Implement SeparatedSyntaxList.Count(Func<T, bool>) (#78348)

CSharpAttributeData.IsTargetEarlyAttribute was calling the Linq version of this list, and thus boxing the SeparatedSyntaxList enumerator, showing up as 5.9 MB (0.1%) of allocations in our OOP process during solution load.

Not a huge win, but it's nice to add to the pit of success for using this type.

Modifications to CSharpAttributeData started with a small cleanup to IsTargetEarlyAttribute to demonstrate the Count() usage. I then looked to see what other Linq usage was in the file, and it made sense to change those too.

See PR for image with defails about allocations being addressed.

Author: ToddGrun

src/Compilers/CSharp/Portable/Symbols/Attributes/AttributeData.cs

@@ -6,16 +6,14 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
-using System.Linq;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Text;
-using System.Reflection;
 using Microsoft.CodeAnalysis.CodeGen;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
-using Microsoft.CodeAnalysis;
-using System.Diagnostics.CodeAnalysis;
 
 namespace Microsoft.CodeAnalysis.CSharp.Symbols
 {
@@ -90,7 +88,7 @@ internal static bool IsTargetEarlyAttribute(NamedTypeSymbol attributeType, Attri
             Debug.Assert(!attributeType.IsErrorType());
 
             int argumentCount = (attributeSyntax.ArgumentList != null) ?
-                attributeSyntax.ArgumentList.Arguments.Count<AttributeArgumentSyntax>((arg) => arg.NameEquals == null) :
+                attributeSyntax.ArgumentList.Arguments.Count(static (arg) => arg.NameEquals == null) :
                 0;
             return AttributeData.IsTargetEarlyAttribute(attributeType, argumentCount, description);
         }
@@ -138,7 +136,7 @@ internal bool IsSecurityAttribute(CSharpCompilation compilation)
             {
                 string className = this.AttributeClass.ToDisplayString(SymbolDisplayFormat.TestFormat);
 
-                if (!this.CommonConstructorArguments.Any() & !this.CommonNamedArguments.Any())
+                if (this.CommonConstructorArguments.IsEmpty && this.CommonNamedArguments.IsEmpty)
                 {
                     return className;
                 }
@@ -343,7 +341,7 @@ private DeclarativeSecurityAction DecodeSecurityAttributeAction(Symbol targetSym
             Debug.Assert(this.IsSecurityAttribute(compilation));
 
             var ctorArgs = this.CommonConstructorArguments;
-            if (!ctorArgs.Any())
+            if (ctorArgs.IsEmpty)
             {
                 // NOTE:    Security custom attributes must have a valid SecurityAction as its first argument, we have none here.
                 // NOTE:    Ideally, we should always generate 'CS7048: First argument to a security attribute must be a valid SecurityAction' for this case.
@@ -365,7 +363,7 @@ private DeclarativeSecurityAction DecodeSecurityAttributeAction(Symbol targetSym
             }
             else
             {
-                TypedConstant firstArg = ctorArgs.First();
+                TypedConstant firstArg = ctorArgs[0];
                 var firstArgType = (TypeSymbol?)firstArg.TypeInternal;
                 if (firstArgType is object && firstArgType.Equals(compilation.GetWellKnownType(WellKnownType.System_Security_Permissions_SecurityAction)))
                 {

src/Compilers/Core/Portable/Syntax/SeparatedSyntaxListExtensions.cs

@@ -0,0 +1,27 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace Microsoft.CodeAnalysis
+{
+    internal static class SeparatedSyntaxListExtensions
+    {
+        internal static int Count<TNode>(this SeparatedSyntaxList<TNode> list, Func<TNode, bool> predicate)
+            where TNode : SyntaxNode
+        {
+            int n = list.Count;
+            int count = 0;
+            for (int i = 0; i < n; i++)
+            {
+                if (predicate(list[i]))
+                {
+                    count++;
+                }
+            }
+
+            return count;
+        }
+    }
+}