Add a ValueStringBuilder for .NET 6 (#7962)

## Summary of changes

Adds a `ValueStringBuilder` implementation, based on the one used
internally in the .NET runtime

## Reason for change

- We can stackalloc the `Span<char>`
- It's a bit faster than our existing `StringBuilderCache`
implementation, and in some places that matters.

## Implementation details

- Made in .NET 6 only for simplicity. We _could_ expose it earlier, but
I wanted this for the updated aspnetcore integration, so .NET6+ only for
now is good enough
- It's not without risks in its usage, so we have to be careful about
things like passing it around (i.e. avoid doing that completely, for
safety)
- Uses an array pool backed implementation (again, built into .NET 6)

## Test coverage

Imported the unit tests from the runtime too

## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-842

Part of a stack

- #7962 👈
- #7963
- #7964
- #7966
- #7965

Author: andrewlock

tracer/src/Datadog.Trace.Trimming/build/Datadog.Trace.Trimming.xml

@@ -672,6 +672,7 @@
       <type fullname="System.IO.UnmanagedMemoryStream" />
       <type fullname="System.IObservable`1" />
       <type fullname="System.IObserver`1" />
+      <type fullname="System.ISpanFormattable" />
       <type fullname="System.Lazy`1" />
       <type fullname="System.Math" />
       <type fullname="System.MethodAccessException" />

tracer/src/Datadog.Trace/Util/ValueStringBuilder.cs

@@ -0,0 +1,316 @@
+// <copyright file="ValueStringBuilder.cs" company="Datadog">
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2 License.
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
+// </copyright>
+
+#if NET6_0_OR_GREATER
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Buffers;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+#nullable enable
+
+namespace Datadog.Trace.Util
+{
+    internal ref struct ValueStringBuilder
+    {
+        private char[]? _arrayToReturnToPool;
+        private Span<char> _chars;
+        private int _pos;
+
+        public ValueStringBuilder(Span<char> initialBuffer)
+        {
+            _arrayToReturnToPool = null;
+            _chars = initialBuffer;
+            _pos = 0;
+        }
+
+        public ValueStringBuilder(int initialCapacity)
+        {
+            _arrayToReturnToPool = ArrayPool<char>.Shared.Rent(initialCapacity);
+            _chars = _arrayToReturnToPool;
+            _pos = 0;
+        }
+
+        /// <summary>Gets the underlying storage of the builder.</summary>
+        public Span<char> RawChars => _chars;
+
+        public int Length
+        {
+            get => _pos;
+            set
+            {
+                _pos = value;
+            }
+        }
+
+        public int Capacity => _chars.Length;
+
+        public ref char this[int index]
+        {
+            get
+            {
+                return ref _chars[index];
+            }
+        }
+
+        public void EnsureCapacity(int capacity)
+        {
+            // This is not expected to be called this with negative capacity
+
+            // If the caller has a bug and calls this with negative capacity, make sure to call Grow to throw an exception.
+            if ((uint)capacity > (uint)_chars.Length)
+            {
+                Grow(capacity - _pos);
+            }
+        }
+
+        /// <summary>
+        /// Ensures that the builder is terminated with a NUL character.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void NullTerminate()
+        {
+            EnsureCapacity(_pos + 1);
+            _chars[_pos] = '\0';
+        }
+
+        /// <summary>
+        /// Get a pinnable reference to the builder.
+        /// Does not ensure there is a null char after <see cref="Length"/>
+        /// This overload is pattern matched in the C# 7.3+ compiler so you can omit
+        /// the explicit method call, and write eg "fixed (char* c = builder)"
+        /// </summary>
+        public ref char GetPinnableReference()
+        {
+            return ref MemoryMarshal.GetReference(_chars);
+        }
+
+        public override string ToString()
+        {
+            string s = _chars.Slice(0, _pos).ToString();
+            Dispose();
+            return s;
+        }
+
+        public ReadOnlySpan<char> AsSpan() => _chars.Slice(0, _pos);
+
+        public ReadOnlySpan<char> AsSpan(int start) => _chars.Slice(start, _pos - start);
+
+        public ReadOnlySpan<char> AsSpan(int start, int length) => _chars.Slice(start, length);
+
+        public void Insert(int index, char value, int count)
+        {
+            if (_pos > _chars.Length - count)
+            {
+                Grow(count);
+            }
+
+            int remaining = _pos - index;
+            _chars.Slice(index, remaining).CopyTo(_chars.Slice(index + count));
+            _chars.Slice(index, count).Fill(value);
+            _pos += count;
+        }
+
+        public void Insert(int index, string? s)
+        {
+            if (s == null)
+            {
+                return;
+            }
+
+            int count = s.Length;
+
+            if (_pos > (_chars.Length - count))
+            {
+                Grow(count);
+            }
+
+            int remaining = _pos - index;
+            _chars.Slice(index, remaining).CopyTo(_chars.Slice(index + count));
+            s
+#if !NET
+                .AsSpan()
+#endif
+                .CopyTo(_chars.Slice(index));
+            _pos += count;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Append(char c)
+        {
+            int pos = _pos;
+            Span<char> chars = _chars;
+            if ((uint)pos < (uint)chars.Length)
+            {
+                chars[pos] = c;
+                _pos = pos + 1;
+            }
+            else
+            {
+                GrowAndAppend(c);
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Append(string? s)
+        {
+            if (s == null)
+            {
+                return;
+            }
+
+            int pos = _pos;
+            // very common case, e.g. appending strings from NumberFormatInfo like separators, percent symbols, etc.
+            if (s.Length == 1 && (uint)pos < (uint)_chars.Length)
+            {
+                _chars[pos] = s[0];
+                _pos = pos + 1;
+            }
+            else
+            {
+                AppendSlow(s);
+            }
+        }
+
+        private void AppendSlow(string s)
+        {
+            int pos = _pos;
+            if (pos > _chars.Length - s.Length)
+            {
+                Grow(s.Length);
+            }
+
+            s
+#if !NET
+                .AsSpan()
+#endif
+                .CopyTo(_chars.Slice(pos));
+            _pos += s.Length;
+        }
+
+        public void Append(char c, int count)
+        {
+            if (_pos > _chars.Length - count)
+            {
+                Grow(count);
+            }
+
+            Span<char> dst = _chars.Slice(_pos, count);
+            for (int i = 0; i < dst.Length; i++)
+            {
+                dst[i] = c;
+            }
+
+            _pos += count;
+        }
+
+        public void Append(scoped ReadOnlySpan<char> value)
+        {
+            int pos = _pos;
+            if (pos > _chars.Length - value.Length)
+            {
+                Grow(value.Length);
+            }
+
+            value.CopyTo(_chars.Slice(_pos));
+            _pos += value.Length;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Span<char> AppendSpan(int length)
+        {
+            int origPos = _pos;
+            if (origPos > _chars.Length - length)
+            {
+                Grow(length);
+            }
+
+            _pos = origPos + length;
+            return _chars.Slice(origPos, length);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void AppendAsLowerInvariant(scoped ReadOnlySpan<char> value)
+        {
+            int pos = _pos;
+            if (pos > _chars.Length - value.Length)
+            {
+                Grow(value.Length);
+            }
+
+            value.ToLowerInvariant(_chars.Slice(_pos));
+            _pos += value.Length;
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void GrowAndAppend(char c)
+        {
+            Grow(1);
+            Append(c);
+        }
+
+        /// <summary>
+        /// Resize the internal buffer either by doubling current buffer size or
+        /// by adding <paramref name="additionalCapacityBeyondPos"/> to
+        /// <see cref="_pos"/> whichever is greater.
+        /// </summary>
+        /// <param name="additionalCapacityBeyondPos">
+        /// Number of chars requested beyond current position.
+        /// </param>
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void Grow(int additionalCapacityBeyondPos)
+        {
+            const uint ArrayMaxLength = 0x7FFFFFC7; // same as Array.MaxLength
+
+            // Increase to at least the required size (_pos + additionalCapacityBeyondPos), but try
+            // to double the size if possible, bounding the doubling to not go beyond the max array length.
+            int newCapacity = (int)Math.Max(
+                (uint)(_pos + additionalCapacityBeyondPos),
+                Math.Min((uint)_chars.Length * 2, ArrayMaxLength));
+
+            // Make sure to let Rent throw an exception if the caller has a bug and the desired capacity is negative.
+            // This could also go negative if the actual required length wraps around.
+            char[] poolArray = ArrayPool<char>.Shared.Rent(newCapacity);
+
+            _chars.Slice(0, _pos).CopyTo(poolArray);
+
+            char[]? toReturn = _arrayToReturnToPool;
+            _chars = _arrayToReturnToPool = poolArray;
+            if (toReturn != null)
+            {
+                ArrayPool<char>.Shared.Return(toReturn);
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Dispose()
+        {
+            char[]? toReturn = _arrayToReturnToPool;
+            this = default; // for safety, to avoid using pooled array if this instance is erroneously appended to again
+            if (toReturn != null)
+            {
+                ArrayPool<char>.Shared.Return(toReturn);
+            }
+        }
+
+        internal void AppendSpanFormattable<T>(T value, string? format = null, IFormatProvider? provider = null)
+            where T : ISpanFormattable
+        {
+            if (value.TryFormat(_chars.Slice(_pos), out int charsWritten, format, provider))
+            {
+                _pos += charsWritten;
+            }
+            else
+            {
+                Append(value.ToString(format, provider));
+            }
+        }
+    }
+}
+#endif