Fix 'use var' with spans (#79348)

Author: CyrusNajmabadi

src/Analyzers/CSharp/Tests/UseImplicitOrExplicitType/UseImplicitTypeTests.cs

@@ -2097,9 +2097,11 @@ static void M()
             }
             """, new TestParameters(options: ImplicitTypeEverywhere()));
 
-    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/22768")]
-    public Task DoNotSuggestVarOnStackAllocExpressions_SpanType_NestedConditional()
-        => TestMissingInRegularAndScriptAsync("""
+    [Fact]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/22768")]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/79337")]
+    public Task DoSuggestVarOnStackAllocExpressions_SpanType_NestedConditional()
+        => TestInRegularAndScript1Async("""
             using System;
             namespace System
             {
@@ -2115,11 +2117,29 @@ static void M(bool choice)
                     [|Span<int>|] x = choice ? stackalloc int [10] : stackalloc int [100];
                 }
             }
+            """, """
+            using System;
+            namespace System
+            {
+                public readonly ref struct Span<T> 
+                {
+                    unsafe public Span(void* pointer, int length) { }
+                }
+            }
+            class C
+            {
+                static void M(bool choice)
+                {
+                    var x = choice ? stackalloc int [10] : stackalloc int [100];
+                }
+            }
             """, new TestParameters(options: ImplicitTypeEverywhere()));
 
-    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/22768")]
-    public Task DoNotSuggestVarOnStackAllocExpressions_SpanType_NestedCast()
-        => TestMissingInRegularAndScriptAsync("""
+    [Fact]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/22768")]
+    [WorkItem("https://github.com/dotnet/roslyn/issues/79337")]
+    public Task DoSuggestVarOnStackAllocExpressions_SpanType_NestedCast()
+        => TestInRegularAndScript1Async("""
             using System;
             namespace System
             {
@@ -2135,6 +2155,22 @@ static void M()
                     [|Span<int>|] x = (Span<int>)stackalloc int [100];
                 }
             }
+            """, """
+            using System;
+            namespace System
+            {
+                public readonly ref struct Span<T> 
+                {
+                    unsafe public Span(void* pointer, int length) { }
+                }
+            }
+            class C
+            {
+                static void M()
+                {
+                    var x = (Span<int>)stackalloc int [100];
+                }
+            }
             """, new TestParameters(options: ImplicitTypeEverywhere()));
 
     [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/22768")]

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Utilities/TypeStyle/CSharpUseImplicitTypeHelper.cs

@@ -122,18 +122,14 @@ SyntaxKind.ForStatement or
             if (variableDeclaration.Variables is not [{ Initializer.Value: var initializer } variable])
                 return false;
 
-            // Do not suggest var replacement for stackalloc span expressions.
-            // This will change the bound type from a span to a pointer.
-            if (!variableDeclaration.Type.IsKind(SyntaxKind.PointerType))
+            // Do not suggest var replacement for stackalloc span expressions. This will change the bound type from a
+            // span to a pointer.  Note: this only applies to `var v = stackalloc ...;`  If `stackalloc` is anywhere
+            // lower (including `var v = (stackalloc ...);`), then this is will be a span, and it will be ok to change
+            // to use 'var'.
+            if (!variableDeclaration.Type.IsKind(SyntaxKind.PointerType) &&
+                initializer is StackAllocArrayCreationExpressionSyntax)
             {
-                var containsStackAlloc = initializer
-                    .DescendantNodesAndSelf(descendIntoChildren: node => node is not AnonymousFunctionExpressionSyntax)
-                    .Any(node => node.IsKind(SyntaxKind.StackAllocArrayCreationExpression));
-
-                if (containsStackAlloc)
-                {
-                    return false;
-                }
+                return false;
             }
 
             if (AssignmentSupportsStylePreference(