Tracking Issue for fn const BuildHasherDefault::new()

[krtab]

Feature gate: #![feature(build_hasher_default_const_new)]

This is a tracking issue for a new new class method for BuildHasherDefault. It allows creating a BuildHasherDefault in const contexts.
Because HashMap::with_hasher constness is being stabilized in #118427 this will in turn allow creating empty HashMap<K,V,BuildHasherDefault<H>> in const context for any H: Default + Hasher.

Public API

impl<H> BuildHasherDefault<H> {
    pub const fn new() -> Self;
}

Steps / History

• Implementation: Add fn const BuildHasherDefault::new #123198
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

@rustbot claim

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[RalfJung]

@rust-lang/libs-api with #132541 on its way into the compiler, we are quite close to stabilizing #102575. However, actually creating a HashMap in const context with just that feature is kind of awkward:

    struct MyBuildDefaultHasher;
    impl BuildHasher for MyBuildDefaultHasher {
        type Hasher = DefaultHasher;

        fn build_hasher(&self) -> Self::Hasher {
            DefaultHasher::new()
        }
    }

    const H: HashMap<(), (), MyBuildDefaultHasher> = HashMap::with_hasher(MyBuildDefaultHasher);

So, I propose that we also stabilize BuildHasherDefault, tracked right here, so that this can instead be written as:

    const H: HashMap<(), (), BuildHasherDefault<DefaultHasher>> =
        HashMap::with_hasher(BuildHasherDefault::new());

[RalfJung]

Cc @rust-lang/wg-const-eval

[m-ou-se]

@rfcbot merge

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

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]

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

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.