Improve caching in XmlToDescriptionGenerator (#1010)

Author: stephentoub

src/ModelContextProtocol.Analyzers/EquatableArray.cs

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections;
+
+namespace ModelContextProtocol.Analyzers;
+
+/// <summary>An immutable, equatable array.</summary>
+/// <typeparam name="T">The type of values in the array.</typeparam>
+internal readonly struct EquatableArray<T> : IEnumerable<T>, IEquatable<EquatableArray<T>>
+{
+    /// <summary>The underlying <typeparamref name="T"/> array.</summary>
+    private readonly T[]? _array;
+
+    /// <param name="source">The source to enumerate and wrap.</param>
+    public EquatableArray(IEnumerable<T> source) => _array = source.ToArray();
+
+    /// <param name="source">The source to wrap.</param>
+    public EquatableArray(T[] array) => _array = array;
+
+    /// <summary>Gets a reference to an item at a specified position within the array.</summary>
+    /// <param name="index">The index of the item to retrieve a reference to.</param>
+    /// <returns>A reference to an item at a specified position within the array.</returns>
+    public ref readonly T this[int index] => ref NonNullArray[index];
+
+    /// <summary>Gets the backing array.</summary>
+    private T[] NonNullArray => _array ?? [];
+
+    /// <summary>Gets the length of the current array.</summary>
+    public int Length => NonNullArray.Length;
+
+    /// <inheritdoc/>
+    public bool Equals(EquatableArray<T> other) => NonNullArray.SequenceEqual(other.NonNullArray);
+
+    /// <inheritdoc/>
+    public override bool Equals(object? obj) =>  obj is EquatableArray<T> array && Equals(array);
+
+    /// <inheritdoc/>
+    public override int GetHashCode()
+    {
+        int hash = 17;
+        foreach (T item in NonNullArray)
+        {
+            hash = hash * 31 + (item?.GetHashCode() ?? 0);
+        }
+
+        return hash;
+    }
+
+    /// <inheritdoc/>
+    IEnumerator<T> IEnumerable<T>.GetEnumerator() => ((IEnumerable<T>)NonNullArray).GetEnumerator();
+
+    /// <inheritdoc/>
+    IEnumerator IEnumerable.GetEnumerator() => NonNullArray.GetEnumerator();
+}