Make `Vec::new` a `const fn` #50233

mark-i-m · 2018-04-25T21:40:08Z

RawVec::empty/_in are a hack. They're there because if size_of::<T> == 0 { !0 } else { 0 } is not allowed in const yet. However, because RawVec is unstable, the empty/empty_in constructors can be removed when #49146 is done...

rust-highfive · 2018-04-25T21:40:11Z

r? @kennytm

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rust-highfive · 2018-04-25T22:43:15Z

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
  Downloading https://files.pythonhosted.org/packages/4a/7d/9c0f35dc594b78137929523f9632ec649b69af2d30ca6a7a8c60d414b23a/awscli-1.15.9-py2.py3-none-any.whl (1.3MB)
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---
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Collecting botocore==1.10.9 (from awscli)
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/packages/urllib3/util/ssl_.py:122: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. You can upgrade to a newer version of Python to solve this. For more information, see https://urllib3.readthedocs.io/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
  Downloading https://files.pythonhosted.org/packages/fe/13/45a8eeb5d78e2578b8e55df58e3921369efe51eaa57a9d2a36e7bef45bcc/botocore-1.10.9-py2.py3-none-any.whl (4.2MB)
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---
[00:54:20] ......i........................................................................................test [run-pass] run-pass/mir_heavy_promoted.rs has been running for over 60 seconds
[00:54:23] .....
[00:54:55] ....................................................................................................
[00:55:24] .......................................................................ii...........................
[00:56:17] ..................................i....................................................i.ii........test [run-pass] run-pass/saturating-float-casts.rs has been running for over 60 seconds
[00:56:59] ...............................................................................................iiiii
[00:57:25] ii..................................................................................................
[00:57:55] ....................................................................................................
[00:57:55] ....................................................................................................
[00:58:20] ..............................................................................................F.....
[00:58:38] failures:
[00:58:38] 
[00:58:38] ---- [run-pass] run-pass/vec-const-new.rs stdout ----
[00:58:38]  
[00:58:38]  
[00:58:38] error: compilation failed!
[00:58:38] status: exit code: 101
[00:58:38] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/run-pass/vec-const-new.rs" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass" "--target=x86_64-unknown-linux-gnu" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/vec-const-new.stage2-x86_64-unknown-linux-gnu" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/vec-const-new.stage2-x86_64-unknown-linux-gnu.aux"
[00:58:38] ------------------------------------------
[00:58:38] 
[00:58:38] ------------------------------------------
[00:58:38] stderr:
[00:58:38] stderr:
[00:58:38] ------------------------------------------
[00:58:38] error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
[00:58:38]    |
[00:58:38]    |
[00:58:38] 13 | const MY_VEC: Vec<usize> = Vec::new();
[00:58:38] 
[00:58:38] error: aborting due to previous error
[00:58:38] 
[00:58:38] For more information about this error, try `rustc --explain E0015`.
---
[00:58:38] 
[00:58:38] thread 'main' panicked at 'Some tests failed', tools/compiletest/src/main.rs:488:22
[00:58:38] 
[00:58:38] 
[00:58:38] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnuWed, 25 Apr 2018 22:43:06 GMT

The command "date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
" exited with 0.
travis_fold:start:after_failure.1

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

oli-obk · 2018-04-26T04:39:44Z

src/liballoc/vec.rs

@@ -324,7 +324,7 @@ impl<T> Vec<T> {
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new() -> Vec<T> {


You forgot the actual change of this PR 😆

Oh. 🤦‍♂️ Thanks!

kennytm · 2018-04-26T19:54:06Z

src/liballoc/vec.rs

@@ -322,9 +322,9 @@ impl<T> Vec<T> {
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new() -> Vec<T> {
+    pub const fn new() -> Vec<T> {


Please mark this with #[rustc_const_unstable(feature = "const_vec_new")] so we don't need to insta-stable this.

@kennytm Done.

rust-highfive · 2018-04-27T04:45:24Z

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:59:27] .......i....................................................................................test [run-pass] run-pass/mir_heavy_promoted.rs has been running for over 60 seconds
[00:59:31] ........
[01:00:05] ....................................................................................................
[01:00:34] ........................................................................ii..........................
[01:01:28] ....................................i....................................................i.ii.test [run-pass] run-pass/saturating-float-casts.rs has been running for over 60 seconds
[01:02:16] .................................................................................................iii
[01:02:43] iiii................................................................................................
[01:03:14] ....................................................................................................
[01:03:39] ................................................................................................F...

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

oli-obk · 2018-04-27T06:03:34Z

The test needs #![feature(const_vec_new)]

mbrubeck · 2018-04-27T15:08:06Z

This changes the behavior for zero-sized types. Before this change, Vec::<()>::new() returns a vector with capacity usize::MAX. After this change, it returns a vector with capacity zero. I'm not sure how much we care about preserving the old behavior, but it seems worth noting explicitly.

If we want to preserve the current optimization for zero-sized types, then instead of getting rid of the if we could replace it with a const_if! macro that expands to something like [0, !0][(size_of::<T>() == 0) as usize].

alexreg · 2018-04-27T15:43:50Z

if statements are coming to const-land very soon, it's worth noting!

mark-i-m · 2018-04-29T01:59:10Z

@alexreg would it be possible to get an ETA on that? What does "very soon" mean exactly? If it will be in the next few weeks or so, then I think it would be worth it to wait for that. Otherwise, we could use the const_if! approach temporarily.

mark-i-m · 2018-04-29T02:07:04Z

@mbrubeck That approach doesn't quite work:

error[E0401]: can't use type parameters from outer function
  --> liballoc/raw_vec.rs:63:52
   |
56 | impl<T, A: Alloc> RawVec<T, A> {
   |      - type variable from outer function
...
59 |     pub const fn new_in(a: A) -> Self {
   |                  ------ try adding a local type parameter in this method instead
...
63 |         const CAP: usize = [0, !0][(mem::size_of::<T>() == 0) as usize];
   |                                                    ^ use of type variable from outer function

alexreg · 2018-04-29T03:23:05Z

@mark-i-m Can't given guarantees to be honest, since this is work that @eddyb is taking over from a branch of mine. But without putting pressure on him, a few weeks sounds feasible. I'll leave it up to you if you just want to use const_if! for now. :-)

oli-obk · 2018-04-29T09:08:47Z

If you don't use an intermediate const, then the trick should work. Const if is still ways off. So use the array trick

alexreg · 2018-04-29T13:35:42Z

@oli-obk Not really. It's on @eddyb's to-do list, after a couple more small PRs. We've discussed it recently. I've already laid the groundwork for it.

oli-obk · 2018-04-29T13:37:52Z

Still. the trick works now. No need to block this PR

alexreg · 2018-04-29T13:43:40Z

Yeah, I don't dispute that. ;-)

mark-i-m · 2018-04-29T22:14:28Z

Hmm... tidy is segfaulting on my machine, so I can't run tests at the moment...

rust-highfive · 2018-04-29T22:23:24Z

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:start:tidy
tidy check
[00:04:42] 
[00:04:42] 
[00:04:42] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:42] expected success, got: signal: 11
[00:04:42] 
[00:04:42] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:42] Build completed unsuccessfully in 0:01:52
[00:04:42] Build completed unsuccessfully in 0:01:52
[00:04:42] Makefile:79: recipe for target 'tidy' failed
[00:04:42] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1e03c84c
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

mark-i-m · 2018-04-29T22:26:41Z

Oh 🤦‍♂️

mark-i-m · 2018-04-29T23:28:44Z

@kennytm I think this is ready for another review

kennytm · 2018-04-30T16:39:39Z

@bors r+

bors · 2018-04-30T16:39:40Z

📌 Commit f9f9923 has been approved by kennytm

Make `Vec::new` a `const fn` `RawVec::empty/_in` are a hack. They're there because `if size_of::<T> == 0 { !0 } else { 0 }` is not allowed in `const` yet. However, because `RawVec` is unstable, the `empty/empty_in` constructors can be removed when rust-lang#49146 is done...

Rollup of 7 pull requests Successful merges: - #50233 (Make `Vec::new` a `const fn`) - #50312 (Add more links in panic docs) - #50316 (Fix some broken links in docs.) - #50325 (Add a few more tests for proc macro feature gating) - #50327 (Display correct unused field suggestion for nested struct patterns) - #50330 (check that #[used] is used only on statics) - #50344 (Update Cargo to 2018-04-28 122fd5be5201913d42e219e132d6569493583bca) Failed merges:

F001 · 2018-05-05T05:24:44Z

impl String {
    pub fn new() -> String {
        String { vec: Vec::new() }
    }
}

Given that String::new() is based on Vec::new(), I think it is reasonable to make it a const fn correspondingly.
If that is the case, I'd be glad to create a pull request.

oli-obk · 2018-05-05T07:31:08Z

Good idea!

Make `String::new()` const Following the steps of rust-lang#50233 , make `String::new()` a `const fn`.

Initially, rust-lang#50233 accidentally changed the capacity of empty ZST. This was pointed out during code review. This commit adds a test to prevent capacity of ZST vectors from accidentally changing to prevent that from happening again.

Test capacity of ZST vector Initially, rust-lang#50233 accidentally changed the capacity of empty ZST. This was pointed out during code review. This commit adds a test to prevent capacity of ZST vectors from accidentally changing to prevent that from happening again.

Make Vec::new const

256096d

rust-highfive assigned kennytm Apr 25, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 25, 2018

make RawVec::empty const

a2105b8

oli-obk reviewed Apr 26, 2018

View reviewed changes

make Vec::new const :P

20ef0e0

kennytm reviewed Apr 26, 2018

View reviewed changes

not insta-stable

c122b3a

feature on test

0212e02

use const trick

e5280e4

mark-i-m changed the title ~~[WIP] Make Vec::new a const fn~~ Make Vec::new a const fn Apr 29, 2018

heh, logic is hard

f9f9923

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 Apr 30, 2018

kennytm mentioned this pull request Apr 30, 2018

Rollup of 7 pull requests #50345

Merged

bors merged commit f9f9923 into rust-lang:master Apr 30, 2018

F001 mentioned this pull request May 5, 2018

Make String::new() const #50460

Merged

kennytm added a commit to kennytm/rust that referenced this pull request May 9, 2018

Rollup merge of rust-lang#50460 - F001:const_string, r=kennytm

1f4718a

Make `String::new()` const Following the steps of rust-lang#50233 , make `String::new()` a `const fn`.

mark-i-m deleted the const_vec branch November 14, 2018 16:40

KamilaBorowska mentioned this pull request Dec 11, 2018

Test capacity of ZST vector #56713

Merged

jonas-schievink mentioned this pull request Aug 30, 2019

Tracking issue for making Vec::new a const fn #64022

Closed

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Make `Vec::new` a `const fn` #50233

Make `Vec::new` a `const fn` #50233

mark-i-m commented Apr 25, 2018

rust-highfive commented Apr 25, 2018

rust-highfive commented Apr 25, 2018

oli-obk Apr 26, 2018

mark-i-m Apr 26, 2018

kennytm Apr 26, 2018

mark-i-m Apr 27, 2018

rust-highfive commented Apr 27, 2018

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mbrubeck commented Apr 27, 2018

alexreg commented Apr 27, 2018 •

edited

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alexreg commented Apr 29, 2018

mark-i-m commented Apr 29, 2018

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mark-i-m commented Apr 29, 2018

mark-i-m commented Apr 29, 2018

kennytm commented Apr 30, 2018

bors commented Apr 30, 2018

F001 commented May 5, 2018

oli-obk commented May 5, 2018

		@@ -324,7 +324,7 @@ impl<T> Vec<T> {
		#[stable(feature = "rust1", since = "1.0.0")]
		pub fn new() -> Vec<T> {

Make Vec::new a const fn #50233

Make Vec::new a const fn #50233

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oli-obk commented May 5, 2018

Make `Vec::new` a `const fn` #50233

Make `Vec::new` a `const fn` #50233

alexreg commented Apr 27, 2018 •

edited

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