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Update impl-trait.md (#249)

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Adding an extra speaker note why `impl Display` is used very appropriately here and what would go wrong if we tried `T: Display`.
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Igor Petruk 2023-01-23 13:41:14 +00:00 committed by GitHub
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src/generics/impl-trait.md
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@ -28,4 +28,11 @@ The meaning of `impl Trait` is a bit different in the different positions.
without naming the type. This can be useful when you don't want to expose the concrete type in a
public API.
This example is great, because it uses `impl Display` twice. It helps to explain that
nothing here enforces that it is _the same_ `impl Display` type. If we used a single
`T: Display`, it would enforce the constraint that input `T` and return `T` type are the same type.
It would not work for this particular function, as the type we expect as input is likely not
what `format!` returns. If we wanted to do the same via `: Display` syntax, we'd need two
independent generic parameters.
</details>