Tracking Issue for const_convert #88674

usbalbin · 2021-09-05T20:32:34Z

Feature gate: #![feature(const_convert)]

This is a tracking issue for constifying some conversions (see #87852 for numeric conversions). Among other things this enables using the ?-operator on Option and Result in const contexts for trivial conversions. Note that this does not add any new implementations, it only constifies the existing ones.

Simple example:

const fn foo() -> Result<(), FooError> {
    let bar = try_thing()?; // A function that returns Result<_, FooError>
    do_stuff_with(bar)?;    // A function that returns Result<_, FooError>
    Ok(())
}

Public API

Constifies the following impls

impl<T, U> const Into<U> for T where U: ~const From<T>;
impl<T> const From<T> for T;

impl<T> const ops::Try for Option<T>;
impl<T> const ops::FromResidual for Option<T>;

impl<T, E> const ops::Try for Result<T, E>;
impl<T, E, F> const ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> where F: ~const From<E>;


impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T where T: ~const AsMut<U>;

impl<T, U> const TryInto<U> for T where U: ~const TryFrom<T>;
impl<T, U> const TryFrom<U> for T where U: ~const Into<T>;

Steps / History

Implementation: Constify ?-operator for Result and Option #86853
More impls Extend const_convert to rest of blanket core::convert impls #92382
Blocked on Deprecated tracking issue for closed RFC 2632, impl const Trait for Ty and ~const (tilde const) syntax (const_trait_impl) #67792 and Tracking Issue for try_trait_v2, A new design for the ? desugaring (RFC#3058) #84277
Final comment period (FCP)
Stabilization PR

Unresolved Questions

None yet.

The text was updated successfully, but these errors were encountered:

Constify ?-operator for Result and Option Try to make `?`-operator usable in `const fn` with `Result` and `Option`, see rust-lang#74935 . Note that the try-operator itself was constified in rust-lang#87237. TODO * [x] Add tests for const T -> T conversions * [x] cleanup commits * [x] Remove `#![allow(incomplete_features)]` * [?] Await decision in rust-lang#86808 - I'm not sure * [x] Await support for parsing `~const` in bootstrapping compiler * [x] Tracking issue(s)? - rust-lang#88674

Make more `From` impls `const` (libcore) Adding `const` to `From` implementations in the core. `rustc_const_unstable` attribute is not added to unstable implementations. Tracking issue: rust-lang#88674 <details> <summary>Done</summary><div> - `T` from `T` - `T` from `!` - `Option<T>` from `T` - `Option<&T>` from `&Option<T>` - `Option<&mut T>` from `&mut Option<T>` - `Cell<T>` from `T` - `RefCell<T>` from `T` - `UnsafeCell<T>` from `T` - `OnceCell<T>` from `T` - `Poll<T>` from `T` - `u32` from `char` - `u64` from `char` - `u128` from `char` - `char` from `u8` - `AtomicBool` from `bool` - `AtomicPtr<T>` from `*mut T` - `AtomicI(bits)` from `i(bits)` - `AtomicU(bits)` from `u(bits)` - `i(bits)` from `NonZeroI(bits)` - `u(bits)` from `NonZeroU(bits)` - `NonNull<T>` from `Unique<T>` - `NonNull<T>` from `&T` - `NonNull<T>` from `&mut T` - `Unique<T>` from `&mut T` - `Infallible` from `!` - `TryIntError` from `!` - `TryIntError` from `Infallible` - `TryFromSliceError` from `Infallible` - `FromResidual for Option<T>` </div></details> <details> <summary>Remaining</summary><dev> - `NonZero` from `NonZero` These can't be made const at this time because these use Into::into. https://github.com/rust-lang/rust/blob/master/library/core/src/convert/num.rs#L393 - `std`, `alloc` There may still be many implementations that can be made `const`. </div></details>

clarfonthey · 2021-12-29T01:53:38Z

~~Note: impl<T, U> const Into<U> for T where U: ~const From<T>; is not actually included here; I add it with #92382.~~

Just kidding; I forgot to rebase when I thought I had. ><

Extend const_convert to rest of blanket core::convert impls This adds constness to all the blanket impls in `core::convert` under the existing `const_convert` feature, tracked by rust-lang#88674. Existing impls under that feature: ```rust impl<T> const From<T> for T; impl<T, U> const Into<U> for T where U: ~const From<T>; impl<T> const ops::Try for Option<T>; impl<T> const ops::FromResidual for Option<T>; impl<T, E> const ops::Try for Result<T, E>; impl<T, E, F> const ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> where F: ~const From<E>; ``` Additional impls: ```rust impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T where T: ~const AsRef<U>; impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T where T: ~const AsRef<U>; impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T where T: ~const AsMut<U>; impl<T, U> const TryInto<U> for T where U: ~const TryFrom<T>; impl<T, U> const TryFrom<U> for T where U: ~const Into<T>; ```

These implementations are trivially const - since they only create the enum structure without running any non-const code. const_convert is tracked by rust-lang#88674

…low, r=scottmcm Extend const_convert with const {FormResidual, Try} for ControlFlow. Very small change so I just used the existing `const_convert` feature flag. rust-lang#88674 Newly const API: ``` impl<B, C> const ops::Try for ControlFlow<B, C>; impl<B, C> const ops::FromResidual for ControlFlow<B, C>; ``` `@usbalbin` I hope it is ok that I added to your feature.

G33KatWork · 2023-04-22T20:14:30Z

With commit 76dbe29 the feature const_convert has been completely removed from the core library. I understand that it's unstable, but it's also not exactly documented why it got removed. I would've at least expected a discussion about this in this tracking issue.

Does anybody know more about this?

usbalbin · 2023-04-22T20:25:04Z

I believe that might be #110395

RalfJung · 2024-09-08T06:46:16Z

This has been entirely removed by #110393.

usbalbin added C-tracking-issue Category: An issue tracking the progress of sth. like the implementation of an RFC T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Sep 5, 2021

usbalbin mentioned this issue Sep 5, 2021

Constify ?-operator for Result and Option #86853

Merged

5 tasks

lilasta mentioned this issue Oct 18, 2021

Make more From impls const (libcore) #90009

Merged

clarfonthey mentioned this issue Dec 29, 2021

Extend const_convert to rest of blanket core::convert impls #92382

Merged

nipunn1313 mentioned this issue Jul 25, 2022

Extend const_convert for Cow on From<&str> and From<String> #99733

Closed

onestacked mentioned this issue Sep 22, 2022

Extend const_convert with const {FormResidual, Try} for ControlFlow. #102144

Merged

rust-lang deleted a comment from atifnimran Dec 25, 2022

fee1-dead added the F-const_trait_impl `#![feature(const_trait_impl)]` label Mar 24, 2023

afternoon mentioned this issue Apr 23, 2023

Feature const_convert no longer available with rust 1.69 nightly alexeden/adafruit-seesaw#1

Closed

RalfJung closed this as completed Sep 8, 2024

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Tracking Issue for const_convert #88674

Tracking Issue for const_convert #88674

usbalbin commented Sep 5, 2021 •

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clarfonthey commented Dec 29, 2021 •

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G33KatWork commented Apr 22, 2023

usbalbin commented Apr 22, 2023 •

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RalfJung commented Sep 8, 2024

Tracking Issue for const_convert #88674

Tracking Issue for const_convert #88674

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usbalbin commented Sep 5, 2021 • edited Loading

Public API

Steps / History

Unresolved Questions

clarfonthey commented Dec 29, 2021 • edited Loading

G33KatWork commented Apr 22, 2023

usbalbin commented Apr 22, 2023 • edited Loading

RalfJung commented Sep 8, 2024

usbalbin commented Sep 5, 2021 •

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clarfonthey commented Dec 29, 2021 •

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usbalbin commented Apr 22, 2023 •

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