Stringbuilder.Append should have an IFormattable overload

[jrmoreno1]

Background and Motivation

StringBuilder has several overloads, but none of them work with an interpolated string without first turning it into a string. Given that purpose of the StringBuilder is performance, it seems like turning the interpolation into a string first is going to give a performance hit.

Proposed API

namespace System.Text
{
     public sealed class StringBuilder : System.Runtime.Serialization.ISerializable
     {
+        public StringBuilder AppendFormat(IFormattable)
     }
}

Usage Examples

const string lastName = "Moreno";
const string firstName = "John";
var sb = new StringBuilder();
IFormattable name = $"{lastName}, {firstName}";
sb.AppendFormat(name);

-->

Risks

It's possible that the implementation for the overload would be slower than sb.AppendFormat("{0}, {1}", lastName, firstName);

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

This is addressed by #51653. With it, you can just write:

sb.Append($"{lastName}, {firstName}");

and the right things happen.

Closing as a dup of #50635. Thanks.