From 0ab9fe93f5695a096ba26d62fb24082bd024128f Mon Sep 17 00:00:00 2001
From: Timothy Makkison <timmakkison@gmail.com>
Date: Fri, 15 Sep 2023 11:33:18 +0100
Subject: [PATCH] feat: use `IEnumerable` `Dictionary` constructor

---
 .../DictionaryMappingBuilder.cs               | 31 +++++++++++++
 .../Mapping/DictionaryTest.cs                 | 44 ++-----------------
 2 files changed, 35 insertions(+), 40 deletions(-)

diff --git a/src/Riok.Mapperly/Descriptors/MappingBuilders/DictionaryMappingBuilder.cs b/src/Riok.Mapperly/Descriptors/MappingBuilders/DictionaryMappingBuilder.cs
index c12866fbb4..e2d2a2c30f 100644
--- a/src/Riok.Mapperly/Descriptors/MappingBuilders/DictionaryMappingBuilder.cs
+++ b/src/Riok.Mapperly/Descriptors/MappingBuilders/DictionaryMappingBuilder.cs
@@ -37,6 +37,9 @@ or CollectionType.IDictionary
                 or CollectionType.IReadOnlyDictionary
         )
         {
+            if (TryGetFromEnumerable(ctx, keyMapping, valueMapping) is { } toDictionary)
+                return toDictionary;
+
             var targetDictionarySymbol = ctx.Types.Get(typeof(Dictionary<,>)).Construct(keyMapping.TargetType, valueMapping.TargetType);
             ctx.ObjectFactories.TryFindObjectFactory(ctx.Source, ctx.Target, out var dictionaryObjectFactory);
             return new ForEachSetDictionaryMapping(
@@ -135,6 +138,34 @@ private static (INewInstanceMapping, INewInstanceMapping)? BuildKeyValueMapping(
         return (keyMapping, valueMapping);
     }
 
+    private static INewInstanceMapping? TryGetFromEnumerable(
+        MappingBuilderContext ctx,
+        INewInstanceMapping keyMapping,
+        INewInstanceMapping valueMapping
+    )
+    {
+        if (!keyMapping.IsSynthetic || !valueMapping.IsSynthetic || keyMapping.TargetType.IsNullable())
+            return null;
+
+        // use .NET Core 2+ Dictionary constructor if value and key mapping is synthetic
+        var enumerableType = ctx.Types.Get(typeof(IEnumerable<>));
+        var dictionaryType = ctx.Types.Get(typeof(Dictionary<,>));
+
+        var fromEnumerableCtor = dictionaryType.Constructors.FirstOrDefault(
+            x =>
+                x.Parameters.Length == 1
+                && SymbolEqualityComparer.Default.Equals(((INamedTypeSymbol)x.Parameters[0].Type).ConstructedFrom, enumerableType)
+        );
+
+        if (fromEnumerableCtor != null)
+        {
+            var constructedDictionary = dictionaryType.Construct(keyMapping.TargetType, valueMapping.TargetType);
+            return new CtorMapping(ctx.Source, constructedDictionary);
+        }
+
+        return null;
+    }
+
     private static INamedTypeSymbol? GetExplicitIndexer(MappingBuilderContext ctx)
     {
         if (
diff --git a/test/Riok.Mapperly.Tests/Mapping/DictionaryTest.cs b/test/Riok.Mapperly.Tests/Mapping/DictionaryTest.cs
index a9adfc042f..cacc3c986f 100644
--- a/test/Riok.Mapperly.Tests/Mapping/DictionaryTest.cs
+++ b/test/Riok.Mapperly.Tests/Mapping/DictionaryTest.cs
@@ -24,16 +24,7 @@ public void DictionaryToSameDictionaryDeepCloning()
         TestHelper
             .GenerateMapper(source)
             .Should()
-            .HaveSingleMethodBody(
-                """
-                var target = new global::System.Collections.Generic.Dictionary<string, long>(source.Count);
-                foreach (var item in source)
-                {
-                    target[item.Key] = item.Value;
-                }
-                return target;
-                """
-            );
+            .HaveSingleMethodBody("return new global::System.Collections.Generic.Dictionary<string, long>(source);");
     }
 
     [Fact]
@@ -126,16 +117,7 @@ public void KeyValueEnumerableToIDictionary()
         TestHelper
             .GenerateMapper(source)
             .Should()
-            .HaveSingleMethodBody(
-                """
-                var target = new global::System.Collections.Generic.Dictionary<string, int>();
-                foreach (var item in source)
-                {
-                    target[item.Key] = item.Value;
-                }
-                return target;
-                """
-            );
+            .HaveSingleMethodBody("return new global::System.Collections.Generic.Dictionary<string, int>(source);");
     }
 
     [Fact]
@@ -145,16 +127,7 @@ public void CustomDictionaryToIDictionary()
         TestHelper
             .GenerateMapper(source)
             .Should()
-            .HaveSingleMethodBody(
-                """
-                var target = new global::System.Collections.Generic.Dictionary<string, int>(source.Count);
-                foreach (var item in source)
-                {
-                    target[item.Key] = item.Value;
-                }
-                return target;
-                """
-            );
+            .HaveSingleMethodBody("return new global::System.Collections.Generic.Dictionary<string, int>(source);");
     }
 
     [Fact]
@@ -164,16 +137,7 @@ public void CustomKeyValueListToIDictionary()
         TestHelper
             .GenerateMapper(source)
             .Should()
-            .HaveSingleMethodBody(
-                """
-                var target = new global::System.Collections.Generic.Dictionary<string, int>(source.Count);
-                foreach (var item in source)
-                {
-                    target[item.Key] = item.Value;
-                }
-                return target;
-                """
-            );
+            .HaveSingleMethodBody("return new global::System.Collections.Generic.Dictionary<string, int>(source);");
     }
 
     [Fact]