Added optimized IMemoryGroup<T>.GetEnumerator() method

src/ImageSharp/Memory/DiscontiguousBuffers/IMemoryGroup{T}.cs

@@ -33,5 +33,15 @@ namespace SixLabors.ImageSharp.Memory
         /// the image buffers internally.
         /// </remarks>
         bool IsValid { get; }
+
+        /// <summary>
+        /// Returns a value-type implementing an allocation-free enumerator of the memory groups in the current
+        /// instance. The return type shouldn't be used directly: just use a <see langword="foreach"/> block on
+        /// the <see cref="IMemoryGroup{T}"/> instance in use and the C# compiler will automatically invoke this
+        /// method behind the scenes. This method takes precedence over the <see cref="IEnumerable{T}.GetEnumerator"/>
+        /// implementation, which is still available when casting to one of the underlying interfaces.
+        /// </summary>
+        /// <returns>A new <see cref="MemoryGroupEnumerator{T}"/> instance mapping the current <see cref="Memory{T}"/> values in use.</returns>
+        new MemoryGroupEnumerator<T> GetEnumerator();
     }
 }

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroupView{T}.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers;
 using System.Collections;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
@@ -37,6 +38,7 @@ namespace SixLabors.ImageSharp.Memory
 
         public int Count
         {
+            [MethodImpl(InliningOptions.ShortMethod)]
             get
             {
                 this.EnsureIsValid();
@@ -73,7 +75,15 @@ namespace SixLabors.ImageSharp.Memory
             }
         }
 
-        public IEnumerator<Memory<T>> GetEnumerator()
+        /// <inheritdoc/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public MemoryGroupEnumerator<T> GetEnumerator()
+        {
+            return new MemoryGroupEnumerator<T>(this);
+        }
+
+        /// <inheritdoc/>
+        IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
         {
             this.EnsureIsValid();
             for (int i = 0; i < this.Count; i++)
@@ -82,7 +92,8 @@ namespace SixLabors.ImageSharp.Memory
             }
         }
 
-        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+        /// <inheritdoc/>
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<Memory<T>>)this).GetEnumerator();
 
         internal void Invalidate()
         {

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.Consumed.cs

@@ -3,13 +3,16 @@
 
 using System;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
     internal abstract partial class MemoryGroup<T>
     {
-        // Analogous to the "consumed" variant of MemorySource
+        /// <summary>
-        private sealed class Consumed : MemoryGroup<T>
+        /// A <see cref="MemoryGroup{T}"/> implementation that consumes the underlying memory buffers.
+        /// </summary>
+        public sealed class Consumed : MemoryGroup<T>, IEnumerable<Memory<T>>
         {
             private readonly Memory<T>[] source;
 
@@ -20,11 +23,23 @@ namespace SixLabors.ImageSharp.Memory
                 this.View = new MemoryGroupView<T>(this);
             }
 
-            public override int Count => this.source.Length;
+            public override int Count
+            {
+                [MethodImpl(InliningOptions.ShortMethod)]
+                get => this.source.Length;
+            }
 
             public override Memory<T> this[int index] => this.source[index];
 
-            public override IEnumerator<Memory<T>> GetEnumerator()
+            /// <inheritdoc/>
+            [MethodImpl(InliningOptions.ShortMethod)]
+            public override MemoryGroupEnumerator<T> GetEnumerator()
+            {
+                return new MemoryGroupEnumerator<T>(this);
+            }
+
+            /// <inheritdoc/>
+            IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
             {
                 /* The runtime sees the Array class as if it implemented the
                  * type-generic collection interfaces explicitly, so here we

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.Owned.cs

@@ -9,10 +9,12 @@ using System.Runtime.CompilerServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
-    // Analogous to the "owned" variant of MemorySource
     internal abstract partial class MemoryGroup<T>
     {
-        private sealed class Owned : MemoryGroup<T>
+        /// <summary>
+        /// A <see cref="MemoryGroup{T}"/> implementation that owns the underlying memory buffers.
+        /// </summary>
+        public sealed class Owned : MemoryGroup<T>, IEnumerable<Memory<T>>
         {
             private IMemoryOwner<T>[] memoryOwners;
 
@@ -30,6 +32,7 @@ namespace SixLabors.ImageSharp.Memory
 
             public override int Count
             {
+                [MethodImpl(InliningOptions.ShortMethod)]
                 get
                 {
                     this.EnsureNotDisposed();
@@ -46,7 +49,15 @@ namespace SixLabors.ImageSharp.Memory
                 }
             }
 
-            public override IEnumerator<Memory<T>> GetEnumerator()
+            /// <inheritdoc/>
+            [MethodImpl(InliningOptions.ShortMethod)]
+            public override MemoryGroupEnumerator<T> GetEnumerator()
+            {
+                return new MemoryGroupEnumerator<T>(this);
+            }
+
+            /// <inheritdoc/>
+            IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
             {
                 this.EnsureNotDisposed();
                 return this.memoryOwners.Select(mo => mo.Memory).GetEnumerator();
@@ -70,7 +81,7 @@ namespace SixLabors.ImageSharp.Memory
                 this.IsValid = false;
             }
 
-            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            [MethodImpl(InliningOptions.ShortMethod)]
             private void EnsureNotDisposed()
             {
                 if (this.memoryOwners is null)

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.cs

@@ -6,7 +6,6 @@ using System.Buffers;
 using System.Collections;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
-using SixLabors.ImageSharp.Memory.Internals;
 
 namespace SixLabors.ImageSharp.Memory
 {
@@ -48,10 +47,21 @@ namespace SixLabors.ImageSharp.Memory
         public abstract void Dispose();
 
         /// <inheritdoc />
-        public abstract IEnumerator<Memory<T>> GetEnumerator();
+        public abstract MemoryGroupEnumerator<T> GetEnumerator();
 
         /// <inheritdoc />
-        IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+        IEnumerator<Memory<T>> IEnumerable<Memory<T>>.GetEnumerator()
+        {
+            /* This method is implemented in each derived class.
+             * Implementing the method here as non-abstract and throwing,
+             * then reimplementing it explicitly in each derived class, is
+             * a workaround for the lack of support for abstract explicit
+             * interface method implementations in C#. */
+            throw new NotImplementedException($"The type {this.GetType()} needs to override IEnumerable<Memory<T>>.GetEnumerator()");
+        }
+
+        /// <inheritdoc />
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<Memory<T>>)this).GetEnumerator();
 
         /// <summary>
         /// Creates a new memory group, allocating it's buffers with the provided allocator.