Announce `if` and `match` in constants on nightly #461

ecstatic-morse · 2019-11-20T01:50:03Z

This is a rough draft of a post announcing that if and match will be available on nightly. Obviously this can't be merged before rust-lang/rust#66507.

cc @rust-lang/wg-const-eval

rust-highfive · 2019-11-20T01:50:14Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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posts/inside-rust/2019-11-23-const-if-match.md

Centril · 2019-11-20T14:37:39Z

posts/inside-rust/2019-11-23-const-if-match.md

+This change will allow a great number of standard library functions to be
+made `const`. If you like, you can help with this process! There's a [list of
+numeric functions][const-int] that can be constified with little effort. Be
+aware that things move slowly in the standard library, and it's always best


I think we can be quite liberal with accepting const fn-ification on nightly -- stabilizing is a different matter tho. I think it's in our interest to use the standard library as a proving ground for the feature. So I think we should be more encouraging than "slowly" suggests wrt. unstable stuff.

Constification PRs have been closed because they don't have a motivating use case. I agree with your position on this, but is there consensus? Regardless, "slowly" is unnecessarily pejorative, and I'll change it.

I believe in the case you mention, it's more the case that we know that it is not useful in const contexts yet rather than requiring an elaborate justification. Here I think "proving out the language feature" is a useful justification in-and-of-itself. The libs team will have an opportunity to reject stabilization and revert things (if e.g. it incurred hacks), so I think we should be good. My interest here is to have a light-weight process and avoid bureaucracy.

Okay, I've removed that sentence entirely. Do we want to suggest including a concrete example? For many functions that will be overkill.

Maybe we could give an example of how you constify something by pointing to some merged PR which introduced rustc_const_unstable?

How about rust-lang/rust#63123?

Yeah that works; rust-lang/rust#61635 is another good example.

I used rust-lang/rust#61635 as the example and explicitly mention rustc_const_unstable.

Co-Authored-By: lzutao <taolzu@gmail.com>

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

oli-obk · 2019-11-22T20:06:55Z

posts/inside-rust/2019-11-23-const-if-match.md

+`debug_assert_ne`) remain forbidden, since they need to call `Debug::fmt` on
+their arguments.
+
+Also forbidden are looping constructs, `while`, `for`, and `loop`, which will


Maybe also note that for loops need trait methods, too and that you can emulate loops with recursion as shown above

oli-obk · 2019-11-22T20:09:17Z

posts/inside-rust/2019-11-23-const-if-match.md

+
+## What took you so long?
+
+[Miri], which rust uses under the hood for compile-time function evaluation,


This is not quite right. Both const eval and miri use the miri engine. The miri engine is essentially a VM that runs MIR as "bytecode". But it's right in the sense that miri showcases what you can do when you take off the safety padding.

So change "Miri" to "miri engine"? Also, how do we capitalize miri at the start of a sentence?

Uh. I think it.s even supposed to be capitalized in the middle of sentences. I do forget. cc @solson

I've typically used "Miri" when referring to the project and miri when referring to the binary or crate name, but I probably haven't been consistent myself.

oli-obk · 2019-11-22T20:10:15Z

posts/inside-rust/2019-11-23-const-if-match.md

+}
+```
+
+However, it is sometimes okay for a `const` to contain a *type* that allows


It's also OK for constants to have their base value have interior mutability since each use site creates its own copy of it

"a const to contain a reference to a type that..."?

Yea that's good

oli-obk · 2019-11-22T20:11:04Z

posts/inside-rust/2019-11-23-const-if-match.md

+inside the `const fn`, we don't detect that all fields of `tuple` have been moved
+out of, and thus conservatively assume that the drop impl for `tuple` will run.
+While this particular case is trivial, there are other, more complex ones that
+would require a more expensive solution. It is an open question how precise we


Maybe note that the solution would make cases not needing it also more expensive

This also separates the discussion of looping constructs from that of `const` trait methods and the `?` operator.

ecstatic-morse · 2019-11-22T21:28:55Z

@oli-obk Pushed a commit addressing each of your concerns.

posts/inside-rust/2019-11-23-const-if-match.md

Co-Authored-By: lqd <remy.rakic+github@gmail.com>

ecstatic-morse · 2019-11-24T18:34:32Z

@nikomatsakis (or anybody else with merge rights) #![feature(const_if_match)] is now available on the latest nightly, so we should publish this.

rust-highfive assigned nikomatsakis Nov 20, 2019

tesuji reviewed Nov 20, 2019

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Centril reviewed Nov 20, 2019

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ecstatic-morse and others added 9 commits November 20, 2019 11:07

Announce if and match in constants

0fde6ec

Fix assert example

7f36264

Co-Authored-By: lzutao <taolzu@gmail.com>

I've seen those English dramas, too

3592422

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

Add team to front matter

fb2ab59

Split out const fn example

23748d5

Reword call for participation

07782e8

Fix link

4b22d22

Remove trailing whitespace

1b53e7c

Remove acknowledgements section lest I forget someone

cbbfaa6

ecstatic-morse force-pushed the patch-1 branch from 183f13e to cbbfaa6 Compare November 20, 2019 19:07

Explain constification process

b2fa25c

ecstatic-morse mentioned this pull request Nov 22, 2019

Enable if and match in constants behind a feature flag rust-lang/rust#66507

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oli-obk reviewed Nov 22, 2019

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ecstatic-morse added 4 commits November 22, 2019 13:24

Talk about recursion as a substitute for looping

9652c80

This also separates the discussion of looping constructs from that of `const` trait methods and the `?` operator.

Link to Miri engine in rustc guide, not Miri project

4867fa7

reference to a *type*

0365205

Explicitly mention perf tradeoff with more precise analysis

8e25725

lqd reviewed Nov 22, 2019

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posts/inside-rust/2019-11-23-const-if-match.md Outdated Show resolved Hide resolved

Fix typos in new changes

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Co-Authored-By: lqd <remy.rakic+github@gmail.com>

oli-obk approved these changes Nov 23, 2019

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Centril approved these changes Nov 23, 2019

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Update post date

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pietroalbini merged commit dfff7a6 into rust-lang:master Nov 25, 2019

ecstatic-morse mentioned this pull request Nov 27, 2019

Tracking Issue for more const int functions rust-lang/rust#53718

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Announce `if` and `match` in constants on nightly #461

Announce `if` and `match` in constants on nightly #461

ecstatic-morse commented Nov 20, 2019

rust-highfive commented Nov 20, 2019

Centril Nov 20, 2019

ecstatic-morse Nov 20, 2019 •

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Centril Nov 20, 2019

ecstatic-morse Nov 20, 2019

Centril Nov 20, 2019

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ecstatic-morse Nov 22, 2019

oli-obk Nov 22, 2019

oli-obk Nov 22, 2019

ecstatic-morse Nov 22, 2019

oli-obk Nov 22, 2019

solson Nov 24, 2019

oli-obk Nov 22, 2019

ecstatic-morse Nov 22, 2019

oli-obk Nov 22, 2019

oli-obk Nov 22, 2019

ecstatic-morse commented Nov 22, 2019

ecstatic-morse commented Nov 24, 2019


		## What took you so long?

		[Miri], which rust uses under the hood for compile-time function evaluation,

Announce if and match in constants on nightly #461

Announce if and match in constants on nightly #461

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ecstatic-morse commented Nov 22, 2019

ecstatic-morse commented Nov 24, 2019

Announce `if` and `match` in constants on nightly #461

Announce `if` and `match` in constants on nightly #461

ecstatic-morse Nov 20, 2019 •

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