entry_and_modify -

# `entry_and_modify`

The tracking issue for this feature is: #44733

This introduces a new method for the Entry API of maps (`std::collections::HashMap` and `std::collections::BTreeMap`), so that occupied entries can be modified before any potential inserts into the map.

For example:

    #![feature(entry_and_modify)]
    # fn main() {
    use std::collections::HashMap;

    struct Foo {
        new: bool,
    }

    let mut map: HashMap<&str, Foo> = HashMap::new();

    map.entry("quux")
       .and_modify(|e| e.new = false)
       .or_insert(Foo { new: true });
    # }

This is not possible with the stable API alone since inserting a default before modifying the `new` field would mean we would lose the default state:

    # fn main() {
    use std::collections::HashMap;

    struct Foo {
        new: bool,
    }

    let mut map: HashMap<&str, Foo> = HashMap::new();

    map.entry("quux").or_insert(Foo { new: true }).new = false;
    # }

In the above code the `new` field will never be `true`, even though we only intended to update that field to `false` for previously extant entries.

To achieve the same effect as `and_modify` we would have to manually match against the `Occupied` and `Vacant` variants of the `Entry` enum, which is a little less user-friendly, and much more verbose:

    # fn main() {
    use std::collections::HashMap;
    use std::collections::hash_map::Entry;

    struct Foo {
        new: bool,
    }

    let mut map: HashMap<&str, Foo> = HashMap::new();

    match map.entry("quux") {
        Entry::Occupied(entry) => {
            entry.into_mut().new = false;
        },
        Entry::Vacant(entry) => {
            entry.insert(Foo { new: true });
        },
    };
    # }