Better error message for check_receiver_cap

[adri326]

This addresses issue #2986 by:

• giving a local equivalent to the this->x viewpoint adaptation (which is what is printed on the last line of the "Infos" section)
• giving a hint when the this->x viewpoint adaptation turns into a box/val/iso in the current context while also trying to access a ref method

As an example, this is what the error from the code included in the original issue looks like on 0.33.1:

Error:
/_/main.pony:19:8: receiver type is not a subtype of target type
    fox()
       ^
    Info:
    /_/main.pony:19:5: receiver type: this->Fox ref
        fox()
        ^
    /_/main.pony:28:3: target type: Fox ref
      fun ref apply() =>
      ^
    /_/main.pony:10:12: Fox box is not a subtype of Fox ref: box is not a subcap of ref
      let fox: Fox ref

And this is what the error looks like with this patch:

Error:
/_/main.pony:19:8: receiver type is not a subtype of target type
    fox()
       ^
    Info:
    /_/main.pony:19:5: receiver type: this->Fox ref (which becomes 'Fox box' in this context)
        fox()
        ^
    /_/main.pony:28:3: target type: Fox ref
      fun ref apply() =>
      ^
    /_/main.pony:10:12: Fox box is not a subtype of Fox ref: box is not a subcap of ref
      let fox: Fox ref
               ^
    /_/main.pony:17:3: you are trying to change state in a box function; this would be possible in a ref function
      fun apply() =>
      ^

[SeanTAllen]

@adri326 can you update the PR title to be something that would be meaningful in a CHANGELOG entry?

[adri326]

@SeanTAllen there.

[SeanTAllen]

Thanks @adri326.

[mfelsche]

Nice one!

[SeanTAllen]

Thanks for the improvement @adri326.