CODEWITHSUNDEEP
rustpattern-matching
user2292040
user2292040

Reputation: 315

How can the ref keyword be avoided when pattern matching in a function taking &self or &mut self?

The Rust book calls the ref keyword "legacy". As I want to follow the implicit advice to avoid ref, how can I do it in the following toy example? You can find the code also on the playground.

struct OwnBox(i32);

impl OwnBox {
    fn ref_mut(&mut self) -> &mut i32 {
        match *self {
            OwnBox(ref mut i) => i,
        }

        // This doesn't work. -- Even not, if the signature of the signature of the function is
        // adapted to take an explcit lifetime 'a and use it here like `&'a mut i`.
        // match *self {
        //     OwnBox(mut i) => &mut i,
        // }

        // This doesn't work
        // match self {
        //     &mut OwnBox(mut i) => &mut i,
        // }
    }
}

Upvotes: 2

Views: 989

Answers (1)

E_net4
E_net4

Reputation: 29993

Since self is of type &mut Self, it is enough to match against itself, while omitting ref entirely. Either dereferencing it with *self or adding & to the match arm would cause an unwanted move.

fn ref_mut(&mut self) -> &mut i32 {
    match self {
        OwnBox(i) => i,
    }
}

For newtypes such as this one however, &mut self.0 would have been enough.

This is thanks to RFC 2005 — Match Ergonomics.

Upvotes: 6

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