also discussions around a FillableArray type in 0.3.x that would solve this problem.