Improve associated constant item CTFE timing section #1147

XrXr · 2022-01-27T20:29:14Z

@RalfJung pointed out in a comment that the previous phrasing of the
sentence can read like it is giving guarantees about free constant
definitions always undergoing CTFE, even when unused. That seems to be
how the compiler behaves right now, but it's unclear whether it's
intentional.

Be more precise and don't talk about free constants at all.

Related to this, I'm not sure if rust-lang/rust#89006
stabilized the fact that unamed constants (in contrast to named but unused free constants) always undergo CTFE. It's part
of the issue, and it's used in the release notes to demo the feature, so I had assumed that it does. Going through the RFC and the issue though, it does not state it explicitly, so it's possible that nothing with regards to when CTFE is guaranteed to happen was actually stabilized.

I'm fairly new to the language overall, but maybe it's time that I try to write an RFC about the whens of CTFE?

@RalfJung

@RalfJung pointed out in a [comment] that the previous phrasing of the sentence can read like it is giving guarantees about free constant definitions always undergoing CTFE, even when unused. That seems to be how the compiler behaves right now, but it's unclear whether it's intentional. Be more precise and don't talk about free constants at all. [comment]: rust-lang#1120 (comment)

RalfJung · 2022-01-27T20:44:00Z

I'm fairly new to the language overall, but maybe it's time that I try to write an RFC about the whens of CTFE?

If you want to help with this, I think the first step is to contact the lang team (e.g. in their Zulip stream) -- I am not sure if an RFC is even required; possibly an FCP (final comment period) is enough to turn this current implementation fact into a stable guarantee.

ehuss · 2022-02-02T18:49:16Z

@oli-obk / @RalfJung Are you OK with this change?

oli-obk · 2022-02-02T20:55:15Z

src/items/associated-items.md

-Unlike [free] constants, associated constant definitions undergo
-[constant evaluation] only when referenced.
+Associated constant definitions undergo [constant evaluation] only when
+referenced.


And only after monomorphization. Nothing is actually done on generic constants, even if we could attempt it nowadays

I pushed d0b33a2 and 37e3c15 to try to explain this.

This makes the example a bit bigger, but makes the reason why constants don't immediately undergo CTFE more conspicuous. When there is computation in the definition that use generic parameters, there is not enough information to fully evaluate the definition eagerly. I made it a `compile_fail` example so the run button on the page gives reader useful outputs.

ehuss

Thanks!

Update books ## nomicon 3 commits in 9493715a6280a1f74be759c7e1ef9999b5d13e6f..90993eeac93dbf9388992de92965f99cf6f29a03 2022-01-27 19:00:32 -0800 to 2022-02-13 12:44:12 +0900 - Fix a small typo in exception-safety.md (rust-lang/nomicon#341) - Make `Vec::new` public in vec-alloc.md (rust-lang/nomicon#336) - Fix a syntax error in leaking.md (rust-lang/nomicon#335) ## reference 6 commits in 411c2f0d5cebf48453ae2d136ad0c5e611d39aec..70fc73a6b908e08e66aa0306856c5211312f6c05 2022-01-30 12:46:37 -0800 to 2022-02-14 19:33:01 -0800 - Document pre-Rust-2021 special case for IntoIterator method lookup (rust-lang/reference#1154) - Mention std::is_aarch64_feature_detected (rust-lang/reference#1061) - Fix link to the Bastion of the Turbofish (rust-lang/reference#1161) - Improve associated constant item CTFE timing section (rust-lang/reference#1147) - document `#![feature(const_generics_defaults)]` (rust-lang/reference#1098) - Update patterns allowed in @ patterns (rust-lang/reference#1158) ## book 6 commits in 98904efaa4fc968db8ff59cf2744d9f7ed158166..67b768c0b660a069a45f0e5d8ae2f679df1022ab 2022-01-29 21:22:31 -0500 to 2022-02-09 21:52:41 -0500 - Snapshot of ch18 for nostarch - Remove mention of destructuring references as that's not covered currently - Add note that exhaustiveness checking doesn't extend to match guards - Change match guard example to actually be unexpressable with patterns alone - Corrected listing number from 9-10 to 9-13 - Remove duplicate paragraph after No Starch related changes ## rustc-dev-guide 3 commits in 8763adb..62f5839 2022-01-26 14:01:40 -0800 to 2022-02-11 08:42:50 -0500 - Correction, building stage3 compiler (rust-lang/rustc-dev-guide#1298) - Triage some date references (rust-lang/rustc-dev-guide#1293) - mention test folders for cfg(bootstrap) (rust-lang/rustc-dev-guide#1294)

oli-obk reviewed Feb 2, 2022

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XrXr added 2 commits February 5, 2022 10:10

Mention associated const CTFE interactions with generics

d0b33a2

XrXr changed the title ~~Don't implicitly talk about free constant CTFE timing~~ Improve associated constant item CTFE timing section Feb 5, 2022

oli-obk approved these changes Feb 6, 2022

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ehuss approved these changes Feb 8, 2022

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ehuss merged commit e4ed2b5 into rust-lang:master Feb 8, 2022

XrXr deleted the precision-in-language branch February 8, 2022 17:07

ehuss mentioned this pull request Feb 16, 2022

Update books rust-lang/rust#94045

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Improve associated constant item CTFE timing section #1147

Improve associated constant item CTFE timing section #1147

XrXr commented Jan 27, 2022 •

edited

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RalfJung commented Jan 27, 2022

ehuss commented Feb 2, 2022

oli-obk Feb 2, 2022

XrXr Feb 5, 2022

ehuss left a comment

Improve associated constant item CTFE timing section #1147

Improve associated constant item CTFE timing section #1147

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XrXr commented Jan 27, 2022 • edited Loading

RalfJung commented Jan 27, 2022

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XrXr commented Jan 27, 2022 •

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