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

mark-i-m · Apr 25, 2018

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 · Apr 25, 2018

r? @kennytm

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rust-highfive · Apr 25, 2018

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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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 · Apr 26, 2018

oli-obk reviewed Apr 26, 2018

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oli-obk · Apr 26, 2018

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 · Apr 26, 2018

kennytm reviewed Apr 26, 2018

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kennytm · Apr 26, 2018

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 · Apr 27, 2018

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.

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[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 · Apr 27, 2018

The test needs #![feature(const_vec_new)]

mbrubeck · Apr 27, 2018

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 · Apr 27, 2018

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

mark-i-m · Apr 29, 2018

@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 · Apr 29, 2018

@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 · Apr 29, 2018

@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 · Apr 29, 2018

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 · Apr 29, 2018

@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 · Apr 29, 2018

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

alexreg · Apr 29, 2018

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

mark-i-m · Apr 29, 2018

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

rust-highfive · Apr 29, 2018

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 · Apr 29, 2018

Oh 🤦‍♂️

mark-i-m · Apr 29, 2018

@kennytm I think this is ready for another review

kennytm · Apr 30, 2018

@bors r+

bors · Apr 30, 2018

📌 Commit f9f9923 has been approved by kennytm

F001 · May 5, 2018

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 · May 5, 2018

Good idea!

Make Vec::new const

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rust-highfive assigned kennytm Apr 25, 2018

rust-highfive added the S-waiting-on-review label Apr 25, 2018

make RawVec::empty const

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a2105b8

make Vec::new const :P

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20ef0e0

not insta-stable

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c122b3a

feature on test

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0212e02

use const trick

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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

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f9f9923

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Rollup of 7 pull requests #50345

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Make `String::new()` const #50460

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Test capacity of ZST vector #56713

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

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Rollup of 7 pull requests #50345

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Make `String::new()` const #50460

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Test capacity of ZST vector #56713

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alexreg commented Apr 27, 2018 •

edited

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