Use implicit (not explicit) rules for promotability by default in `const fn` #75502

ecstatic-morse · 2020-08-13T21:25:15Z

For crater run. See rust-lang/const-eval#54 (comment).

ecstatic-morse · 2020-08-13T22:01:33Z

bors · 2020-08-13T22:01:44Z

⌛ Trying commit 860d599d5b11db275a829a3f898560405fa84bbe with merge f6e2f78768c57172b99c3e85892f867894dc7e30...

bors · 2020-08-13T22:43:46Z

☀️ Try build successful - checks-actions, checks-azure
Build commit: f6e2f78768c57172b99c3e85892f867894dc7e30 (f6e2f78768c57172b99c3e85892f867894dc7e30)

ecstatic-morse · 2020-08-13T23:37:08Z

@craterbot check

cc @RalfJung. Unfortunately the queue is long.

craterbot · 2020-08-13T23:37:11Z

👌 Experiment pr-75502 created and queued.
🤖 Automatically detected try build f6e2f78768c57172b99c3e85892f867894dc7e30
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

RalfJung · 2020-08-14T06:09:38Z

src/librustc_mir/transform/promote_consts.rs

+            Some(hir::ConstContext::Static(_) | hir::ConstContext::Const)
+        );
+
+        if !self.explicit && !context_uses_explicit_promotion_rules {


self.explicit is already encoding whether we use explicit promotion, and that's what we check everywhere else in this file. Why does this one check need special treatment?

Ah I see, it's the self.const_kind.is_none() here that introduced the exception. IMO that should just be if !self.explicit... but that doesn't seem how we currently organize things.

However, this means that all const_kind.is_none() are wrong. Promotion should behave the same for const_kind == Some(ConstFn) and const_kind == None.

Most of these checks are for things that are not allowed in a const context (without feature gates). Presumably this logic is left over from when promotion and const-checking happened in the same module.

Ah, that's a good point.

Still, it wouldn't hurt to adjust them all to call something like is_const_only or so (meaning "this code will only ever be executed during CTFE), just to make sure we haven't missed any of them.

RalfJung · 2020-08-14T06:24:00Z

Let's also cc @oli-obk.

I must still be missing something. This line sets implicit promotion rules for lifetime extension:

https://github.com/rust-lang/rust/blob/860d599d5b11db275a829a3f898560405fa84bbe/src/librustc_mir/transform/promote_consts.rs#L801

Where is the code that overwrites this to use explicit promotion rules for lifetime extension in const/static items and const fn bodies?

EDIT: Found the answer.

RalfJung · 2020-08-14T08:42:32Z

Give the length of the queue, I wonder if it's not worth to crater all the cases where const fn are treated differently from fn at once. Basically, no more const_kind.is_{some,none}. Those are the most critical points, I think.

Btw the PR title needs adjusting -- this is not about explicit vs implicit promotion (as witnessed by the code), it's about promotion depending on const_kind.

craterbot · 2020-08-29T04:36:51Z

🚧 Experiment pr-75502 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2020-08-31T16:32:56Z

🎉 Experiment pr-75502 is completed!
📊 13 regressed and 8 fixed (117373 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

ecstatic-morse · 2020-08-31T16:44:47Z

There appears to be a single, legit root regression in the x86_64 crate when the const_fn feature is enabled.

https://github.com/rust-osdev/x86_64/blob/fe8d09c7edeaa399d6390da31de8aa7c4819731a/src/structures/paging/page_table.rs#L190-L194

This could be patched relatively easily, or we could revive the attempts to treat non-Copy array initializers as explicit promotion contexts.

It seems that we could change these semantics without too severe of an impact on the ecosystem. @RalfJung what's the next step here?

RalfJung · 2020-09-01T12:56:06Z

There appears to be a single, legit root regression in the x86_64 crate when the const_fn feature is enabled.

That's great news!
I do not even see the use of promotion there? ENTRY_COUNT is a const.

EDIT: D'oh, it's for the initializer, not the length. Can you submit a PR to work around this?

It seems that we could change these semantics without too severe of an impact on the ecosystem. @RalfJung what's the next step here?

We nominate this for lang-team discussion and try to get the FCP process started.

Also please rebase and resolve the conflicts.

RalfJung · 2020-09-01T12:57:41Z

@rust-lang/lang the summary for the FCP is that this does most of what it takes to fix #75586. I view this as an instance of "we accidentally stabilized too much". For further information, see that issue; for the big picture, see rust-lang/const-eval#53.

Hi! Rust compiler team member here. When the `const_fn` feature is enabled, the implementation of `PageTable::new` relies on a compiler bug to work. Specifically, `PageTableEntry::new` should not be eligible for promotion as part of an array initializer. See rust-lang/rust#75502 for more information. This bug may be fixed in the future, which will cause this crate to stop compiling.

RalfJung · 2020-09-17T07:05:38Z

Sorry, I meant 65ee8a787d33dd5a0f0695d81508acdd9e4c8f3c.

ecstatic-morse · 2020-09-18T16:08:03Z

@bors r+

bors · 2020-09-18T16:08:04Z

📌 Commit e03f416 has been approved by ecstatic-morse

RalfJung · 2020-09-18T16:11:42Z

Let's reflect the reviewer.
@bors r- r=RalfJung

bors · 2020-09-18T16:11:45Z

📌 Commit e03f416 has been approved by RalfJung

Rollup of 14 pull requests Successful merges: - rust-lang#73963 (deny(unsafe_op_in_unsafe_fn) in libstd/path.rs) - rust-lang#75099 (lint/ty: move fns to avoid abstraction violation) - rust-lang#75502 (Use implicit (not explicit) rules for promotability by default in `const fn`) - rust-lang#75580 (Add test for checking duplicated branch or-patterns) - rust-lang#76310 (Add `[T; N]: TryFrom<Vec<T>>` (insta-stable)) - rust-lang#76400 (Clean up vec benches bench_in_place style) - rust-lang#76434 (do not inline black_box when building for Miri) - rust-lang#76492 (Add associated constant `BITS` to all integer types) - rust-lang#76525 (Add as_str() to string::Drain.) - rust-lang#76636 (assert ScalarMaybeUninit size) - rust-lang#76749 (give *even better* suggestion when matching a const range) - rust-lang#76757 (don't convert types to the same type with try_into (clippy::useless_conversion)) - rust-lang#76796 (Give a better error message when x.py uses the wrong stage for CI) - rust-lang#76798 (Build fixes for RISC-V 32-bit Linux support) Failed merges: r? `@ghost`

…tatic-morse Some promotion cleanup Based on top of both rust-lang#75502 and rust-lang#75585, this does some cleanup of the promotion code. The last 2 commits are new. * Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes rust-lang#75586. * ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~ * Promote `&mut []` everywhere, not just in non-`const` functions, for consistency. * Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~ This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from @rust-lang/wg-const-eval .

…tic-morse Some promotion cleanup Based on top of both rust-lang#75502 and rust-lang#75585, this does some cleanup of the promotion code. The last 2 commits are new. * Remove the remaining cases where `const fn` is treated different from `fn`. This means no longer promoting ptr-to-int casts, raw ptr operations, and union field accesses in `const fn` -- or anywhere, for that matter. These are all unstable in const-context so this should not break any stable code. Fixes rust-lang#75586. * ~~Promote references to statics even outside statics (i.e., in functions) for consistency.~~ * Promote `&mut []` everywhere, not just in non-`const` functions, for consistency. * Explain why we do not promote deref's of statics outside statics. ~~(This is the only remaining direct user of `const_kind`.)~~ This can only land once the other two PRs land; I am mostly putting this up already because I couldn't wait ;) and to get some feedback from `@rust-lang/wg-const-eval` .

lcnr · 2020-10-05T11:06:31Z

This is an expected breaking change so it probably makes sense to quickly mention this in the release notes

Clean up some of the pkgsrc Makefile, there's still lots in here that should just be deleted though. Switch SunOS to the illumos bootstrap by default. Version 1.48.0 (2020-11-19) ========================== Language -------- - [The `unsafe` keyword is now syntactically permitted on modules.][75857] This is still rejected *semantically*, but can now be parsed by procedural macros. Compiler -------- - [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158] This tells `rustc` whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on `windows-gnu`, `linux-musl`, and `wasi` platforms.) - [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.][77386] Note: If you're using cargo you must explicitly pass the `--target` flag. - [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`, and `&Stderr`.][76275] - [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310] - [The `matches!` macro now supports having a trailing comma.][74880] - [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194] - [The `RefCell::{replace, replace_with, clone}` methods now all use `#[track_caller]`.][77055] Stabilized APIs --------------- - [`slice::as_ptr_range`] - [`slice::as_mut_ptr_range`] - [`VecDeque::make_contiguous`] - [`future::pending`] - [`future::ready`] The following previously stable methods are now `const fn`'s: - [`Option::is_some`] - [`Option::is_none`] - [`Option::as_ref`] - [`Result::is_ok`] - [`Result::is_err`] - [`Result::as_ref`] - [`Ordering::reverse`] - [`Ordering::then`] Cargo ----- Rustdoc ------- - [You can now link to items in `rustdoc` using the intra-doc link syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate a link to `std::future`'s documentation. See ["Linking to items by name"][intradoc-links] for more information. - [You can now specify `#[doc(alias = "<alias>")]` on items to add search aliases when searching through `rustdoc`'s UI.][75740] Compatibility Notes ------------------- - [Promotion of references to `'static` lifetime inside `const fn` now follows the same rules as inside a `fn` body.][75502] In particular, `&foo()` will not be promoted to `'static` lifetime any more inside `const fn`s. - [Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.][27675] - [When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.][54121] - [Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check.][77153] This may cause errors if your macro wasn't correctly handling recursive nonterminal tokens. - [`&mut` references to non zero-sized types are no longer promoted.][75585] - [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]` in places where they have no effect.][73461] - [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166] - [`mem::uninitialized` will now panic if any inner types inside a struct or enum disallow zero-initialization.][71274] - [`#[target_feature]` will now error if used in a place where it has no effect.][78143] - [Foreign exceptions are now caught by `catch_unwind` and will cause an abort.][70212] Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the [`catch_unwind`] documentation for further information. Internal Only ------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Building `rustc` from source now uses `ninja` by default over `make`.][74922] You can continue building with `make` by setting `ninja=false` in your `config.toml`. - [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030] - [Made `ensure_sufficient_stack()` non-generic][76680] [78143]: rust-lang/rust#78143 [76680]: rust-lang/rust#76680 [76030]: rust-lang/rust#76030 [70212]: rust-lang/rust#70212 [27675]: rust-lang/rust#27675 [54121]: rust-lang/rust#54121 [71274]: rust-lang/rust#71274 [77386]: rust-lang/rust#77386 [77153]: rust-lang/rust#77153 [77055]: rust-lang/rust#77055 [76275]: rust-lang/rust#76275 [76310]: rust-lang/rust#76310 [76420]: rust-lang/rust#76420 [76158]: rust-lang/rust#76158 [75857]: rust-lang/rust#75857 [75585]: rust-lang/rust#75585 [75740]: rust-lang/rust#75740 [75502]: rust-lang/rust#75502 [74880]: rust-lang/rust#74880 [74922]: rust-lang/rust#74922 [74430]: rust-lang/rust#74430 [74194]: rust-lang/rust#74194 [73461]: rust-lang/rust#73461 [73166]: rust-lang/rust#73166 [intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html [`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some [`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none [`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref [`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok [`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err [`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref [`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse [`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then [`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range [`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range [`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous [`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html [`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html

Pkgsrc changes: * Compensate for files being moved around upstream. * Introduce optional, on-by-default semi-static building of cargo, using the internal curl and openssl sources. This reduces the dynamic dependencies of cargo and therefore the rust package itself. Ref. options.mk. * The 1.47.0 bootstrap kits have been re-built with the above option turned on, so no longer depends on curl or openssl from pkgsrc and/or from earlier OS or pkgsrc versions. This should hopefully fix installation of rust with non-default PREFIX, ref. PR#54453. Upstream changes: Version 1.48.0 (2020-11-19) ========================== Language -------- - [The `unsafe` keyword is now syntactically permitted on modules.][75857] This is still rejected *semantically*, but can now be parsed by procedural macros. Compiler -------- - [Stabilised the `-C link-self-contained=<yes|no>` compiler flag.][76158] This tells `rustc` whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on `windows-gnu`, `linux-musl`, and `wasi` platforms.) - [You can now use `-C target-feature=+crt-static` on `linux-gnu` targets.] [77386] Note: If you're using cargo you must explicitly pass the `--target` flag. - [Added tier 2\* support for `aarch64-unknown-linux-musl`.][76420] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`io::Write` is now implemented for `&ChildStdin` `&Sink`, `&Stdout`, and `&Stderr`.][76275] - [All arrays of any length now implement `TryFrom<Vec<T>>`.][76310] - [The `matches!` macro now supports having a trailing comma.][74880] - [`Vec<A>` now implements `PartialEq<[B]>` where `A: PartialEq<B>`.][74194] - [The `RefCell::{replace, replace_with, clone}` methods now all use `#[track_caller]`.][77055] Stabilized APIs --------------- - [`slice::as_ptr_range`] - [`slice::as_mut_ptr_range`] - [`VecDeque::make_contiguous`] - [`future::pending`] - [`future::ready`] The following previously stable methods are now `const fn`'s: - [`Option::is_some`] - [`Option::is_none`] - [`Option::as_ref`] - [`Result::is_ok`] - [`Result::is_err`] - [`Result::as_ref`] - [`Ordering::reverse`] - [`Ordering::then`] Cargo ----- Rustdoc ------- - [You can now link to items in `rustdoc` using the intra-doc link syntax.][74430] E.g. ``/// Uses [`std::future`]`` will automatically generate a link to `std::future`'s documentation. See ["Linking to items by name"][intradoc-links] for more information. - [You can now specify `#[doc(alias = "<alias>")]` on items to add search aliases when searching through `rustdoc`'s UI.][75740] Compatibility Notes ------------------- - [Promotion of references to `'static` lifetime inside `const fn` now follows the same rules as inside a `fn` body.][75502] In particular, `&foo()` will not be promoted to `'static` lifetime any more inside `const fn`s. - [Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.][27675] - [When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.][54121] - [Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check.][77153] This may cause errors if your macro wasn't correctly handling recursive nonterminal tokens. - [`&mut` references to non zero-sized types are no longer promoted.][75585] - [`rustc` will now warn if you use attributes like `#[link_name]` or `#[cold]` in places where they have no effect.][73461] - [Updated `_mm256_extract_epi8` and `_mm256_extract_epi16` signatures in `arch::{x86, x86_64}` to return `i32` to match the vendor signatures.][73166] - [`mem::uninitialized` will now panic if any inner types inside a struct or enum disallow zero-initialization.][71274] - [`#[target_feature]` will now error if used in a place where it has no effect.][78143] - [Foreign exceptions are now caught by `catch_unwind` and will cause an abort.][70212] Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the [`catch_unwind`] documentation for further information. Internal Only ------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Building `rustc` from source now uses `ninja` by default over `make`.][74922] You can continue building with `make` by setting `ninja=false` in your `config.toml`. - [cg_llvm: `fewer_names` in `uncached_llvm_type`][76030] - [Made `ensure_sufficient_stack()` non-generic][76680] [78143]: rust-lang/rust#78143 [76680]: rust-lang/rust#76680 [76030]: rust-lang/rust#76030 [70212]: rust-lang/rust#70212 [27675]: rust-lang/rust#27675 [54121]: rust-lang/rust#54121 [71274]: rust-lang/rust#71274 [77386]: rust-lang/rust#77386 [77153]: rust-lang/rust#77153 [77055]: rust-lang/rust#77055 [76275]: rust-lang/rust#76275 [76310]: rust-lang/rust#76310 [76420]: rust-lang/rust#76420 [76158]: rust-lang/rust#76158 [75857]: rust-lang/rust#75857 [75585]: rust-lang/rust#75585 [75740]: rust-lang/rust#75740 [75502]: rust-lang/rust#75502 [74880]: rust-lang/rust#74880 [74922]: rust-lang/rust#74922 [74430]: rust-lang/rust#74430 [74194]: rust-lang/rust#74194 [73461]: rust-lang/rust#73461 [73166]: rust-lang/rust#73166 [intradoc-links]: https://doc.rust-lang.org/rustdoc/linking-to-items-by-name.html [`catch_unwind`]: https://doc.rust-lang.org/std/panic/fn.catch_unwind.html [`Option::is_some`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_some [`Option::is_none`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.is_none [`Option::as_ref`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref [`Result::is_ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_ok [`Result::is_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.is_err [`Result::as_ref`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.as_ref [`Ordering::reverse`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.reverse [`Ordering::then`]: https://doc.rust-lang.org/std/cmp/enum.Ordering.html#method.then [`slice::as_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_ptr_range [`slice::as_mut_ptr_range`]: https://doc.rust-lang.org/std/primitive.slice.html#method.as_mut_ptr_range [`VecDeque::make_contiguous`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.make_contiguous [`future::pending`]: https://doc.rust-lang.org/std/future/fn.pending.html [`future::ready`]: https://doc.rust-lang.org/std/future/fn.ready.html

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RalfJung reviewed Aug 14, 2020

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RalfJung mentioned this pull request Aug 16, 2020

Treat const fn like fn for promotion. #75586

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Aug 31, 2020

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ecstatic-morse changed the title ~~Use implicit (not explicit) rules for promotability in const fn~~ Use implicit (not explicit) rules for promotability by default in const fn Sep 1, 2020

Default to implicit (not explicit) rules for promotability in const fn

e03f416

ecstatic-morse force-pushed the implicit-promotion-in-const-fn branch from 7258290 to e03f416 Compare September 17, 2020 15:39

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 18, 2020

bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 18, 2020

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RalfJung mentioned this pull request Sep 19, 2020

Rollup of 14 pull requests #76912

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bors merged commit f62ba52 into rust-lang:master Sep 19, 2020

rustbot added this to the 1.48.0 milestone Sep 19, 2020

RalfJung mentioned this pull request Sep 24, 2020

Regression for #![feature(const_in_array_repeat_expressions)] #77138

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SNCPlay42 mentioned this pull request Oct 5, 2020

Previously working code doesn't work on nightly (zero-sized types) #77563

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lcnr added the relnotes Marks issues that should be documented in the release notes of the next release. label Oct 5, 2020

ecstatic-morse deleted the implicit-promotion-in-const-fn branch October 6, 2020 01:42

RalfJung added the A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) label Dec 1, 2024

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Use implicit (not explicit) rules for promotability by default in `const fn` #75502

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RalfJung commented Aug 14, 2020 • edited Loading

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lcnr commented Oct 5, 2020 • edited Loading

Use implicit (not explicit) rules for promotability by default in `const fn` #75502

Use implicit (not explicit) rules for promotability by default in `const fn` #75502

ecstatic-morse commented Aug 13, 2020 •

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