Use manual foreach instead of AddRange method for certain collect…

…ion expression spreads (#74630) * Use manual `foreach` instead of `AddRange` method when collection type of spreading element doesn't implement `ICollection<T>`, but has a `struct` enumerator
dotnet · Aug 14, 2024 · e1496f4 · e1496f4
1 parent 74270d4
commit e1496f4
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diff --git a/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs b/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs
@@ -1067,6 +1067,21 @@ private BoundExpression CreateAndPopulateList(BoundCollectionExpression node, Ty
                         if (addRangeMethod is null)
                             return false;
 
+                        if (spreadElement.EnumeratorInfoOpt is { } enumeratorInfo)
+                        {
+                            var iCollectionOfTType = _compilation.GetSpecialType(SpecialType.System_Collections_Generic_ICollection_T);
+                            var iCollectionOfElementType = iCollectionOfTType.Construct(enumeratorInfo.ElementType);
+                            var discardedUseSiteInfo = CompoundUseSiteInfo<AssemblySymbol>.Discarded;
+
+                            // If collection has a struct enumerator but doesn't implement ICollection<T>
+                            // then manual `foreach` is always more efficient then using `AddRange` method
+                            if (enumeratorInfo.GetEnumeratorInfo.Method.ReturnType.IsValueType &&
+                                !enumeratorInfo.CollectionType.ImplementsInterface(iCollectionOfElementType, ref discardedUseSiteInfo))
+                            {
+                                return false;
+                            }
+                        }
+
                         var type = rewrittenSpreadOperand.Type!;
 
                         var useSiteInfo = GetNewCompoundUseSiteInfo();