Background

It seems logical that we should support appending Span<char> and ReadOnlySpan<char> to a StringBuilder, since the point of both data types is to reduce allocations. The only way to do this with the current APIs is to loop through the span one char at a time and call Append on each of them, which is pretty slow.

Proposed API

namespace System.Text
{
    public sealed class StringBuilder
    {
        // Span is implicitly convertible to ReadOnlySpan
        public StringBuilder Append(ReadOnlySpan<char> value);
    }
}

cc @jkotas, @KrzysztofCwalina, @justinvp