StringBuilder.Append($"Foo{a++}") unexpected allocations

[Wh1teDuke]

Given this snippet:

using System.Text;

var sb = new StringBuilder(20);
var buff = new char[12];

var oldIndex = GC.GetGCMemoryInfo().Index;
var a = 0;
while (true)
{
    sb.Clear().Append($"Foo{a++}").CopyTo(0, buff, sb.Length); // .Append("Foo").Append(a++) // Works as expected
    Console.WriteLine(buff.AsSpan(0, sb.Length));
    if (a > 5_000_000) break;
}
var newIndex = GC.GetGCMemoryInfo().Index;
Console.WriteLine(newIndex - oldIndex);

I'd expect the last output to be 0, but it isn't. Using DefaultInterpolatedStringHandler I get zero collections:

using System.Runtime.CompilerServices;

var a = 0;
var oldIndex = GC.GetGCMemoryInfo().Index;
while (true)
{
    Foo.Bar($"Foo{a++}");
    if (a > 5_000_000) break;
}
var newIndex = GC.GetGCMemoryInfo().Index;
Console.WriteLine(newIndex - oldIndex);

internal static class Foo
{
    public static void Bar(ref DefaultInterpolatedStringHandler handler)
    {
        Console.WriteLine(handler.Text);
        handler.Clear();
    }
}

[stephentoub]

I just tried with this on .NET 10:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Text;

BenchmarkSwitcher.FromAssembly(typeof(Benchmarks).Assembly).Run(args);

[MemoryDiagnoser(false)]
public class Benchmarks
{
    private readonly StringBuilder _sb = new (20);
    private readonly char[] _buff = new char[12];

    [Benchmark]
    [Arguments(42)]
    public void WithInterpolation(int a) => _sb.Clear().Append($"Foo{a++}").CopyTo(0, _buff, _sb.Length);

    [Benchmark]
    [Arguments(42)]
    public void WithoutInterpolation(int a) => _sb.Clear().Append("Foo").Append(a++).CopyTo(0, _buff, _sb.Length);
}

and I don't see any allocation difference:

Method	a	Mean	Error	StdDev	Allocated
WithInterpolation	42	12.26 ns	0.176 ns	0.165 ns	-
WithoutInterpolation	42	10.21 ns	0.094 ns	0.079 ns	-

[Wh1teDuke]

My mistake, looks like the IDE tricked me, should have tested from the terminal before.