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Tracking Issue for const_convert #88674

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2 of 5 tasks
usbalbin opened this issue Sep 5, 2021 · 3 comments
Open
2 of 5 tasks

Tracking Issue for const_convert #88674

usbalbin opened this issue Sep 5, 2021 · 3 comments
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C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. F-const_trait_impl `#![feature(const_trait_impl)]` T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.

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@usbalbin
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usbalbin commented Sep 5, 2021 •
edited

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

Unresolved Questions

  • None yet.
@usbalbin usbalbin added C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Sep 5, 2021
@usbalbin usbalbin mentioned this issue Sep 5, 2021
Merged
5 tasks
bors added a commit to rust-lang-ci/rust that referenced this issue Sep 30, 2021
@bors
Auto merge of rust-lang#86853 - usbalbin:const_try, r=oli-obk
c6007fd 
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
@lilasta lilasta mentioned this issue Oct 18, 2021
Merged
GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this issue Oct 19, 2021
@GuillaumeGomez
Rollup merge of rust-lang#90009 - woppopo:const_from_more, r=dtolnay
d6af6c9 
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>
JohnTitor added a commit to JohnTitor/rust that referenced this issue Oct 19, 2021
@JohnTitor
Rollup merge of rust-lang#90009 - woppopo:const_from_more, r=dtolnay
9f2ad0a 
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 clarfonthey mentioned this issue Dec 29, 2021
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@clarfonthey
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clarfonthey commented Dec 29, 2021 •
edited

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. ><

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Jan 14, 2022
@matthiaskrgr
Rollup merge of rust-lang#92382 - clarfonthey:const_convert, r=scottmcm
89ea551 
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>;
```
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Jan 15, 2022
@matthiaskrgr
Rollup merge of rust-lang#92382 - clarfonthey:const_convert, r=scottmcm
1d9ab77 
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>;
```
nipunn1313 added a commit to nipunn1313/rust that referenced this issue Jul 25, 2022
@nipunn1313
Extend const_convert for Cow on From<&str> and From<String>
ebd1467 
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
@nipunn1313 nipunn1313 mentioned this issue Jul 25, 2022
Open
@onestacked onestacked mentioned this issue Sep 22, 2022
Merged
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Sep 22, 2022
@matthiaskrgr
Rollup merge of rust-lang#102144 - chriss0612:const_convert_control_f…
2337063 
…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.
@rust-lang rust-lang deleted a comment from atifnimran Dec 25, 2022
@fee1-dead fee1-dead added the F-const_trait_impl `#![feature(const_trait_impl)]` label Mar 24, 2023
@G33KatWork
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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
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usbalbin commented Apr 22, 2023 •
edited

I believe that might be #110395

@afternoon afternoon mentioned this issue Apr 23, 2023
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