Brendan Zabarauskas on Twitter: "@KevinHoffman @rustlang You might need a higher ranked lifetime: type Callback = dyn for<'a, 'b>Fn(&'a str, &'b [u8]) -> Result<Vec<u8>> + Sync + Send + 'static;" / Twitter

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and

You might need a higher ranked lifetime: type Callback = dyn for<'a, 'b>Fn(&'a str, &'b [u8]) -> Result<Vec<u8>> + Sync + Send + 'static;

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and

Oh wait, I think twitter.com/RReverser/stat is the right answer. You'll need to do `Box<Callback>` or `&Callback` etc.

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Replying to @KevinHoffman and @rustlang

Well it's an unsized type, so like with any unsized type, you need to either store a reference to Callback or a Box<Callback> in your actual struct.

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Playing in the playground Rust it seems to elide the higher ranked lifetimes automatically.

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