Vector<T> should have a ctor that accepts ReadOnlySpan<T>

[GrabYourPitchforks]

Currently the vector type has the following span-based ctor overload.

public struct Vector<T> {
    public Vector(Span<T> values);
}

We should add a readonly span-based ctor overload so that vectors can be read from immutable data buffers, such as a ReadOnlySpan<char> backed by a String. This will make it easier to vectorize text processing APIs without falling back to the dangerous Unsafe or MemoryMarshal classes.

public struct Vector<T> {
    /* new ctor overload proposal */
    public Vector(ReadOnlySpan<T> values);

    /* new instance method proposals */
    public void CopyTo(Span<T> destination);
    public bool TryCopyTo(Span<T> destination);
}

[GrabYourPitchforks]

I updated the proposal with CopyTo / TryCopyTo, since if we're reading vectors from buffers it'll also be good to write vectors to buffers. :)

[4creators]

Perhaps we should add above APIs to HWIntrinsics VectorNNN<T> what partially would heal wounds of some devs as discussed in dotnet/corefx#18926

[evilguest]

Where can I see the Vector(Span) constructor? The one available in System.Numerics.Vectors 4.1.4.0 does have only the T[]-based constructors :(

[Joe4evr]

@evilguest https://apisof.net/catalog/System.Numerics.Vector%3CT%3E..ctor(Span%3CT%3E)

[dangi12012]

THIS NEADS TO HAPPEN!
Vector should have a ctor that accepts ReadOnlySpan
Vector should MUST also support CopyTo(Span mySpan)
since if we get it from a buffer we would like to store the results there too ;)

[terrajobst]

Video

@GrabYourPitchforks also suggested these APIs:

public struct Vector<T> {
    /* new ctor overload proposal */
    public Vector(ReadOnlySpan<byte> values);
   
    /* new instance method proposals */
    public void CopyTo(Span<byte> destination);
    public bool TryCopyTo(Span<byte> destination);
}

This would avoid consumers having to re-interpret case the spans. We don't know what happens when the Vector<T> is instantiated with byte though. @GrabYourPitchforks, please take a look and file another item.

[GrabYourPitchforks]

I also checked out the Vector<byte> behavior with regard to the dueling overloads. In this case, if you call new Vector<byte>(new ReadOnlySpan<byte>(new byte[32])), the compiler prefers the ROS<byte> overload instead of the ROS<T> overload. (The same is true for CopyTo.) This behavior is perfectly fine since both the ROS<byte> and the ROS<T> ctors have the exact same behaviors when T = byte.