Should a reference to an enum be dereferenced before it is matched?

问题

As a newcomer to Rust, I've stumbled upon two apparently valid ways of running a match on a reference type.

I've defined an enum:

enum Color {
    Red,
    Yellow,
    Green,
    Teal,
    Blue,
    Purple,
}

I want to implement a function that works on a &self reference of an instance of this enum.

I can see two ways to write such a function:

impl Color {
    // Approach #1: Match the reference, using references in each pattern
    fn contains_red(&self) -> bool {
        match self {
            &Color::Red => true,
            &Color::Yellow => true,
            &Color::Purple => true,
            _ => false,
        }
    }

    // Approach #2: Dereference &self and match the patterns directly
    fn contains_blue(&self) -> bool {
        match *self {
            Color::Blue => true,
            Color::Teal => true,
            Color::Purple => true,
            _ => false,
        }
    }
}

I expected that dereferencing &self would be counted as a move, and would cause errors if I called color.contains_blue() on the same instance twice in a row, but this doesn't seem to be the case.

Are these approaches functionally identical? Would one of them break down if I were matching more complex objects?

回答1:

You can't move out of an immutable reference, so you don't have to worry about that being the case.

It's worth considering that one would expect matches to work somewhat akin to (Partial)Eq, and that takes &self. In other words, one would expect it to take a reference implicitly unless forced. This is easily confirmed with a little experimentation.

It's worth noting that *self is not a move - it's a reference to a memory location. It is thus an lvalue. Ergo,

When the head expression is an lvalue, the match does not allocate a temporary location (however, a by-value binding may copy or move from the lvalue).

https://doc.rust-lang.org/reference.html#match-expressions

If a temporary location is not allocated, a move cannot occur. Thus the behaviour is guaranteed. As the parenthetical notes, internal data can still get moved out from destructuring pattern, which would cause a problem. This, however, is true regardless of whether you are matching on self or *self.

Using *self seems to be an informal idiom, and should be preferred.

回答2:

I personally prefer the second. It conveys the intent better, in my opinion.

回答3:

As of Rust 1.26, the idiomatic solution is neither; you don't have to dereference the value or add & to the patterns:

fn contains_blue(&self) -> bool {
    match self {
        Color::Blue => true,
        Color::Teal => true,
        Color::Purple => true,
        _ => false,
    }
}

This is thanks to improved match ergonomics.

You can also use pattern alternation in this case:

match self {
    Color::Blue | Color::Teal | Color::Purple => true,
    _ => false,
}

来源：https://stackoverflow.com/questions/35921015/should-a-reference-to-an-enum-be-dereferenced-before-it-is-matched

标签

pattern-matching

rust

dereference