Thank you for the contribution. Before I dive into a detailed review, I think we should first align on the intended behavior.
As far as I can see, the current approach doesn’t work with existing target mappings (and [MappingTarget]), right? See Riok.Mapperly.Tests.Mapping.UserMethodAdditionalParametersTest.ExistingTarget...

At the moment, Mapperly matches additional mapping parameters by name. I believe we should also match them by name when passing them to downstream mapping methods.

How does your approach handle the following scenario:

record A(ANested Nested);
record B(BNested Nested);
record ANested(int ValueA);
record BNested(int ValueB, int Ctx);

public partial B Map(A src, int ctx);

Does this automatically generate private BNested MapToB(B src, int ctx) and pass the ctx along?
How does this work if there are multiple levels of nesting with an in-between level which is user-defined without the additional parameters?

@latonz All of these are valid use cases. I will investigate and fix them.
My implementation was mostly about passing parameters to [MapProperty] methods and using the correct Map method for the Project method with parameters.
Or maybe these mappings can be fixed in a separate PR?

These issues can be addressed in a separate PR (which might make more sense, otherwise this one could become too big). However, the overall concept of how everything should work needs to be clarified first.

However, the overall concept of how everything should work needs to be clarified first.

Sure.

At the moment, Mapperly matches additional mapping parameters by name. I believe we should also match them by name when passing them to downstream mapping methods.

In this PR, I match parameters by name and type. I think, it would make sense to match them only by name and map them if needed.

As far as I can see, the current approach doesn’t work with existing target mappings (and [MappingTarget]), right?

I didn't consider [MappingTarget]. But it would make sense to support it.

Does this automatically generate private BNested MapToB(B src, int ctx) and pass the ctx along?

Currently, this functionality does not work. And I think, it would be extremely tricky to do this automatically. Maybe it is better to only support this when mapping methods are manually defined.

Currently, some diagnostics are incorrect. I'll fix those.

Hi, thank you for opening this. I am also looking forward to this feature.

When I was thinking about how to implement this, I thought to support only existing "extended" mapping:

record A(ANested Nested);
record B(BNested Nested);
record ANested(int ValueA);
record BNested(int ValueB, int Ctx);

public partial B Map(A src, int ctx);
private partial BNested Map(ANested src, int ctx); // <-- this is mandatory

If there is no BNested Map(ANested src, int ctx); the B Map(A src, int ctx) does not try to pass the parameter to the nested objects and simply reports "no target for ctx". I find such an automated search a bit dangerous.

Sounds good to me:

• Match by name
• Don't generate methods with additional parameters (only user-defined methods with additional parameters are supported)
• Support existing target mappings and [MappingTarget]

I’ve converted this PR to a draft. Please mark it as ready for review once it’s complete and request a review from me. Thank you!

Sorry, I don't have time to continue with this right now.
And it needs a different approach anyway. With the current approach, there is no easy way to set a mapping parameter as used.
I'm closing this for now. I will open a new PR, if I find time to work on this more.

Thank you for your efforts anyway!