T is decided by the caller, but you provide MyStruct from within your function. If MyTrait was implemented for u8 and the user decides to pass a function that takes fn(u8) as input, it would not get a u8 value as argument, but a value of MyStruct, breaking the contract of bar's signature. Generic type arguments are placeholders for concrete types (like u8 or MyStruct). If you want to pass something that implements MyTrait from inside your function, don't use a generic type argument, but either impl Trait or a trait object. Unfortunately for you, you can't use impl Trait in function pointers, so you'd have to use a trait object instead:
impl Trait in argument position is a generic. In cases where it is allowed today, replacing a generic T with impl Trait in argument position means the same thing, only no one can name the latter. 
It's not inconceivable that fn(impl Trait)specifically will be special-cased to for<T: Trait> fn(T) some day, like how fn(&u8) means for<'x> fn(&'x u8). But that would be something new and not what you linked to, as for<T> types don't exist in Rust (so far).
Switching between the two is often still a breaking change though, because it changes the turbofish signature of the function, if turbofishable. ↩︎