Tracking Issue for `const_default_impls`

[fee1-dead]

Feature gate: #![feature(const_default_impls)]

This is a tracking issue for const Default impls in the standard library.

Public API

// core::array
impl<T> const Default for [T; 0];

// core::default
impl const Default for ();
impl const Default for bool;
impl const Default for char;
impl const Default for usize;
impl const Default for u8;
impl const Default for u16;
impl const Default for u32;
impl const Default for u64;
impl const Default for u128;
impl const Default for isize;
impl const Default for i8;
impl const Default for i16;
impl const Default for i32;
impl const Default for i64;
impl const Default for i128;
impl const Default for f32;
impl const Default for f64;

// core::hash
impl<H> const Default for BuildHasherDefault<H>;

// core::iter
impl<T> const Default for Empty<T>;

// core::marker
impl<T: ?Sized> const Default for PhantomData<T>;

// core::option
impl<T> const Default for Option<T>;

// core::slice
impl<T> const Default for &[T];
impl<T> const Default for &mut [T];

// core::str
impl const Default for &str;

// core::sync::atomic
impl const Default for AtomicBool;
impl<T> const Default for AtomicPtr<T>;

// alloc::boxed
impl<T> const Default for Box<[T]>;
impl const Default for Box<str>;

// alloc::string
impl const Default for String;

// alloc::vec
impl<T> const Default for Vec<T>;

// std::lazy
impl<T> const Default for SyncOnceCell<T>;

Steps / History

• Implementation: constified implementations of Default #86808, impl const Default for Box<[T]> and Box<str> #95947
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• None yet.

[kartva]

What's the status of this issue?

[Liamolucko]

It's dependent on #67792.

[C0D3-M4513R]

I stumbled across this pr, as I was trying to implement my own const Default implementations and ran into various errors:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=36cb0ae9a38d3d57000dce3173d38df2

What am I doing wrong here?

[fee1-dead]

See #110395.

[dtolnay]

Based on rg 'const Default' library all these impls have been rolled back as part of #110393.

I'll close the issue since this isn't currently tracking any API requiring Libs-API team attention, but the list of impls at the top of this issue may be useful if const impls ever get reinstated, and we can reopen the issue (or create a new one) at that point.