Confusion around Sizedness bound for trait objects

[runiq]

Hi, long-time reader, first-time poster :)

I got confused by two apparently conflicting bits in your whirlwind tour on Rust stdlib traits. In the chapter on subtraits and supertraits, you mention that only types can implement traits. Then, in the chapter on sizedness, you mention that all traits have an implicit ?Sized bound like this:

trait Trait: ?Sized {}

Which, given that only types can implement traits according to the chapter on subtraits and supertraits, would be desugared into something like this:

trait Trait where Self: ?Sized {}

Now, to me that implies that wnenever I would implement any trait for a type, that type would automatically gain the ?Sized bound. But that obviously cannot be true. So—what am I missing here? 🤔

[pretzelhammer]

Now, to me that implies that whenever I would implement any trait for a type, that type would automatically gain the ?Sized bound. But that obviously cannot be true. So—what am I missing here? 🤔

I think your confusion might stem from how you're interpreting ?Sized. So if ?Sized meant "is not sized" then yes. as an auto-bound on all trait impls it would not make sense since a lot of types are sized, however what it actually means is "may not be sized" and any type may or may not be sized, just like any boolean may be true or may be false :)

In short: some types are Sized, some are types !Sized, and all types are ?Sized, since ?Sized is the superset of Sized + !Sized. Does that answer your question? If not feel free to ask follow-up questions :)

[runiq]

So basically, there's some "special sauce" for the 'relaxed bound' syntax that makes it behave somewhat differently from normal trait bounds?

If so, yes, that clears it up. Thanks! :)

[pretzelhammer]

Haha, you can call it "special sauce" if you want to but yes, a "relaxed bound" or a "widening bound" is different than a regular bound because it relaxes constraints instead of adding constraints, so it will always increase the number of types which can pass the constraints.