Stabilize `Vec::new` and `String::new` as `const fn`s #64028

Centril · 2019-08-30T23:17:27Z

Centril · 2019-08-30T23:19:43Z

src/liballoc/raw_vec.rs

+    /// If you change `RawVec<T>::new` or dependencies, please take care to not
+    /// introduce anything that would truly violate `min_const_fn`.
+    ///
+    /// NOTE: We could avoid this hack and check conformance with some


I'm pretty unhappy about this hack since I think carries the risk that we might stabilize things we don't want in const eval inadvertently and I have WIP more principled solution in https://github.com/Centril/rust/commits/stabilize-vec-new-const which I think we should seriously consider instead (but which can change into after this PR if necessary).

Centril · 2019-08-30T23:20:24Z

cc @alexcrichton for T-libs FCP

alexcrichton · 2019-09-03T14:23:04Z

@rfcbot fcp merge

rfcbot · 2019-09-03T14:23:05Z

Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rfcbot · 2019-09-03T16:03:58Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2019-09-13T16:08:10Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

alexcrichton · 2019-09-16T14:08:59Z

r=me with a rebase

Centril · 2019-09-16T15:04:36Z

r? @alexcrichton @bors r=alexcrichton rollup

bors · 2019-09-16T15:04:38Z

📌 Commit 9b3e11f has been approved by alexcrichton

@oli-obk

…excrichton Stabilize `Vec::new` and `String::new` as `const fn`s Closes rust-lang#64022. r? @oli-obk

@ghost

Rollup of 10 pull requests Successful merges: - #63955 (Make sure interned constants are immutable) - #64028 (Stabilize `Vec::new` and `String::new` as `const fn`s) - #64119 (ci: ensure all tool maintainers are assignable on issues) - #64444 (fix building libstd without backtrace feature) - #64446 (Fix build script sanitizer check.) - #64451 (when Miri tests are not passing, do not add Miri component) - #64467 (Hide diagnostics emitted during --cfg parsing) - #64497 (Don't print the "total" `-Ztime-passes` output if `--prints=...` is also given) - #64499 (Use `Symbol` in two more functions.) - #64504 (use println!() instead of println!("")) Failed merges: r? @ghost

RalfJung · 2019-09-16T21:54:57Z

src/liballoc/raw_vec.rs

+            // FIXME(mark-i-m): use `cap` when ifs are allowed in const
+            cap: [0, !0][(mem::size_of::<T>() == 0) as usize],
+            a: Global,
+        }


@ecstatic-morse @oli-obk are you keeping a list somewhere for all the awful hacks we have in libstd to work around const qualif limitations? This would be another one (there's also #63786).

Maybe we should have a special string for "const related awful hack"? FIXME(awful_const_hack) :D

We can make an issue also...

We could also just tag PRs like this as const-workaround-if on GitHub, and someone can go through at the tagged PRs once the required operations are allowed.

If we're going with a label, I'd make it a bit more general: const-hack.

Presumably there could be a const-hack-loop and const-hack-mut-ref if anyone gets really frisky.

@ecstatic-morse I was wondering if we couldn't use one label for all of them?

I don't care so much. I thought it would be easier to go back and remove the labels if we know exactly which limitation is being worked around, but the absence of conditionals is probably the only thing that can be circumvented with these kinds of hacks.

Added a const-hack label. Please let me know if there are some PRs I missed.

I have a todo entry for reviewing all const fns after we get loops and conditions, I'm certain we'll miss some by whitelisting what to check

@Centril thanks!

@oli-obk Let's just go through the tag then after you are done with that review, to double-check.

jadbox · 2019-09-19T15:31:55Z

Does this improve the performance of String and Vec creation?

Centril · 2019-09-19T15:34:15Z

I wouldn't expect it to have any perf impact one way or the other; it's just copying some code.

Pkgsrc changes: * Remove patch which no longer applies (but what about RPATH?) * Adapt a few patches to changed files upstream. Upstream changes: Version 1.39.0 (2019-11-07) =========================== Language -------- - [You can now create `async` functions and blocks with `async fn`, `async move {}`, and `async {}` respectively, and you can now call `.await` on async expressions.][63209] - [You can now use certain attributes on function, closure, and function pointer parameters.][64010] These attributes include `cfg`, `cfg_attr`, `allow`, `warn`, `deny`, `forbid` as well as inert helper attributes used by procedural macro attributes applied to items. e.g. ```rust fn len( #[cfg(windows)] slice: &[u16], #[cfg(not(windows))] slice: &[u8], ) -> usize { slice.len() } ``` - [You can now take shared references to bind-by-move patterns in the `if` guards of `match` arms.][63118] e.g. ```rust fn main() { let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]); match array { nums // ---- `nums` is bound by move. if nums.iter().sum::<u8>() == 10 // ^------ `.iter()` implicitly takes a reference to `nums`. => { drop(nums); // ----------- Legal as `nums` was bound by move and so we have ownership. } _ => unreachable!(), } } ``` Compiler -------- - [Added tier 3\* support for the `i686-unknown-uefi` target.][64334] - [Added tier 3 support for the `sparc64-unknown-openbsd` target.][63595] - [rustc will now trim code snippets in diagnostics to fit in your terminal.] [63402] **Note** Cargo currently doesn't use this feature. Refer to [cargo#7315][cargo/7315] to track this feature's progress. - [You can now pass `--show-output` argument to test binaries to print the output of successful tests.][62600] \* Refer to Rust's [platform support page][forge-platform-support] for more information on Rust's tiered platform support. Libraries --------- - [`Vec::new` and `String::new` are now `const` functions.][64028] - [`LinkedList::new` is now a `const` function.][63684] - [`str::len`, `[T]::len` and `str::as_bytes` are now `const` functions.][63770] - [The `abs`, `wrapping_abs`, and `overflowing_abs` numeric functions are now `const`.][63786] Stabilized APIs --------------- - [`Pin::into_inner`] - [`Instant::checked_duration_since`] - [`Instant::saturating_duration_since`] Cargo ----- - [You can now publish git dependencies if supplied with a `version`.] [cargo/7237] - [The `--all` flag has been renamed to `--workspace`.][cargo/7241] Using `--all` is now deprecated. Misc ---- - [You can now pass `-Clinker` to rustdoc to control the linker used for compiling doctests.][63834] Compatibility Notes ------------------- - [Code that was previously accepted by the old borrow checker, but rejected by the NLL borrow checker is now a hard error in Rust 2018.][63565] This was previously a warning, and will also become a hard error in the Rust 2015 edition in the 1.40.0 release. - [`rustdoc` now requires `rustc` to be installed and in the same directory to run tests.][63827] This should improve performance when running a large amount of doctests. - [The `try!` macro will now issue a deprecation warning.][62672] It is recommended to use the `?` operator instead. - [`asinh(-0.0)` now correctly returns `-0.0`.][63698] Previously this returned `0.0`. [62600]: rust-lang/rust#62600 [62672]: rust-lang/rust#62672 [63118]: rust-lang/rust#63118 [63209]: rust-lang/rust#63209 [63402]: rust-lang/rust#63402 [63565]: rust-lang/rust#63565 [63595]: rust-lang/rust#63595 [63684]: rust-lang/rust#63684 [63698]: rust-lang/rust#63698 [63770]: rust-lang/rust#63770 [63786]: rust-lang/rust#63786 [63827]: rust-lang/rust#63827 [63834]: rust-lang/rust#63834 [63927]: rust-lang/rust#63927 [63933]: rust-lang/rust#63933 [63934]: rust-lang/rust#63934 [63938]: rust-lang/rust#63938 [63940]: rust-lang/rust#63940 [63941]: rust-lang/rust#63941 [63945]: rust-lang/rust#63945 [64010]: rust-lang/rust#64010 [64028]: rust-lang/rust#64028 [64334]: rust-lang/rust#64334 [cargo/7237]: rust-lang/cargo#7237 [cargo/7241]: rust-lang/cargo#7241 [cargo/7315]: rust-lang/cargo#7315 [`Pin::into_inner`]: https://doc.rust-lang.org/std/pin/struct.Pin.html#method.into_inner [`Instant::checked_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_duration_since [`Instant::saturating_duration_since`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.saturating_duration_since

Centril added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Aug 30, 2019

rust-highfive assigned oli-obk Aug 30, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 30, 2019

Centril added this to the 1.39 milestone Aug 30, 2019

Centril commented Aug 30, 2019

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Centril force-pushed the stabilize-alloc-new-2 branch from 01cbb77 to 9ffe706 Compare August 31, 2019 00:20

Centril added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 3, 2019

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Sep 3, 2019

rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Sep 3, 2019

Centril removed the needs-fcp This change is insta-stable, so needs a completed FCP to proceed. label Sep 6, 2019

rfcbot added the finished-final-comment-period The final comment period is finished for this PR / Issue. label Sep 13, 2019

rfcbot removed the final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. label Sep 13, 2019

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Centril added 2 commits September 16, 2019 16:45

Const-stabilize Vec::new.

d5fe583

Const-stabilize String::new.

9b3e11f

Centril force-pushed the stabilize-alloc-new-2 branch from 9ffe706 to 9b3e11f Compare September 16, 2019 15:03

rust-highfive assigned alexcrichton and unassigned oli-obk Sep 16, 2019

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 Sep 16, 2019

Centril added a commit to Centril/rust that referenced this pull request Sep 16, 2019

Rollup merge of rust-lang#64028 - Centril:stabilize-alloc-new-2, r=al…

be327a8

…excrichton Stabilize `Vec::new` and `String::new` as `const fn`s Closes rust-lang#64022. r? @oli-obk

Centril mentioned this pull request Sep 16, 2019

Rollup of 10 pull requests #64510

Merged

bors merged commit 9b3e11f into rust-lang:master Sep 16, 2019

Centril deleted the stabilize-alloc-new-2 branch September 16, 2019 20:02

RalfJung reviewed Sep 16, 2019

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Centril added the const-hack label Sep 17, 2019

burrbull mentioned this pull request Sep 17, 2019

Replace pdf_path & gs_stack with Vec crlf0710/tectonic#57

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matthiaskrgr mentioned this pull request Dec 12, 2019

const eval Vec::new() the same as vec![] #54475

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Stabilize `Vec::new` and `String::new` as `const fn`s #64028

Stabilize `Vec::new` and `String::new` as `const fn`s #64028

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Stabilize Vec::new and String::new as const fns #64028

Stabilize Vec::new and String::new as const fns #64028

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jadbox commented Sep 19, 2019

Centril commented Sep 19, 2019

Stabilize `Vec::new` and `String::new` as `const fn`s #64028

Stabilize `Vec::new` and `String::new` as `const fn`s #64028

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