Tracking issue for RFC 2632 `?const` trait bound opt-out #67794

ecstatic-morse · 2020-01-02T00:38:25Z

This is the tracking issue for the ?const opt-out for trait bounds described in rust-lang/rfcs#2632. The primary tracking issue for that RFC is #67792. The feature gate for it is const_trait_bound_opt_out.

This RFC has not yet been accepted. It is being implemented on a provisional basis to evaluate the potential fallout.

cc #57563

Implementation steps:

Implement (Implement ?const opt-out for trait bounds #68140)
Forbid ?const outside const fn or impl const Trait

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

@oli-obk

Parse the syntax described in RFC 2632 This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out. I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled? @oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking? cc rust-lang#67792 rust-lang#67794 r? @Centril

@oli-obk

Parse the syntax described in RFC 2632 This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out. I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled? @oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking? cc rust-lang#67792 rust-lang#67794 r? @Centril

@oli-obk

Parse the syntax described in RFC 2632 This adds support for both `impl const Trait for Ty` and `?const Trait` bound syntax from rust-lang/rfcs#2632 to the parser. For now, both modifiers end up in a newly-added `constness` field on `ast::TraitRef`, although this may change once the implementation is fleshed out. I was planning on using `delay_span_bug` when this syntax is encountered during lowering, but I can't write `should-ice` UI tests. I emit a normal error instead, which causes duplicates when the feature gate is not enabled (see the `.stderr` files for the feature gate tests). Not sure what the desired approach is; Maybe just do nothing when the syntax is encountered with the feature gate is enabled? @oli-obk I went with `const_trait_impl` and `const_trait_bound_opt_out` for the names of these features. Are these to your liking? cc rust-lang#67792 rust-lang#67794 r? @Centril

@oli-obk

Support `?const` opt-out for trait bounds For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](rust-lang/rfcs#2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment. cc #67794 r? @oli-obk

@oli-obk

…i-obk Implement `?const` opt-out for trait bounds For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](rust-lang/rfcs#2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment. Prior to this PR, the constness of a trait bound/impl was stored in `TraitRef`. Now, the constness of an `impl` is stored in `ast::ItemKind::Impl` and the constness of a bound in `ast::TraitBoundModifer`. Additionally, constness of trait bounds is now stored in an additional field of `ty::Predicate::Trait`, and the combination of the constness of the item along with any `TraitBoundModifier` determines the constness of the bound in accordance with the RFC. Encoding the constness of impls at the `ty` level is left for a later PR. After a discussion in \#wg-grammar on Discord, it was decided that the grammar should not encode the mutual exclusivity of trait bound modifiers. The grammar for trait bound modifiers remains `[?const] [?]`. To encode this, I add a dummy variant to `ast::TraitBoundModifier` that is used when the syntax `?const ?` appears. This variant causes an error in AST validation and disappears during HIR lowering. cc #67794 r? @oli-obk

@oli-obk

…t-out, r=oli-obk Implement `?const` opt-out for trait bounds For now, such bounds are treated exactly the same as unprefixed ones in all contexts. [RFC 2632](rust-lang/rfcs#2632) does not specify whether such bounds are forbidden outside of `const` contexts, so they are allowed at the moment. Prior to this PR, the constness of a trait bound/impl was stored in `TraitRef`. Now, the constness of an `impl` is stored in `ast::ItemKind::Impl` and the constness of a bound in `ast::TraitBoundModifer`. Additionally, constness of trait bounds is now stored in an additional field of `ty::Predicate::Trait`, and the combination of the constness of the item along with any `TraitBoundModifier` determines the constness of the bound in accordance with the RFC. Encoding the constness of impls at the `ty` level is left for a later PR. After a discussion in \#wg-grammar on Discord, it was decided that the grammar should not encode the mutual exclusivity of trait bound modifiers. The grammar for trait bound modifiers remains `[?const] [?]`. To encode this, I add a dummy variant to `ast::TraitBoundModifier` that is used when the syntax `?const ?` appears. This variant causes an error in AST validation and disappears during HIR lowering. cc rust-lang#67794 r? @oli-obk

clarfonthey · 2021-09-01T00:44:20Z

From what I understand, this was recently changed to ~const but none of the tilde-const PRs link back to this one, and the issue name/description should be updated?

clarfonthey · 2021-09-01T00:45:30Z

cc @oli-obk @fee1-dead since from what I understand you two know the most about this change

fee1-dead · 2021-09-01T02:52:31Z

The opt-out is no longer needed. The previous scheme was:

const fn a<T: /* inferred const bound */ Foo + /* opt-out normal trait bound */ ?const Bar>

And the current scheme is now:

const fn a<T: /* normal trait bound */ Foo + /* const bound if called in a const context */ ~const Bar>

This issue should be closed, but it can be reopened if somehow we decide to use ?const opt-out again.

jplatte · 2021-09-01T08:48:39Z

So where is ~const tracked then? #67792?

oli-obk · 2021-09-01T09:00:20Z

yea, const_trait_impl requires the opt in syntax, otherwise you can't write anything but trivial impls

clarfonthey · 2021-09-01T16:07:59Z

So... is there no dedicated tracking issue for ~const, then?

Mostly because I don't see any easy place someone can look to know what the new syntax means.

oli-obk · 2021-09-01T22:56:29Z

I updated the const_trait_impl tracking issue to link to the latest RFC text

ecstatic-morse mentioned this issue Jan 2, 2020

Tracking issue for RFC 2632, impl const Trait for Ty and ~const (tilde const) syntax #67792

Open

11 tasks

ecstatic-morse added A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. labels Jan 2, 2020

Centril added the T-lang Relevant to the language team, which will review and decide on the PR/issue. label Jan 2, 2020

ecstatic-morse mentioned this issue Jan 3, 2020

Parse the syntax described in RFC 2632 #67820

Merged

ecstatic-morse changed the title ~~Tracking issue for RFC 2362, ?const trait bound opt-out~~ Tracking issue for RFC 2632 ?const trait bound opt-out Jan 3, 2020

oli-obk added the F-const_trait_bound_opt_out label Jan 5, 2020

ecstatic-morse mentioned this issue Jan 12, 2020

Implement ?const opt-out for trait bounds #68140

Merged

jonas-schievink added the A-const-eval Area: constant evaluation (mir interpretation) label Mar 28, 2020

jplatte mentioned this issue May 2, 2020

Tracking issue for RFC 2342, "Allow if and match in constants" #49146

Closed

4 tasks

jonas-schievink mentioned this issue Jun 7, 2020

Array range indexes don't work in const eval #73083

Closed

mcy mentioned this issue Aug 14, 2020

Building Tock with stable rustc tock/tock#1654

Open

28 tasks

Lokathor mentioned this issue Aug 23, 2020

Meta tracking issue for const fn #57563

Open

25 tasks

josephlr mentioned this issue Sep 29, 2020

Fix build error on latest nightly rust-osdev/x86_64#186

Merged

RalfJung mentioned this issue Apr 18, 2021

further split up const_fn feature flag #84310

Merged

oli-obk closed this as completed Sep 1, 2021

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Tracking issue for RFC 2632 `?const` trait bound opt-out #67794

Tracking issue for RFC 2632 `?const` trait bound opt-out #67794

ecstatic-morse commented Jan 2, 2020 •

edited by oli-obk

clarfonthey commented Sep 1, 2021

clarfonthey commented Sep 1, 2021

fee1-dead commented Sep 1, 2021

jplatte commented Sep 1, 2021

oli-obk commented Sep 1, 2021

clarfonthey commented Sep 1, 2021

oli-obk commented Sep 1, 2021

Tracking issue for RFC 2632 ?const trait bound opt-out #67794

Tracking issue for RFC 2632 ?const trait bound opt-out #67794

Comments

ecstatic-morse commented Jan 2, 2020 • edited by oli-obk

clarfonthey commented Sep 1, 2021

clarfonthey commented Sep 1, 2021

fee1-dead commented Sep 1, 2021

jplatte commented Sep 1, 2021

oli-obk commented Sep 1, 2021

clarfonthey commented Sep 1, 2021

oli-obk commented Sep 1, 2021

Tracking issue for RFC 2632 `?const` trait bound opt-out #67794

Tracking issue for RFC 2632 `?const` trait bound opt-out #67794

ecstatic-morse commented Jan 2, 2020 •

edited by oli-obk