[Feature Request] [mojo-lang] Reflection capabilities for functions in parameter space

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

I don't know if this is viable to implement in the compiler, but I thought I'd write this up in case it can.

# a trait that all functions conform to
trait AnyDef: ...

def do_something[func: AnyDef]():
    comptime for type in parameter_types(func): ...
    comptime for type in argument_types(func): ...
    comptime ret_type = return_type(func)
    ...

Bonus, if we could even treat effects as traits we could maybe build generic code around things like

def do_something[func: AnyDef]():
    comptime if conforms_to(func, thin): ...

What is your motivation for this change?

• modular/modular#6521, part of the problem is that we don't have a way to know when a function's arguments or return type can be trivially converted from cpython directly to Mojo and do a roundtrip through the Python representation
• The potential for this to change how we do generic functional code

Any other details?

I don't have enough examples off the top of by head, but this feels like a feature that will prove very powerful in several applications

I'll add @owenhilyard and @soraros in case they have some input

Start with the Mojo roadmap and the existing compiler support for parameter-space functions, traits, effects, and compile-time introspection; no files or tests are named in the request. Define the supported reflection model and identify the relevant compiler entry points and tests before implementation, with completion requiring agreed behavior for parameter types, argument types, return types, and effects.