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Rust then lets you convert a reference &RcBox<String> to a fat reference &RcBox<Display>

From

Cover of Programming Rust

Note

could do with detailed explanations here; nothing in the book so far explain why we can coerce a &Foo<Ty> to &Foo<Tr> when Ty implements Tr.

More info: https://doc.rust-lang.org/std/marker/trait.Unsize.html