Pre-RFC: User-provided default field values

[steffahn]

@atagunov

The way it's been formulated is giving me a vague feeling something is not right. But I cannot to put a finger on it. If re-formulated to simple "if"-s it will be easier to work with I think.

For the record, I had the same feeling initially, but figured out it wasn't really the "if and only if" that was confusing me, but mostly the actual contents of what's being said. (E.g. compare my last answer above.) Do you feel the "if and only if"s in my proposed alternative, i.e.

@steffahn

• A user can construct a value using .. if (and only if) all fields are visible and all left-out fields have default values in the corresponding struct/enum item, and the struct or enum- variant is not #[non_exhaustive] , or local to the current crate.
• A user has to add .. to struct expressions or enum variant expression, if the structs/enums is external and declares .. in its struct/enum item (in the corresponding variant).
• A struct/enum item can declare .. (for a struct, or for an enum variant) if and only if the type and all of its fields are public , and the struct or enum- variant is not #[non_exhaustive] .

is confusing you, too?

Actually, I'm just noticing, that the second one of those lacks the "only if" part, so perhaps "must" or "has to" together with "if and only if" did make me (uncouciously) feel uncomfortable after all, and I dropped it.

Note that "if and only if" or "if" is, in practice, often used interchangeably in definitions in maths, as far as I'm aware. (Interchangeable only in definitions, when it's clear from context that it actually means to be a precise definition, and definitely not in theorems!)

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@atagunov

@jhpratt

My point is that changing it to just "if" alters the meaning in a significant way.

Of course. But you can re-write it to two separate "if" sentences. I feel it will likely improve everybody's understanding.

I don't feel like that would add any clarity. In any case, I agree that this is not quite relevant when the point is to figure out the actual rules, not the best way to present them.