Examples of coercions are wrong

[martin-t]

Several of the examples on the coercions page claim that 42 gets coerced into i8. However, no conversion takes place, the 42 is i8 already thanks to type inference.

Potentially related PR: #342

[thepowersgang]

A more suitable type pair would be &String and &str (which is a deref coercion), or &Foo and &dyn TraitFooImpls