implement const iterator using `rustc_do_not_const_check` #106541

fee1-dead · 2023-01-06T19:04:42Z

Previous experiment: #102225.

Explanation: rather than making all default methods work under const all at once, this uses rustc_do_not_const_check as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the next method. Any actual calls to the trait methods other than next will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

rustbot · 2023-01-06T19:04:47Z

r? @thomcc

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fee1-dead · 2023-01-13T14:05:06Z

cc @oli-obk for compiler changes

rustbot · 2023-01-13T14:05:24Z

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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onestacked · 2023-01-16T15:23:35Z

If this approach is chosen, how is the plan to remove the workaround later?
Will we start bringing some functions slowly from the "Actual" const implementation in while keeping the rest with #[rustc_do_not_const_check]?

fee1-dead · 2023-01-17T01:16:56Z

Will we start bringing some functions slowly from the "Actual" const implementation in while keeping the rest with #[rustc_do_not_const_check]?

Yes. Once const closures hit beta.

bors · 2023-01-25T20:55:51Z

☔ The latest upstream changes (presumably #103902) made this pull request unmergeable. Please resolve the merge conflicts.

m-ou-se · 2023-01-31T16:12:18Z

This seems okay, as long as it doesn't affect the stable behaviour of course. But, what safety mechanisms in place to make sure it doesn't affect stable behaviour, neither now nor (accidentally) in the future?

joshtriplett · 2023-01-31T16:15:09Z

Could we also have a test to make sure this only affects default method implementations, not the functions overriding those implementations in an impl Iterator for Type?

fee1-dead · 2023-01-31T16:46:35Z

This seems okay, as long as it doesn't affect the stable behaviour of course. But, what safety mechanisms in place to make sure it doesn't affect stable behaviour, neither now nor (accidentally) in the future?

Were you talking about the rustc_do_not_const_check attribute or the process of making Iterator const in general? For the attribute, no codegen/mir would be changed as it is only used by the const checker which will skip those marked with the attribute.

As for the process of making Iterator const, I think we can move forward with a conservative approach of only making it const without using workarounds that would make it harder to maintain. (e.g. safest would be only adding const as function modifier or for const closures) The changes to libcore should be easy to revert as to not add additional maintainer burden.

Could we also have a test to make sure this only affects default method implementations, not the functions overriding those implementations in an impl Iterator for Type?

Yes, I will add that in a moment

onestacked · 2023-01-31T16:47:47Z

A test to make that #[rustc_do_not_const_check] can't be used without the feature gate would probably also be good.

onestacked · 2023-02-04T15:50:05Z

library/core/src/iter/traits/iterator.rs

@@ -218,6 +220,7 @@ pub trait Iterator {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+ #[rustc_do_not_const_check]


This is not needed as this is trivially const anyways.

onestacked · 2023-02-04T15:51:50Z

library/core/src/iter/traits/iterator.rs

@@ -255,6 +258,7 @@ pub trait Iterator {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+ #[rustc_do_not_const_check]


Can be removed using const_closures

onestacked · 2023-02-04T15:54:34Z

library/core/src/iter/traits/iterator.rs

@@ -285,6 +289,7 @@ pub trait Iterator {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+ #[rustc_do_not_const_check]
 fn last(self) -> Option<Self::Item>


Can be removed simply by making the inner function const.
In fact looking at this most of those seem to be no longer needed now that const_closures is in bootstrap.

fee1-dead · 2023-02-15T01:51:40Z

@chriss0612 I still think that this PR should be as minimal as possible and we can make followup PRs for it to do more things.

Are there any concerns that are still blocking this?

@rustbot ready

onestacked · 2023-02-15T12:44:55Z

@chriss0612 I still think that this PR should be as minimal as possible and we can make followup PRs for it to do more things.

Yeah that's fine, I mostly wanted to mention it.

thomcc · 2023-02-22T15:22:39Z

My understanding is this has gotten libs-api approval. The implementation seems fine to me, so

@bors r+

bors · 2023-02-22T15:22:46Z

📌 Commit 3aeb43c has been approved by thomcc

It is now in the queue for this repository.

…o, r=thomcc implement const iterator using `rustc_do_not_const_check` Previous experiment: rust-lang#102225. Explanation: rather than making all default methods work under `const` all at once, this uses `rustc_do_not_const_check` as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the `next` method. Any actual calls to the trait methods other than `next` will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

Rollup of 10 pull requests Successful merges: - rust-lang#106541 (implement const iterator using `rustc_do_not_const_check`) - rust-lang#106918 (Rebuild BinaryHeap on unwind from retain) - rust-lang#106923 (Restore behavior when primary bundle is missing) - rust-lang#108169 (Make query keys `Copy`) - rust-lang#108287 (Add test for bad cast with deferred projection equality) - rust-lang#108370 (std: time: Avoid to use "was created" in elapsed() description) - rust-lang#108377 (Fix ICE in 'duplicate diagnostic item' diagnostic) - rust-lang#108388 (parser: provide better suggestions and errors on closures with braces missing) - rust-lang#108391 (Fix `is_terminal`'s handling of long paths on Windows.) - rust-lang#108401 (diagnostics: remove inconsistent English article "this" from E0107) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

[rust-lang/rust#106541][1] added `#[const_trait]` to `Iterator` as well as internal attribute to bypass the constness check for other methods than `Iterator::next`. As a result, user code can now implement `const Iterator` in a normal way, i.e., just by implementing `Iterator::next`. [1]: rust-lang/rust#106541

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fee1-dead force-pushed the no-const-check-no branch from f402cc9 to 4d31609 Compare January 7, 2023 19:26

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fee1-dead force-pushed the no-const-check-no branch from 4d31609 to 85ffe50 Compare January 13, 2023 13:27

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the8472 mentioned this pull request Jan 14, 2023

Allow using Range as an Iterator in const contexts. #104100

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fee1-dead added 2 commits January 31, 2023 17:09

implement const iterator using rustc_do_not_const_check

6bf2c4d

add and bless tests

3aeb43c

fee1-dead force-pushed the no-const-check-no branch from 87a2cbe to 3aeb43c Compare January 31, 2023 17:27

onestacked mentioned this pull request Feb 3, 2023

Experiment: Const iterrator range with #[rustc_do_not_const_check] #107647

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Dylan-DPC mentioned this pull request Feb 23, 2023

Rollup of 5 pull requests #108393

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Rollup of 10 pull requests #108421

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fee1-dead mentioned this pull request Mar 21, 2023

Tracking Issue for const Iterator impls and related const fns #92476

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implement const iterator using `rustc_do_not_const_check` #106541

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implement const iterator using rustc_do_not_const_check #106541

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implement const iterator using `rustc_do_not_const_check` #106541

implement const iterator using `rustc_do_not_const_check` #106541

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