More informative error for mixed product arrays

[rtjoa]

Give a more informative error message for product arrays whose elements are neither scannable nor external.

Old:

1 | let f_bad (x : #(string * float#)) = make_vect 42 x
                                         ^^^^^^^^^^^^^^
Error: Unboxed product array elements must be external or contain all gc
       scannable types. The product type this function is applied at is
       not external but contains an element of sort float64.

New:

1 | let f_bad (x : #(string * float#)) = make_vect 42 x
                                         ^^^^^^^^^^^^^^
Error: An unboxed product array element must be formed from all
       external types (which are ignored by the gc) or all gc-scannable types.
       But this array operation is peformed for an array whose
       element type is #(string * float#), which is an unboxed product
       that is not external and contains a type with the non-scannable
       layout float64.
       Hint: if the array contents should not be scanned, annotating
       contained abstract types as [mod external] may resolve this error.

[ccasin]

I'll just chime in to say I think I prefer "external" to "non-gc". The former is something with a concrete meaning - it appears in kinds, and that is what we are checking. It will also soon appear in modes. The latter is not used elsewhere, and seems imprecise to me.

Notably, this error message should not be trying to "contrast" these things at all. There is overlap (immediate) on purpose

[goldfirere]

OK. I'm fine with keeping external. But bonus points if the message can be phrased in a way that someone who doesn't know what that means can figure it out from context.

[ccasin]

OK. I'm fine with keeping external. But bonus points if the message can be phrased in a way that someone who doesn't know what that means can figure it out from context.

I think you are suggesting adding a hint or something defining external, which sounds like a reasonable idea to me!

[mshinwell]

The whole point of my original comment here was that "external" seems confusing. I'd like to discuss this further.