Fix make-readonly with spans.

src/Analyzers/CSharp/Analyzers/MakeStructMemberReadOnly/CSharpMakeStructMemberReadOnlyAnalyzer.cs

@@ -14,7 +14,6 @@
 using Microsoft.CodeAnalysis.Operations;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Extensions;
-using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp.MakeStructMemberReadOnly;
 
@@ -331,6 +330,10 @@ private static bool OperationPotentiallyMutatesThis(
 
             if (operation is IInlineArrayAccessOperation)
             {
+                // to determine if this is safe to make into a ReadOnlySpan or not.
+                if (operation.Type.IsSpan())
+                    return true;
+
                 // If we're writing into an inline-array off of 'this'.  Then we can't make this `readonly`.
                 if (CSharpSemanticFacts.Instance.IsWrittenTo(semanticModel, operation.Syntax, cancellationToken))
                     return true;

src/Analyzers/CSharp/Tests/MakeStructMemberReadOnly/MakeStructMemberReadOnlyTests.cs

@@ -2345,4 +2345,33 @@ public int Z
             LanguageVersion = LanguageVersion.CSharp12,
             ReferenceAssemblies = ReferenceAssemblies.Net.Net80,
         }.RunAsync();
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/79384")]
+    public Task TestSpanFromInlineArray()
+        => new VerifyCS.Test
+        {
+            TestCode = """
+                using System.Runtime.CompilerServices;
+
+                public struct Example
+                {
+                    private Buffer _buffer;
+
+                    public void Set(int index, int value)
+                    {
+                        var bufferSpan = _buffer[..0];
+
+                        bufferSpan[0] = value;
+                    }
+                }
+
+                [InlineArray(1)]
+                public struct Buffer
+                {
+                    private int _element0;
+                }
+                """,
+            LanguageVersion = LanguageVersion.CSharp13,
+            ReferenceAssemblies = ReferenceAssemblies.Net.Net90,
+        }.RunAsync();
 }