Make `Default` const and add some `const Default` impls #134628

estebank · 2024-12-21T20:25:57Z

Full list of impl const Default types:

()
bool
char
std::ascii::Char
usize
u8
u16
u32
u64
u128
i8
i16
i32
i64
i128
f16
f32
f64
f128
std::marker::PhantomData
Option
std::iter::Empty
std::ptr::Alignment
&[T]
&mut [T]
&str
&mut str
String
Vec

rustbot · 2024-12-21T20:26:04Z

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

estebank · 2024-12-21T20:26:35Z

Made this for some experiments with default field values, might as well put it somewhere visible as it seems like an obvious next step for the const traits effort.

compiler-errors · 2024-12-21T20:31:31Z

Please make sure this is assigned to someone on @rust-lang/project-const-traits if you take this out of draft.

tests/ui/const-generics/std/const-generics-alloc.rs

bors · 2024-12-24T07:04:27Z

☔ The latest upstream changes (presumably #134716) made this pull request unmergeable. Please resolve the merge conflicts.

estebank · 2024-12-25T00:14:50Z

r? @oli-obk

Let me know what would be needed to make this merge-ready.

fee1-dead · 2024-12-30T15:00:19Z

I'd like to block this on #133999.

rustbot · 2025-02-12T17:48:55Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

bors · 2025-03-02T02:10:52Z

☔ The latest upstream changes (presumably #137752) made this pull request unmergeable. Please resolve the merge conflicts.

apiraino · 2025-03-06T15:53:34Z

This was blocked on #133999 (comment) which is now closed.

@fee1-dead are you still interested in working on a follow-up to #133999 or are there other plans? Or no plans for now?

Just to get a clearer picture of the progress here :-) Thanks

cc @oli-obk

Randl · 2025-06-27T12:37:15Z

Is it still blocked?

oli-obk · 2025-06-27T14:12:47Z

Nope, ready again

fee1-dead · 2025-06-27T14:15:59Z

This was blocked on #133999 (comment) which is now closed.

FTR it was finished up in #135423 by errs

tests/ui/lint/dead-code/unused-struct-derive-default.stderr

library/core/src/default.rs

estebank · 2025-07-01T17:20:16Z

src/tools/clippy/tests/ui/or_fun_call.stderr

-error: function call inside of `unwrap_or`
-  --> tests/ui/or_fun_call.rs:409:21
-   |
-LL |     let _ = opt_foo.unwrap_or(Foo { val: String::default() });
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_else(|| Foo { val: String::default() })`


Should this clippy lint have a check that the value is const constructable in a stable way (which would revert this output change)?

fmease · 2025-07-01T17:36:06Z

library/core/src/cell.rs

@@ -333,7 +333,8 @@ impl<T: Copy> Clone for Cell<T> {
 }

 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Default> Default for Cell<T> {
+#[rustc_const_unstable(feature = "const_default", issue = "67792")]
+impl<T: ~const Default> const Default for Cell<T> {


Suggested change

impl<T: ~const Default> const Default for Cell<T> {

impl<T: [const] Default> const Default for Cell<T> {

Not possible until the bootstrap bump because we use stage 0 rustfmt

These are not parsed against the master branch I rebased on top of? Has the parser change already landed?

The parser changes landed. But rustfmt is bumped when the bootstrap compiler is bumped. At that time everything will automatically be formatted from ~const to [const]

library/core/src/cell.rs

Full list of `impl const Default` types: - () - bool - char - std::ascii::Char - usize - u8 - u16 - u32 - u64 - u128 - i8 - i16 - i32 - i64 - i128 - f16 - f32 - f64 - f128 - std::marker::PhantomData<T> - Option<T> - std::iter::Empty<T> - std::ptr::Alignment - &[T] - &mut [T] - &str - &mut str - String - Vec<T>

The lint should likely still trigger while the const Default impl is unstable.

rustbot assigned wesleywiser Dec 21, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 21, 2024

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estebank force-pushed the const-default branch from f36f4d5 to 00b0c75 Compare December 22, 2024 01:59

oli-obk reviewed Dec 22, 2024

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estebank force-pushed the const-default branch from 00b0c75 to d7cfdf5 Compare December 22, 2024 20:30

estebank force-pushed the const-default branch from d7cfdf5 to 4f6558b Compare December 25, 2024 00:13

rustbot assigned oli-obk and unassigned wesleywiser Dec 25, 2024

fee1-dead mentioned this pull request Dec 30, 2024

Prepare const traits for stabilization rust-lang/rust-project-goals#106

Open

14 tasks

fee1-dead added the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Dec 30, 2024

This was referenced Jan 16, 2025

Breakage related to default field values (RFC 3681) obi1kenobi/cargo-semver-checks#993

Open

Tracking issue for RFC 3681: Default field values #132162

Open

estebank added F-const_trait_impl `#![feature(const_trait_impl)]` F-default_field_values `#![feature(default_field_values)]` labels Jan 18, 2025

compiler-errors mentioned this pull request Jan 19, 2025

[generic_assert] "Constify" the Debug trait #135712

Closed

estebank force-pushed the const-default branch from 3a03fb5 to 78db9d1 Compare February 12, 2025 02:39

estebank marked this pull request as ready for review February 12, 2025 02:39

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estebank force-pushed the const-default branch from 7795f95 to eaaabae Compare June 27, 2025 19:14

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estebank force-pushed the const-default branch from 5deb7c8 to 37eb628 Compare July 1, 2025 17:15

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estebank added 10 commits July 3, 2025 17:59

Update tests

0ae3f87

Add const Default for *Cell

84a6915

Add rustc_const_unstable to Debug

f811624

Update tests

ec41e44

[double check] update clippy test

170f887

The lint should likely still trigger while the const Default impl is unstable.

Remove rustc_trivial_field_reads from trait Default

34d461b

Fix test

9ee6024

Set issue

7cd06dc

fix test

4069e12

estebank force-pushed the const-default branch from c8e3810 to a8848a5 Compare July 3, 2025 18:22

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estebank added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 3, 2025

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Make `Default` const and add some `const Default` impls #134628

Make `Default` const and add some `const Default` impls #134628

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	impl<T: ~const Default> const Default for Cell<T> {
	impl<T: [const] Default> const Default for Cell<T> {

Make Default const and add some const Default impls #134628

Are you sure you want to change the base?

Make Default const and add some const Default impls #134628

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Make `Default` const and add some `const Default` impls #134628

Make `Default` const and add some `const Default` impls #134628