PM> Install-Package ExpressionDebugger

This plugin allows you to perform step-into debugging using Roslyn!

Then add following code on start up (or anywhere before mapping is compiled)

TypeAdapterConfig.GlobalSettings.Compiler = exp => exp.CompileWithDebugInfo();

Now in your mapping code (only in DEBUG mode).

var dto = poco.Adapt<SimplePoco, SimpleDto>(); //<--- you can step-into this function!!

private, protected and internal aren't allowed in debug mode.

We can also see how Mapster generates mapping logic with ToScript method.

var script = poco.BuildAdapter()
                .CreateMapExpression<SimpleDto>()
                .ToScript();

To step-into debugging, you might need to emit file

var opt = new ExpressionCompilationOptions { EmitFile = true };
TypeAdapterConfig.GlobalSettings.Compiler = exp => exp.CompileWithDebugInfo(opt);
...
var dto = poco.Adapt<SimplePoco, SimpleDto>(); //<-- you can step-into this function!!

In RELEASE mode, Roslyn compiler is actually faster than default dynamic compilation by 2x. Here is the result: