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 });
    },
};
# }