[Feature Request] `where` clauses on trait methods

Review Mojo's priorities

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

What is your request?

Allow the following:

trait Trait1:
     comptime OpIsAllowedWith[OtherType: Trait1]: Bool

     def do_operation[OtherType: Trait1](self, other: OtherType) where Self.OpIsAllowedWith[OtherType]:
         ...

@fieldwise_init
struct Struct1(Trait1):
     comptime OpIsAllowedWith[OtherType: Trait1] = Self == OtherType
     
     def do_operation[OtherType: Trait1](self, other: OtherType) where Self.OpIsAllowedWith[OtherType]:
         ...

@fieldwise_init
struct Struct2(Trait1):
      comptime OpIsAllowedWith[OtherType: Trait1] = OtherType == Self or OtherType == Struct3

      def do_operation[OtherType: Trait1](self, other: OtherType) where Self.OpIsAllowedWith[OtherType]:
         ...

@fieldwise_init
struct Struct3(Trait1):
       comptime OpIsAllowedWith[OtherType: Trait1] = OtherType == Self or OtherType == Struct2

      def do_operation[OtherType: Trait1](self, other: OtherType) where Self.OpIsAllowedWith[OtherType]:
         ...

def main():
     var s1_0 = Struct1()
     var s1_1 = Struct1()
     var s2 = Struct2()
     var s3 = Struct3()

     s1_0.do_operation(s1_1) 
     s1_0.do_operation(s2) # compile-time error
     s2.do_operation(s2)
     s2.do_operation(s3)
     s2.do_operation(s1) # compile-time error
     s3.do_operation(s3)

What is your motivation for this change?

This provides a workaround for one of the major use-cases of parametric traits, which is expressing "one to many" type relationships. The current way to do this more or less involves a "please stick this code in the function body" comment on the trait, which isn't great.

In particular, I'd really like this feature for:

• The IO API https://github.com/modular/modular/pull/4728
• Hardware abstraction (RISC-V especially)
• A few extra bits of stdlib complexity moved into plugin hooks

Any other details?

No response