Tracking Issue for `Option::get_or_insert_default`

[camsteffen]

Feature gate: #![feature(option_get_or_insert_default)]

This is a tracking issue for adding Option::get_or_insert_default.

Basically, it is a shorthand for option.get_or_insert_with(Default::default). It differs with unwrap_or_default since it does not consume the Option. This is useful, for example, when you have an Option as a struct field or inside a data structure like Vec<Option<T>>.

Public API

impl<T> Option<T> {
    pub fn get_or_insert_default(&mut self) -> &mut T where T: Default;
}

Steps / History

• Original issue: #55042
• [x] Implementation: #82849
• [x] Renamed from get_or_default to get_or_insert_default: #82977
• [ ] Final commenting period (FCP)
• [ ] Stabilization PR

Unresolved Questions

• None yet.

[m-ou-se]

It's not obvious to me at all from the name that this modifies the Option. It sounds like it'd be the same as unwrap_or_default. Something like get_or_insert_default would be more consistent with the other method names.

[camsteffen]

Hmm I hadn't considered that.

[sffc]

+1 on the name get_or_insert_default

I wanted to use this today, but it's still experimental. :laughing:

[insou22]

+1 -- was looking for this exact method today

[djc]

@rustbot ping libs

Let's see if this works now? Would be nice to have this FCP'ed.

cc @joshtriplett

[rustbot]

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[joshtriplett]

LGTM

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• [x] @Amanieu
• [ ] @BurntSushi
• [x] @dtolnay
• [x] @joshtriplett
• [x] @m-ou-se
• [ ] @yaahc

Concerns:

• is this in the same category as resize_default (https://github.com/rust-lang/rust/issues/82901#issuecomment-1244996427)

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

:bell: This is now entering its final comment period, as per the review above. :bell:

[dtolnay]

@rfcbot concern is this in the same category as resize_default

Many years ago we deleted vec.resize_default(n) because it just wasn't worth having when vec.resize_with(n, T::default) is equally suitable.

• https://github.com/rust-lang/rust/issues/41758
• https://github.com/rust-lang/rust/pull/57656
• https://github.com/rust-lang/rust/pull/77850

It seems like it came down to just not being worth dedicating a whole new method given how rarely resize_with is used in the first place. This is in contrast to option.unwrap_or_default() as shorthand for option.unwrap_or_else(T::default), which is the only existing precedent for T::default getting a dedicated convenience that I know of, and is justified by being used literally over 100× more than resize_with.

I suppose option.get_or_insert_default() is much closer to the resize_with end of this spectrum than the unwrap_or_default end, and option.get_or_insert_with(T::default) would be good enough.

[imp]

My own experience (which may or may not be representative) tells that in every single case I wanted option.get_or_insert_-like functionality I actually wanted option.get_or_insert_default(). So in my books it is much closer to unwrap_or_default than to resize_with. Just my 2 shekels.

[dtolnay]

Be aware that whether or not you want the default value is not the only consideration here. You also require a situation in which T is completely inferrable. The combination of those 2 is less common in my experience (especially if I also take into account experience from similar situations in the Entry API). Extrapolating "every single case I wanted the default" to "therefore I would have found get_or_insert_default valuable if available" is not correct.

use std::collections::BTreeSet;
use std::iter;

fn main() {
    let mut junks = None;
    for (id, value) in iter::repeat(("junk", 99)).take(10) {
        if id == "junk" {
            junks.get_or_insert_with(BTreeSet::default).insert(value);
        }
    }
    println!("{:?}", junks);
}

With get_or_insert_default:

error[E0282]: type annotations needed for `Option<T>`
 --> src/main.rs:5:9
  |
5 |     let mut junks = None;
  |         ^^^^^^^^^
...
8 |             junks.get_or_insert_default().insert(value);
  |                                           ------ type must be known at this point

[imp]

Very good example. However, in my opinion it doesn't lower the value of get_or_insert_default. Just my opinion - I realize others can feel differently.

[Rua]

Is there anything blocking stabilisation of this?