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$ cargo run
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Compiling error-handling v0.1.0 (file:///projects/error-handling)
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error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
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--> src/main.rs:4:48
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3 | fn main() {
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| --------- this function should return `Result` or `Option` to accept `?`
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4 | let greeting_file = File::open("hello.txt")?;
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| ^ cannot use the `?` operator in a function that returns `()`
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= help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
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For more information about this error, try `rustc --explain E0277`.
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error: could not compile `error-handling` (bin "error-handling") due to 1 previous error
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