Introduce `const Trait` (always-const trait bounds) #119099

fmease · 2023-12-18T23:22:48Z

Feature const_trait_impl currently lacks a way to express “always const” trait bounds. This makes it impossible to define generic items like fns or structs which contain types that depend on const method calls (*). While the final design and esp. the syntax of effects / keyword generics isn't set in stone, some version of “always const” trait bounds will very likely form a part of it. Further, their implementation is trivial thanks to the effects backbone.

Not sure if this needs t-lang sign-off though.

(*):

#![feature(const_trait_impl, effects, generic_const_exprs)]

fn compute<T: const Trait>() -> Type<{ T::generate() }> { /*…*/ }

struct Store<T: const Trait>
where
    Type<{ T::generate() }>:,
{
    field: Type<{ T::generate() }>,
}

Lastly, “always const” trait bounds are a perfect fit for generic_const_items.

#![feature(const_trait_impl, effects, generic_const_items)]

const DEFAULT<T: const Default>: T = T::default();

Previously, we (oli, fee1-dead and I) wanted to reinterpret ~const Trait as const Trait in generic const items which would've been quite surprising and not very generalizable.
Supersedes #117530.

cc @oli-obk

As discussed
r? fee1-dead (or compiler)

compiler/rustc_ast/src/ast.rs

fmease · 2023-12-18T23:36:14Z

compiler/rustc_ast_lowering/src/lib.rs

+        let id = lcx.next_node_id();
+        let hir_id = lcx.next_id();


I've moved this {Node,Hir}Id creation below the HirId registration for the hir::ExprKind::Path above to accommodate the control flow, I hope that doesn't break any invariants (?).

No, as long as you didn't move it outside of a nested self.with_* function.

Actually... hm... it's probably fine...

compiler/rustc_ast_passes/src/ast_validation.rs

compiler/rustc_middle/src/ty/mod.rs

compiler-errors

Do we have tests to check that we deny ~const in const items and structs and stuff?

fmease · 2023-12-18T23:40:52Z

Do we have tests to check that we deny ~const in const items and structs and stuff?

Yes, we do: https://github.com/rust-lang/rust/blob/master/tests/ui/rfcs/rfc-2632-const-trait-impl/tilde-const-invalid-places.rs

fmease · 2023-12-19T11:59:18Z

compiler/rustc_parse/src/parser/ty.rs

@@ -786,6 +794,7 @@ impl<'a> Parser<'a> {
            || self.check(&token::Not)
            || self.check(&token::Question)
            || self.check(&token::Tilde)
+            || self.check_keyword(kw::Const)


I need to investigate if this visibly changes the MBE matching behavior for stable users. I hope not since this check is required for correctness.

Doesn't seem to be a problem. I've added a test to ensure that const Trait doesn't regress stable code: tests/ui/rfcs/rfc-2632-const-trait-impl/mbe-bare-trait-objects-const-trait-bounds.rs

It's obvious in hindsight but this doesn't regress the aforementioned test because I haven't (and I won't) add kw::Const & TokenKind::Tilde to can_begin_type.

rustbot · 2023-12-19T13:20:25Z

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

bors · 2023-12-22T05:03:40Z

☔ The latest upstream changes (presumably #119163) made this pull request unmergeable. Please resolve the merge conflicts.

compiler/rustc_ast/src/ast.rs

compiler/rustc_ast/src/token.rs

compiler/rustc_ast_lowering/src/lib.rs

compiler/rustc_ast_lowering/src/item.rs

compiler/rustc_hir/src/hir.rs

compiler/rustc_hir_analysis/messages.ftl

src/librustdoc/html/format.rs

fee1-dead · 2023-12-22T10:56:03Z

compiler/rustc_parse/src/parser/ty.rs

+                self.sess.emit_err(errors::ModifierLifetime {
+                    span,
+                    modifier: "const",
+                    padding: " ",
+                });


I did not see an ui test for the padding suggestion. Could you give me a link to it or add it?

Well, padding didn't actually make sense. I've removed it. I could run-rustfix for the -Zparse-only test tests/ui/parser/bounds-type.rs but I'm not sure if it's worth it esp. since it concerns a maybe-incorrect suggestion.

No need for run-rustfix, just an ui test that would have covered this code path with stderr would be fine. Though removing it also works.

It's a tool-only suggestion atm, hence it doesn't show up on stderr.

…=compiler-errors Clean up `check_consts` and misc fixes 1. Remove most of the logic around erroring with trait methods. I have kept the part resolving it to a concrete impl, as that is used for const stability checks. 2. Turning on `effects` causes ICE with generic args, due to `~const Tr` when `Tr` is not `#[const_trait]` tripping up expectation in code that handles generic args, more specifically here: https://github.com/rust-lang/rust/blob/8681e077b8afa99d60acf8f8470a012a3ce709a5/compiler/rustc_hir_analysis/src/astconv/generics.rs#L377 We set `arg_count.correct` to `Err` to correctly signal that an error has already been reported. 3. UI test blesses. Edit(fmease): Fixes rust-lang#117244 (UI test is in rust-lang#119099 for now). r? compiler-errors

…ompiler-errors Clean up `check_consts` and misc fixes 1. Remove most of the logic around erroring with trait methods. I have kept the part resolving it to a concrete impl, as that is used for const stability checks. 2. Turning on `effects` causes ICE with generic args, due to `~const Tr` when `Tr` is not `#[const_trait]` tripping up expectation in code that handles generic args, more specifically here: https://github.com/rust-lang/rust/blob/8681e077b8afa99d60acf8f8470a012a3ce709a5/compiler/rustc_hir_analysis/src/astconv/generics.rs#L377 We set `arg_count.correct` to `Err` to correctly signal that an error has already been reported. 3. UI test blesses. Edit(fmease): Fixes rust-lang#117244 (UI test is in rust-lang#119099 for now). r? compiler-errors

bors · 2023-12-27T14:22:35Z

💔 Test failed - checks-actions

rust-log-analyzer · 2023-12-27T14:23:11Z

The job dist-aarch64-linux failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmease · 2023-12-27T14:57:31Z

Network error
@bors retry

bors · 2023-12-27T19:24:35Z

⌛ Testing commit 3eb48a3 with merge 88d69b7...

bors · 2023-12-27T21:26:18Z

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing 88d69b7 to master...

rust-timer · 2023-12-27T22:46:14Z

Finished benchmarking commit (88d69b7): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.1%, 0.5%]	2
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	2
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.8%, 0.5%]	3

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.5%, 0.8%]	3
Regressions ❌ (secondary)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[-0.5%, 0.8%]	4

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 670.98s -> 672.366s (0.21%)
Artifact size: 312.28 MiB -> 312.33 MiB (0.02%)

…, r=compiler-errors Pretty-print always-const trait predicates correctly Follow-up to rust-lang#119099. r? fee1-dead

Rollup merge of rust-lang#119476 - fmease:pp-always-const-trait-preds, r=compiler-errors Pretty-print always-const trait predicates correctly Follow-up to rust-lang#119099. r? fee1-dead

ytmimi · 2024-01-21T23:59:19Z

@fmease @compiler-errors @fee1-dead I'm hoping I can get some help. I've bisected rust-lang/rustfmt#6035 back to this PR.

With the following input: m!(const N: usize = 0;);

rustfmt 1.7.0-dev (88d69b7 2023-12-27) outputs:

m!(const N: usize = 0;);

but the commit just before (rustfmt 1.7.0-dev (a861c89 2023-12-27)) outputs:

m!(
    const N: usize = 0;
);

Any idea why the formatting would have changed?

…=calebcartwright,ytmimi rustfmt: Check that a token can begin a nonterminal kind before parsing it as a macro arg r? `@ytmimi` and/or `@calebcartwright` cc `@fmease` I'm putting this on r-l/rust since it should fix the nightly rustfmt version. If you don't care about having this regression until the next rustfmt->rust sync, then I can move that PR over to r-l/rustfmt. --- > Any idea why the formatting would have changed [from rust-lang#119099]? **Copied over explanation:** This has to do with the weirdness of the way that `parse_macro_arg` works. Unlike parsing nonterminal args in a macro-by-example, it eagerly tries, for example, to parse a type without checking that the beginning token may begin a type: https://github.com/rust-lang/rustfmt/blob/bf967319e258acb9b1648a952bba52665eceaf52/src/parse/macros/mod.rs#L54 Contrast this to the nonterminal parsing code, which first checks that the nonterminal may begin with a given token: https://github.com/rust-lang/rust/blob/ef71f1047e04438181d7cb925a833e2ada6ab390/compiler/rustc_parse/src/parser/nonterminal.rs#L47 In rust-lang#119099, `@fmease` implemented a change so that `const Tr` would be parsed as `dyn const Tr` (a trait object to a const trait) in edition 2015. This is okay for the purposes of macros, because he explicitly made sure that `const` did not get added to the list of tokens that may begin a `:ty` nonterminal kind: rust-lang#119099 (comment) However, since rustfmt is not so careful about eagerly parsing macro args before checking that they're legal in macro position, this changed the way that the string of tokens is being parsed into macro args.

…=calebcartwright,ytmimi rustfmt: Check that a token can begin a nonterminal kind before parsing it as a macro arg r? ``@ytmimi`` and/or ``@calebcartwright`` cc ``@fmease`` I'm putting this on r-l/rust since it should fix the nightly rustfmt version. If you don't care about having this regression until the next rustfmt->rust sync, then I can move that PR over to r-l/rustfmt. --- > Any idea why the formatting would have changed [from rust-lang#119099]? **Copied over explanation:** This has to do with the weirdness of the way that `parse_macro_arg` works. Unlike parsing nonterminal args in a macro-by-example, it eagerly tries, for example, to parse a type without checking that the beginning token may begin a type: https://github.com/rust-lang/rustfmt/blob/bf967319e258acb9b1648a952bba52665eceaf52/src/parse/macros/mod.rs#L54 Contrast this to the nonterminal parsing code, which first checks that the nonterminal may begin with a given token: https://github.com/rust-lang/rust/blob/ef71f1047e04438181d7cb925a833e2ada6ab390/compiler/rustc_parse/src/parser/nonterminal.rs#L47 In rust-lang#119099, ``@fmease`` implemented a change so that `const Tr` would be parsed as `dyn const Tr` (a trait object to a const trait) in edition 2015. This is okay for the purposes of macros, because he explicitly made sure that `const` did not get added to the list of tokens that may begin a `:ty` nonterminal kind: rust-lang#119099 (comment) However, since rustfmt is not so careful about eagerly parsing macro args before checking that they're legal in macro position, this changed the way that the string of tokens is being parsed into macro args.

Rollup merge of rust-lang#120218 - compiler-errors:parse_macro_arg, r=calebcartwright,ytmimi rustfmt: Check that a token can begin a nonterminal kind before parsing it as a macro arg r? ``@ytmimi`` and/or ``@calebcartwright`` cc ``@fmease`` I'm putting this on r-l/rust since it should fix the nightly rustfmt version. If you don't care about having this regression until the next rustfmt->rust sync, then I can move that PR over to r-l/rustfmt. --- > Any idea why the formatting would have changed [from rust-lang#119099]? **Copied over explanation:** This has to do with the weirdness of the way that `parse_macro_arg` works. Unlike parsing nonterminal args in a macro-by-example, it eagerly tries, for example, to parse a type without checking that the beginning token may begin a type: https://github.com/rust-lang/rustfmt/blob/bf967319e258acb9b1648a952bba52665eceaf52/src/parse/macros/mod.rs#L54 Contrast this to the nonterminal parsing code, which first checks that the nonterminal may begin with a given token: https://github.com/rust-lang/rust/blob/ef71f1047e04438181d7cb925a833e2ada6ab390/compiler/rustc_parse/src/parser/nonterminal.rs#L47 In rust-lang#119099, ``@fmease`` implemented a change so that `const Tr` would be parsed as `dyn const Tr` (a trait object to a const trait) in edition 2015. This is okay for the purposes of macros, because he explicitly made sure that `const` did not get added to the list of tokens that may begin a `:ty` nonterminal kind: rust-lang#119099 (comment) However, since rustfmt is not so careful about eagerly parsing macro args before checking that they're legal in macro position, this changed the way that the string of tokens is being parsed into macro args.

internal: Parse (nightly) `const` and `async` trait bounds Both of these bound modifiers were added recently: * `const` trait bounds: rust-lang/rust#119099 * `async` trait bounds: rust-lang/rust#120392 The latter will certainly will not do the right thing; namely, `async Fn` needs to be mapped to the `AsyncFn` trait. IDK how to do that, so advice would be appreciated, though perhaps we could land this first so the parser isn't complaining about these bounds?

rustbot assigned fee1-dead Dec 18, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 18, 2023

fmease added F-const_trait_impl `#![feature(const_trait_impl)]` F-effects `#![feature(effects)]` labels Dec 18, 2023

This comment has been minimized.

Sign in to view

fmease commented Dec 18, 2023

View reviewed changes

compiler-errors reviewed Dec 18, 2023

View reviewed changes

fmease force-pushed the always-const-trait-bounds branch from b265a78 to 1ab36f6 Compare December 19, 2023 11:56

rustbot added the A-rustdoc-json Area: Rustdoc JSON backend label Dec 19, 2023

fmease marked this pull request as ready for review December 19, 2023 11:57

fmease removed the A-rustdoc-json Area: Rustdoc JSON backend label Dec 19, 2023

fmease commented Dec 19, 2023

View reviewed changes

This comment has been minimized.

Sign in to view

fmease force-pushed the always-const-trait-bounds branch from 1ab36f6 to 932c309 Compare December 19, 2023 13:20

rustbot added the A-rustdoc-json Area: Rustdoc JSON backend label Dec 19, 2023

fmease mentioned this pull request Dec 19, 2023

Clean up check_consts and misc fixes #119072

Merged

This comment has been minimized.

Sign in to view

fmease force-pushed the always-const-trait-bounds branch 4 times, most recently from 310df46 to 2abd167 Compare December 19, 2023 19:52

fee1-dead requested changes Dec 22, 2023

View reviewed changes

rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Dec 22, 2023

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Dec 27, 2023

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-review Status: Awaiting review from the assignee but also interested parties. labels Dec 27, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Dec 27, 2023

bors merged commit 88d69b7 into rust-lang:master Dec 27, 2023
12 checks passed

rustbot added this to the 1.77.0 milestone Dec 27, 2023

fmease deleted the always-const-trait-bounds branch December 27, 2023 21:33

bors mentioned this pull request Dec 27, 2023

[experiment] turn on effects everywhere #119083

Closed

fmease mentioned this pull request Dec 31, 2023

Pretty-print always-const trait predicates correctly #119476

Merged

fmease added a commit to fmease/rust that referenced this pull request Jan 2, 2024

Rollup merge of rust-lang#119476 - fmease:pp-always-const-trait-preds…

6240bc9

…, r=compiler-errors Pretty-print always-const trait predicates correctly Follow-up to rust-lang#119099. r? fee1-dead

fmease added a commit to fmease/rust that referenced this pull request Jan 3, 2024

Rollup merge of rust-lang#119476 - fmease:pp-always-const-trait-preds…

e8e5a7e

…, r=compiler-errors Pretty-print always-const trait predicates correctly Follow-up to rust-lang#119099. r? fee1-dead

fmease added a commit to fmease/rust that referenced this pull request Jan 3, 2024

Rollup merge of rust-lang#119476 - fmease:pp-always-const-trait-preds…

b41b169

…, r=compiler-errors Pretty-print always-const trait predicates correctly Follow-up to rust-lang#119099. r? fee1-dead

fmease removed the A-rustdoc-json Area: Rustdoc JSON backend label Jan 7, 2024

ytmimi mentioned this pull request Jan 21, 2024

rustfmt stopped formatting const items in macros rust-lang/rustfmt#6035

Open

compiler-errors mentioned this pull request Jan 22, 2024

rustfmt: Check that a token can begin a nonterminal kind before parsing it as a macro arg #120218

Merged

compiler-errors mentioned this pull request Feb 16, 2024

internal: Parse (nightly) const and async trait bounds rust-lang/rust-analyzer#16588

Merged

fmease mentioned this pull request Jul 26, 2024

Tracking issue for generic const items #113521

Open

7 tasks

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Introduce `const Trait` (always-const trait bounds) #119099

Introduce `const Trait` (always-const trait bounds) #119099

fmease commented Dec 18, 2023 •

edited

Loading

This comment has been minimized.

fmease Dec 18, 2023

compiler-errors Dec 18, 2023

compiler-errors Dec 18, 2023

compiler-errors left a comment

fmease commented Dec 18, 2023

fmease Dec 19, 2023

fmease Dec 24, 2023

fmease Dec 27, 2023

This comment has been minimized.

rustbot commented Dec 19, 2023

This comment has been minimized.

bors commented Dec 22, 2023

fee1-dead Dec 22, 2023

fmease Dec 23, 2023

fee1-dead Dec 24, 2023

fmease Dec 24, 2023 •

edited

Loading

bors commented Dec 27, 2023

rust-log-analyzer commented Dec 27, 2023

fmease commented Dec 27, 2023

bors commented Dec 27, 2023

bors commented Dec 27, 2023

rust-timer commented Dec 27, 2023

ytmimi commented Jan 21, 2024

Introduce const Trait (always-const trait bounds) #119099

Introduce const Trait (always-const trait bounds) #119099

Conversation

fmease commented Dec 18, 2023 • edited Loading

This comment has been minimized.

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

compiler-errors left a comment

Choose a reason for hiding this comment

fmease commented Dec 18, 2023

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

This comment has been minimized.

rustbot commented Dec 19, 2023

This comment has been minimized.

bors commented Dec 22, 2023

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

fmease Dec 24, 2023 • edited Loading

Choose a reason for hiding this comment

bors commented Dec 27, 2023

rust-log-analyzer commented Dec 27, 2023

fmease commented Dec 27, 2023

bors commented Dec 27, 2023

bors commented Dec 27, 2023

rust-timer commented Dec 27, 2023

Overall result: no relevant changes - no action needed

ytmimi commented Jan 21, 2024

Introduce `const Trait` (always-const trait bounds) #119099

Introduce `const Trait` (always-const trait bounds) #119099

fmease commented Dec 18, 2023 •

edited

Loading

fmease Dec 24, 2023 •

edited

Loading