Add fast path in string.Concat

src/libraries/Common/tests/Tests/System/StringTests.cs

@@ -317,6 +317,8 @@ void Validate(string result)
             Validate(string.Concat((ReadOnlySpan<string?>)values));
             Validate(string.Concat((IEnumerable<string>)values));
             Validate(string.Concat<string>((IEnumerable<string>)values)); // Call the generic IEnumerable<T>-based overload
+            Validate(string.Concat(values.Select(s => s)));
+            Validate(string.Concat(new List<string?>(values)));
         }
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -152,6 +152,16 @@ public static unsafe string Concat(IEnumerable<string?> values)
         {
             ArgumentNullException.ThrowIfNull(values);
 
+            if (values.GetType() == typeof(List<string?>))
+            {
+                return Concat(CollectionsMarshal.AsSpan((List<string?>)values));
+            }
+
+            if (values is string?[] valuesArray)
+            {
+                return Concat((ReadOnlySpan<string?>)valuesArray);
+            }
+
             using (IEnumerator<string?> en = values.GetEnumerator())
             {
                 if (!en.MoveNext())