I'm addressing the intent of the original question.
No one would ask this question in the case where the struct is generic over a type parameter bounded by the trait, since such a design can only store a homogeneous collection of values of a single concrete type implementing the trait; the question doesn't even make sense in that situation.
The question only arises for a struct that must store a heterogeneous collection of values with different concrete types implementing the trait, in which case a trait object (dyn Trait) is required.
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I'm addressing the intent of the original question.
No one would ask this question in the case where the struct is generic over a type parameter bounded by the trait, since such a design can only store a homogeneous collection of values of a single concrete type implementing the trait; the question doesn't even make sense in that situation.
The question only arises for a struct that must store a heterogeneous collection of values with different concrete types implementing the trait, in which case a trait object (dyn Trait) is required.