Report a diagnostic if the source and target types are equal but the method has MapProperty attributes and deep cloning is disabled

[Shiko1st]

Let's imagine the following entity:

public class Entity
{
    public int Id { get; set; }
    public int Prop { get; set; }
}

and the following mapping class:

[Mapper]
public static class Mapper
{
    // 1
    public static partial Entity Clone(this Entity source);

    // 2
    [MapperIgnoreSource(nameof(Entity.Id))]
    [MapperIgnoreTarget(nameof(Entity.Id))]
    public static partial Entity CloneWithoutId(this Entity source);
}

Actual relevant generated code

Both (1) and (2) methods after generation look in the same way. Attributes on the second method are totally ignored. Both methods return the same instance, their code looks like this:

[global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.2.1.0")]
public static partial global::MyNameSpace.Entity CloneWithoutId(this global::MyNameSpace.Entity source)
{
    return source;
}

Expected relevant generated code

Expected behaviour: at least the second method returns a new instance of Entity, something like this:

[global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.2.1.0")]
public static partial global::MyNameSpace.Entity CloneWithoutId(this global::MyNameSpace.Entity source)
{
    return new Entity
    {
        Prop = source.Prop,
    };
}

Environment (please complete the following information):

• Mapperly Version: 4.2.1
• Nullable reference types: enabled
• .NET Version: 9.0.302
• Target Framework: net9.0
• Compiler Version: 4.14.0-3.25279.5 (995f12b6)
• C# Language Version: 13.0
• IDE: Rider 2025.1.3
• OS: Win10

[latonz]

This is expected behaviour, but I see that the DX here could be improved by reporting a diagnostic that a configuration was applied to a method which will not create a mapping.

To actually clone the entity you need to apply deep cloning: see docs.