Any specification for where clause?

I always though that it will be

where SOME_TYPE : SOME_BOUNDS

but the code shown in RustLatam 2019 - Florian Gilcher: The Power of the “Where” Clause strikes me:

fn takes_into_string<T>(t : T)
where String : From<T>
{
  ...
}

This makes generics more flexible and expressive !

Yes, check out the reference: https://doc.rust-lang.org/reference/items/generics.html#where-clauses

Where clauses provide an another way to specify bounds on type and lifetime parameters as well as a way to specify bounds on types that aren’t type parameters.

Check out the source code for typenum to see some of the crazy type level meta programming you can do.