fix: map existing target sets correctly

src/Riok.Mapperly/Descriptors/MappingBuilders/EnumerableMappingBuilder.cs

@@ -87,10 +87,16 @@ public static class EnumerableMappingBuilder
             return CreateForEach(nameof(Queue<object>.Enqueue));
 
         // create a foreach loop with add calls if source is not an array
-        // and  ICollection.Add(T): void is implemented and not explicit
+        // and void ICollection.Add(T) or bool ISet.Add(T) is implemented and not explicit
         // ensures add is not called and immutable types
-        if (!ctx.Target.IsArrayType() && ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ICollection<>)), AddMethodName))
+        // ISet.Add(T) is explicitly needed as sets implement the ICollection.Add(T) explicit,
+        // and override the add method with new
+        var hasImplicitCollectionAdd = ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ICollection<>)), AddMethodName);
+        var hasImplicitSetAdd = ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ISet<>)), AddMethodName);
+        if (!ctx.Target.IsArrayType() && (hasImplicitCollectionAdd || hasImplicitSetAdd))
+        {
             return CreateForEach(AddMethodName);
+        }
 
         // if a mapping could be created for an immutable collection
         // we diagnostic when it is an existing target mapping
@@ -167,22 +173,22 @@ private static LinqConstructorMapping BuildLinqConstructorMapping(MappingBuilder
         }
 
         // create a foreach loop with add calls if source is not an array
-        // and  ICollection.Add(T): void is implemented and not explicit
-        // ensures add is not called and immutable types
-        if (!ctx.Target.IsArrayType() && ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ICollection<>)), AddMethodName))
-        {
-            var ensureCapacityStatement = EnsureCapacityBuilder.TryBuildEnsureCapacity(ctx.Source, ctx.Target, ctx.Types);
-            return new ForEachAddEnumerableMapping(
-                ctx.Source,
-                ctx.Target,
-                elementMapping,
-                objectFactory,
-                AddMethodName,
-                ensureCapacityStatement
-            );
-        }
+        // and void ICollection.Add(T) or bool ISet.Add(T) is implemented and not explicit
+        // ensures .Add() is not called on immutable types
+        var hasImplicitCollectionAdd = ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ICollection<>)), AddMethodName);
+        var hasImplicitSetAdd = ctx.Target.HasImplicitGenericImplementation(ctx.Types.Get(typeof(ISet<>)), AddMethodName);
+        if (ctx.Target.IsArrayType() || (!hasImplicitCollectionAdd && !hasImplicitSetAdd))
+            return null;
 
-        return null;
+        var ensureCapacityStatement = EnsureCapacityBuilder.TryBuildEnsureCapacity(ctx.Source, ctx.Target, ctx.Types);
+        return new ForEachAddEnumerableMapping(
+            ctx.Source,
+            ctx.Target,
+            elementMapping,
+            objectFactory,
+            AddMethodName,
+            ensureCapacityStatement
+        );
     }
 
     private static (bool CanMapWithLinq, string? CollectMethod) ResolveCollectMethodName(

src/Riok.Mapperly/Helpers/SymbolExtensions.cs

@@ -153,17 +153,20 @@ out bool isExplicit
         }
 
         var interfaceSymbol = typedInterface.GetMembers(symbolName).First();
-
-        var symbolImplementaton = t.FindImplementationForInterfaceMember(interfaceSymbol);
+        var symbolImplementation = t.FindImplementationForInterfaceMember(interfaceSymbol);
 
         // if null then the method is unimplemented
         // symbol implements genericInterface but has not implemented the corresponding methods
-        // this can only occur in unit tests
-        if (symbolImplementaton == null)
-            throw new NotSupportedException("Symbol implementation cannot be null for objects implementing interface.");
+        // this is for example the case for arrays (arrays implement several interfaces at runtime)
+        // and unit tests for which not the full interface is implemented
+        if (symbolImplementation == null)
+        {
+            isExplicit = false;
+            return false;
+        }
 
         // check if symbol is explicit
-        isExplicit = symbolImplementaton switch
+        isExplicit = symbolImplementation switch
         {
             IMethodSymbol methodSymbol => methodSymbol.ExplicitInterfaceImplementations.Any(),
             IPropertySymbol propertySymbol => propertySymbol.ExplicitInterfaceImplementations.Any(),

test/Riok.Mapperly.Tests/Mapping/EnumerableSetTest.cs

@@ -0,0 +1,59 @@
+namespace Riok.Mapperly.Tests.Mapping;
+
+public class EnumerableSetTest
+{
+    [Fact]
+    public void ExistingEnumerableToExistingSet()
+    {
+        var source = TestSourceBuilder.Mapping(
+            "A",
+            "B",
+            "class A { public IEnumerable<int> Values { get; } }",
+            "class B { public ISet<int> Values { get; } }"
+        );
+        TestHelper
+            .GenerateMapper(source)
+            .Should()
+            .HaveSingleMethodBody(
+                """
+                var target = new global::B();
+                foreach (var item in source.Values)
+                {
+                    target.Values.Add(item);
+                }
+
+                return target;
+                """
+            );
+    }
+
+    [Fact]
+    public void ExistingEnumerableToExistingHashSet()
+    {
+        var source = TestSourceBuilder.Mapping(
+            "A",
+            "B",
+            "class A { public IEnumerable<int> Values { get; } }",
+            "class B { public HashSet<int> Values { get; } }"
+        );
+        TestHelper
+            .GenerateMapper(source)
+            .Should()
+            .HaveSingleMethodBody(
+                """
+                var target = new global::B();
+                if (global::System.Linq.Enumerable.TryGetNonEnumeratedCount(source.Values, out var sourceCount))
+                {
+                    target.Values.EnsureCapacity(sourceCount + target.Values.Count);
+                }
+
+                foreach (var item in source.Values)
+                {
+                    target.Values.Add(item);
+                }
+
+                return target;
+                """
+            );
+    }
+}

test/Riok.Mapperly.Tests/Mapping/EnumerableTest.cs

@@ -671,9 +671,9 @@ public void EnumerableToReadOnlyArrayPropertyShouldDiagnostic()
             .Should()
             .HaveSingleMethodBody(
                 """
-            var target = new global::B();
-            return target;
-            """
+                var target = new global::B();
+                return target;
+                """
             )
             .HaveDiagnostic(DiagnosticDescriptors.CannotMapToReadOnlyMember);
     }