Don't use named argument key names as parameters for method_missing calls

[HertzDevil]

Resolves #10381 through #10381 (comment).

method_missing is used in the standard library only by forward_missing_to, which in turn is used only by HTTP::Headers. The specs for it don't exercise this capability beyond calling HTTP::Headers#same?, which can be rewritten without any implicit delegation, and which doesn't have named arguments anyway (it uses #object_id). So the standard library most certainly won't break, but external libraries that refer to call arguments using {{ call.named_args[...].name }} will.

[asterite]

Thank you! ❤️

If it isn't too much work... could you briefly explain what does this change and why? I didn't have time to follow the entire conversation in #10381 which covers a lot of topics.

I guess this is also a breaking change?

From the description I still don't understand why one would need to change name to value when name was working fine before.

[HertzDevil]

The PR makes the named parameters of method_missing's generated defs use _named_arg#{i} instead of copying them from the key names.

The main reasons are:

• A named parameter's external name doesn't need to be a valid identifier, but its internal one must be:

  def foo(*, "a b" a b); end         # unusable inside `method_missing`
  def foo(*, "a b" _named_arg0); end # okay
  
  foo("a b": 0)

• Allowing implicit access to the named arguments by name can potentially shadow other calls:

  class Foo
    def to
      1
    end
  
    forward_missing_to to
  end
  
  Foo.new.upto(to: 3).to_a # => [3]
  Foo.new.upto(3).to_a     # => [1, 2, 3]

  If the internal parameter names for the generated def start with an underscore, as the positional and block parameters already do, then the chances of collision are greatly reduced because these names are reserved by Crystal.
• Ruby's method_missing also accesses the call's named arguments through a double splat parameter, and that parameter's values, not keys, are the actual arguments.

[asterite]

@HertzDevil Thank you again! 💙