Rust

Trick question: what is the return type of a function with body

{
   return || true
}

Let’s try a more difficult one with closures. What’s wrong with this closure

let closure = |x: &str| x;

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You would need to specify a lifetime since you’re returning a borrowed reference. I don’t remember right now if you can annotate lifetimes in closures 😅 but you could do:

let closure = |x: &str| x.to_owned();

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Very warm! Why does this work though?

fn foo(s: &str) -> &str { s }

Yes! This is a wart on closures: lifetime elision works different than for functions for historical reasons and it’s impossible to change it without breaking backwards compatibility.

One possible fix is static ref with trait:

let c: &dyn Fn(&str) -> &str = &|x: &str| x;

Lifetime elision rules. The param s gets a lifetime because its a ref, since there is only one input lifetime it gets assigned to the output. I don’t get why the same doesn’t apply to the closure though. Maybe depends on the context of the closure and how its used 🤔

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Right, but why can’t it do it for the closure?

Because the compiler is smart enough to infer the returned reference lifetime... Because there is only one borrowed reference as a parameter