What does this empty bounds ` where I:` mean?

I found some code use syntax like where I:, for example

struct Foo<I> where I: {
    i: I

This code is written by me for no purpose, but it compiles. Could anyone help what it means and when it is useful to do so?

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It means it applies zero trait bounds to the type I. So, it does nothing in this case.

In some complex cases it can be useful to mention a type (that is more than just a bare type variable), without any bounds, to express “this type must be able to exist”; like where [(); N - 1]: means that the array must be able to exist, i.e. N > 0.


Also, allowing an empty list of bounds makes it easier to write macros and other code that generates code.


It has an additional, mostly undocumented effect on lifetimes -- it makes the lifetime early-bound.