constified implementations of `Default` #86808

fee1-dead · 2021-07-02T10:04:04Z

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rust-highfive · 2021-07-02T10:04:07Z

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library/alloc/src/string.rs

CDirkx · 2021-07-02T17:28:51Z

What would be the policy for changing to const impls in the standard library? Is that something we want to do very conservatively, only to test the implementation for the RFC, or more liberally (or even everywhere where possible). I know of quite some const code that would become a lot more readable when able to use const impl Eq etc., but given that the RFC hasn't been accepted yet I don't know how much the library team wants to commit to this.

library/alloc/src/lib.rs

fee1-dead · 2021-07-16T03:26:33Z

Update: in this zulip thread we felt comfortable marking the feature not incomplete and start experimenting (non generic impls) in libstd.

Mark-Simulacrum · 2021-07-29T02:24:33Z

Update: in this zulip thread we felt comfortable marking the feature not incomplete and start experimenting (non generic impls) in libstd.

Have the incomplete / not yet ready for use parts of the feature gate been split out yet? I'm not sure we should be using it until that has been done: we've run into ongoing trouble with specialization, for example, precisely because of the actually-not-ok bits in min_specialization. I'd prefer to avoid doing so with const trait if there are known limitations.

Regardless, I think a decision from T-libs-api on what the policy for const-unstable trait impls should look like would be merited. I'm a little surprised this is possible since historically impls weren't possible to feature gate - is the story different in the const case for some reason?

fee1-dead · 2021-07-29T03:08:14Z

Update: in this zulip thread we felt comfortable marking the feature not incomplete and start experimenting (non generic impls) in libstd.

Have the incomplete / not yet ready for use parts of the feature gate been split out yet? I'm not sure we should be using it until that has been done: we've run into ongoing trouble with specialization, for example, precisely because of the actually-not-ok bits in min_specialization. I'd prefer to avoid doing so with const trait if there are known limitations.

I don't think we currently have incomplete / not yet ready for use parts. There are actually aspects of the RFC that are yet to be implemented, but I don't think this PR can expose not-ok stuff to users.

I'm a little surprised this is possible since historically impls weren't possible to feature gate - is the story different in the const case for some reason?

AFAIK the stability should be inherited and maybe this is only true for const stability. But I did add a test for this: https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2632-const-trait-impl/staged-api.rs

oli-obk · 2021-07-29T14:08:26Z

is the story different in the const case for some reason?

Yes it is. const stability is a separate system that does not participate in trait resolution, so you don't get type-system unsoundness where a trait is both implemented and not. Instead you just get an error if you call a non-const impl from a const context.

fee1-dead · 2021-08-02T16:46:23Z

Ping @rust-lang/libs-api

@rustbot label T-libs-api -S-waiting-on-review S-waiting-on-team

dtolnay

I think a decision from T-libs-api on what the policy for const-unstable trait impls should look like would be merited.

I don't think we have any objection to starting to land const impls as long as they are properly gated, i.e. stable code cannot be broken by ongoing churn in the const impl feature, all the way through its stabilization or eventual removal. I can't judge that from the PR but if the compiler team asserts it's properly gated, then it's ready.

As for which impls to make const, I think the bar is going to be the same as for const inherent methods: it has to be reasonably usable in const or have an extremely strong justification otherwise. For example we regularly reject const on an inherent method of a type that is impossible to construct in const, even if that method's implementation is otherwise compatible with const.

fee1-dead · 2021-08-03T12:41:33Z

I think:

I am sure that non-generic const impls will be good; (gated behind #[rustc_const_unstable] of course :) )
a bit less sure about generic const fns (should be better after Try filtering out non-const impls when we expect const impls #87375)
not so sure about implementing traits with associated types
Don't think we should constify impls with trait bounds now, because of potential regressions (see this) I need to add a few ui tests tracking this.

Update: Upon digging further, I think it is actually fine to add impls that match any point above. The only case where I am unsure is traits where associated types have bounds.

bors · 2021-08-03T13:40:27Z

☔ The latest upstream changes (presumably #87725) made this pull request unmergeable. Please resolve the merge conflicts.

yaahc · 2021-08-03T19:08:59Z

* I am sure that non-generic const impls will be good;

When you say "non-generic" in this case you mean traits that don't have generic parameters like Default vs From<T>, and not impls that don't have generic parameters like impl Default for Foo vs impl<T> Default for Vec<T> correct? Just trying to gauge if you're indicating that these Default impls should be good to go or not.

fee1-dead · 2021-08-04T01:08:22Z

Sorry, to be clear, I meant that const impls without trait bounds other than Sized (and also : ?Sized bound) will be good, so yes, I think these Default implementations are good to go.

@rustbot label -S-waiting-on-team S-waiting-on-review

The libs-api team agrees to allow const_trait_impl to appear in the standard library as long as stable code cannot be broken (they are properly gated) this means if the compiler teams thinks it's okay, then it's okay. My priority on constifying would be: 1. Non-generic impls (e.g. Default) or generic impls with no bounds 2. Generic functions with bounds (that use const impls) 3. Generic impls with bounds 4. Impls for traits with associated types For people opening constification PRs: please cc me and/or oli-obk.

oli-obk · 2021-08-17T09:39:46Z

@bors r+

bors · 2021-08-17T09:39:47Z

📌 Commit 6bd2ecb has been approved by oli-obk

bors · 2021-08-17T09:41:02Z

⌛ Testing commit 6bd2ecb with merge 1805ef4dd4df410428fb1cd37a47f88781bc469c...

bors · 2021-08-17T13:56:02Z

💥 Test timed out

rust-log-analyzer · 2021-08-17T14:05:59Z

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oli-obk · 2021-08-17T16:05:28Z

@bors retry

bors · 2021-08-17T22:30:11Z

⌛ Testing commit 6bd2ecb with merge adf1688...

bors · 2021-08-18T01:00:10Z

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing adf1688 to master...

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

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jonas-schievink mentioned this pull request Jul 2, 2021

const fn for option copied, take & replace #86828

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fee1-dead mentioned this pull request Jul 29, 2021

Constify ?-operator for Result and Option #86853

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fee1-dead added 2 commits August 17, 2021 07:15

Add ui test

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This was referenced Aug 18, 2021

Add str pointer methods #85816

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Alloc features cleanup #87769

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BTree: remove Ord bound from new #88040

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AerialX mentioned this pull request Jan 11, 2022

Is there an RFC for this? AerialX/const-default.rs#9

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cuviper added this to the 1.56.0 milestone Nov 16, 2023

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constified implementations of `Default` #86808

constified implementations of `Default` #86808

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constified implementations of Default #86808

constified implementations of Default #86808

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constified implementations of `Default` #86808

constified implementations of `Default` #86808

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