Switch libcore array implementations to const generics. #60466

crlf0710 · 2019-05-02T09:01:57Z

Switch libcore array implementations to const generics.

rust-highfive · 2019-05-02T09:02:00Z

(rust_highfive has picked a reviewer for you, use r? to override)

varkor · 2019-05-02T09:25:48Z

There are still issues with array handling for consts generics, so I don't expect this to work quite yet.

rust-highfive · 2019-05-02T09:28:40Z

The job x86_64-gnu-llvm-6.0 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:end:0744c453:start=1556788995527328370,finish=1556788997874399211,duration=2347070841
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:04:27] note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
[00:04:27] 
[00:04:27] note: rustc 1.35.0-beta.1 (2bc1d406d 2019-04-10) running on x86_64-unknown-linux-gnu
[00:04:27] 
[00:04:27] note: compiler flags: -Z external-macro-backtrace -Z unstable-options -Z force-unstable-if-unmarked -C opt-level=2 -C prefer-dynamic -C debug-assertions=y -C codegen-units=1 -C link-args=-Wl,-rpath,$ORIGIN/../lib --crate-type lib
[00:04:27] note: some of the compiler flags provided by cargo are hidden
[00:04:27] 
[00:04:27] error: Could not compile `core`.
[00:04:27] warning: build failed, waiting for other jobs to finish...
---
travis_time:end:00d25034:start=1556789279431610631,finish=1556789279436820375,duration=5209744
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:11d6f9b4
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
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travis_time:start:061ac4c1
travis_time:start:061ac4c1
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:028d532f
$ dmesg | grep -i kill

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)

crlf0710 · 2019-05-02T09:42:37Z

Yes, and it will also be quite a while before the bootstrapping compiler can use const generics :)

Centril · 2019-05-02T10:47:37Z

Even when we have working const generics on nightly we cannot actually land the impls because they would commit us to const generics on stable. As such, I think this PR will end up being closed soon.

varkor · 2019-05-02T10:54:21Z

because they would commit us to const generics on stable

I don't think that's true. We could use const generics unstably in rustc indefinitely to improve Rust's handling of arrays.

Centril · 2019-05-02T10:56:44Z

We could use const generics unstably in rustc indefinitely to improve Rust's handling of arrays.

Elaborate?

varkor · 2019-05-02T11:05:32Z

Using const generics in rustc, but disallowing them in user code allows user code to take advantage of impls on arrays whose lengths are greater than 32, without making any other features of const generics visible.

Centril · 2019-05-02T11:10:10Z

@varkor That still commits you to the underlying quantification over values (but specific to usize) which in my book is exposing const generics in a stable way, just not with the surface syntax. This is no different than using specialization inside the standard library for non-perf purposes (e.g. coherence or to improve dbg!) and we have expressly refused to do that. The same applies here; once const generics is stable we can commit to these impls. Alternatively you can find a way to feature gate implementations and then use it but that seems like a large project.

varkor · 2019-05-02T11:32:22Z

In theory, we could generate these impls with a macro for every value of a usize. Using const generics here doesn't let us do anything we couldn't technically do already in the language (unlike specialisation) — it just makes it sensible.

cramertj · 2019-05-02T15:23:47Z

These particular impls are needed and wanted frequently enough that even if we never landed const generics I'd be heavily in support of an implementation via a custom compiler hack.

rust-highfive · 2019-05-04T15:44:43Z

The job x86_64-gnu-llvm-6.0 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:end:01e02fbb:start=1556982930778490906,finish=1556983019455123521,duration=88676632615
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:27:21]    Compiling cmake v0.1.38
[00:27:21]    Compiling backtrace-sys v0.1.27
[00:27:25]    Compiling std v0.0.0 (/checkout/src/libstd)
[00:27:25]    Compiling rustc_tsan v0.0.0 (/checkout/src/librustc_tsan)
[00:27:25] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>`:
[00:27:25]     |
[00:27:25]     |
[00:27:25] 106 | impl<T, const N: usize> TryFrom<&[T]> for [T; N] where T: Copy {
[00:27:25]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
[00:27:25]    ::: src/libcore/convert.rs:565:1
[00:27:25]     |
[00:27:25]     |
[00:27:25] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:25]     | -------------------------------------------- first implementation here
[00:27:25] 
[00:27:25] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>` for type `&[type error]`:
[00:27:25]     |
[00:27:25]     |
[00:27:25] 115 | impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
[00:27:25]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[type error]`
[00:27:25]    ::: src/libcore/convert.rs:565:1
[00:27:25]     |
[00:27:25]     |
[00:27:25] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:25]     | -------------------------------------------- first implementation here
[00:27:25] 
[00:27:25] error[E0119]: conflicting implementations of trait `convert::TryFrom<&mut [_]>` for type `&mut [type error]`:
[00:27:25]     |
[00:27:25]     |
[00:27:25] 129 | impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
[00:27:25]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut [type error]`
[00:27:25]    ::: src/libcore/convert.rs:565:1
[00:27:25]     |
[00:27:25]     |
[00:27:25] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:25]     | -------------------------------------------- first implementation here
[00:27:25] 
[00:27:25] error[E0119]: conflicting implementations of trait `iter::traits::collect::IntoIterator` for type `&[_; _]`:
[00:27:25]     |
[00:27:25]     |
[00:27:25] 241 | impl<I: Iterator> IntoIterator for I {
[00:27:25]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[_; _]`
[00:27:25]    ::: src/libcore/array.rs:157:1
[00:27:25]     |
[00:27:25]     |
[00:27:25] 157 | impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
[00:27:25]     | ------------------------------------------------------- first implementation here
[00:27:25] error: aborting due to 4 previous errors
[00:27:25] 
[00:27:25] For more information about this error, try `rustc --explain E0119`.
[00:27:25] error: Could not compile `core`.
---
travis_time:end:1261a65e:start=1556984679997518335,finish=1556984680002602124,duration=5083789
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:07894bd8
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:21159d78
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/

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)

ollie27 · 2019-05-04T17:52:13Z

src/libcore/array.rs

+}
+
+#[stable(since = "1.4.0", feature = "array_default")]
+impl<T, const N: usize> Default for [T; N] where T: Default {


The current Default impl for [T; 0] doesn't have a T: Default bound so this would be a breaking change.

In the short term, i think we can put the original definition back. In the long term, how would we solve this? N > 0 or T: !Default or some overlapping specialization?

@Centril

If there was a good way to do this we could remove the bounds on the other [T; 0] impls as well: #52246.

Seems like an obvious candidate for specialization, though unfortunately I'm not aware of any existing efforts towards const-specialization. This is in the "minimal trivially sound" category, though, so again I'd personally be inclined to accept whatever minor hack we need to make it "just work".

I'm surprised that the unconstrained impl Default for [T; 0] even exists given #52246.
Is it some early pre-1.0 addition that was overlooked during stabilization?

@est31 you don't think the pattern from @rodrimati1992 would work for Serde?

@dtolnay it could possibly work. Not sure yet what const generics will allow in their initial version and what they don't. If I put @rodrimati1992 's code into the playpen I get "identifiers may currently not be used for const generics" errors on the true and false expressions. If I replace them with {0u8==0u8} and {0u8!=0u8}, I get tons of ICEs (link).

Let's check again after #60742 and #60839.

Still ICE's as of rustc 1.37.0-nightly (3ade426ed 2019-05-30) . I've filed a bug at #61383

Now there is a compilation error: #61935

rust-highfive · 2019-05-05T04:25:35Z

The job x86_64-gnu-llvm-6.0 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:end:2eecc5a0:start=1557028563534341605,finish=1557028651814816423,duration=88280474818
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:27:42]    Compiling cmake v0.1.38
[00:27:42]    Compiling backtrace-sys v0.1.27
[00:27:45]    Compiling std v0.0.0 (/checkout/src/libstd)
[00:27:45]    Compiling rustc_lsan v0.0.0 (/checkout/src/librustc_lsan)
[00:27:46] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>`:
[00:27:46]     |
[00:27:46]     |
[00:27:46] 106 | impl<T, const N: usize> TryFrom<&[T]> for [T; N] where T: Copy {
[00:27:46]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
[00:27:46]    ::: src/libcore/convert.rs:565:1
[00:27:46]     |
[00:27:46]     |
[00:27:46] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:46]     | -------------------------------------------- first implementation here
[00:27:46] 
[00:27:46] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>` for type `&[type error]`:
[00:27:46]     |
[00:27:46]     |
[00:27:46] 115 | impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
[00:27:46]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[type error]`
[00:27:46]    ::: src/libcore/convert.rs:565:1
[00:27:46]     |
[00:27:46]     |
[00:27:46] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:46]     | -------------------------------------------- first implementation here
[00:27:46] 
[00:27:46] error[E0119]: conflicting implementations of trait `convert::TryFrom<&mut [_]>` for type `&mut [type error]`:
[00:27:46]     |
[00:27:46]     |
[00:27:46] 129 | impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
[00:27:46]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut [type error]`
[00:27:46]    ::: src/libcore/convert.rs:565:1
[00:27:46]     |
[00:27:46]     |
[00:27:46] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:46]     | -------------------------------------------- first implementation here
[00:27:46] 
[00:27:46] error[E0119]: conflicting implementations of trait `iter::traits::collect::IntoIterator` for type `&[_; _]`:
[00:27:46]     |
[00:27:46]     |
[00:27:46] 241 | impl<I: Iterator> IntoIterator for I {
[00:27:46]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[_; _]`
[00:27:46]    ::: src/libcore/array.rs:157:1
[00:27:46]     |
[00:27:46]     |
[00:27:46] 157 | impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
[00:27:46]     | ------------------------------------------------------- first implementation here
[00:27:46] error: aborting due to 4 previous errors
[00:27:46] 
[00:27:46] For more information about this error, try `rustc --explain E0119`.
[00:27:46] error: Could not compile `core`.
---
156508 ./src/llvm-project/clang
144972 ./obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu
144968 ./obj/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release
142416 ./obj/build/bootstrap/debug/incremental/bootstrap-ngvt240vptb4
142412 ./obj/build/bootstrap/debug/incremental/bootstrap-ngvt240vptb4/s-fbx55aqn4o-1jmsec0-li41thz4lrxh
123640 ./src/llvm-project/llvm/test/CodeGen
108532 ./src/llvm-project/lldb
97596 ./src/llvm-project/clang/test
89976 ./src/llvm-emscripten/test/CodeGen

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crlf0710 · 2019-05-07T05:50:10Z

Rebased to master to pick up #53645.

rust-highfive · 2019-05-07T06:17:27Z

The job x86_64-gnu-llvm-6.0 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:end:02cc2a1f:start=1557208150405668102,finish=1557208151158370166,duration=752702064
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:27:52]    Compiling compiler_builtins v0.1.10
[00:27:52]    Compiling cmake v0.1.38
[00:27:52]    Compiling backtrace-sys v0.1.27
[00:27:54]    Compiling std v0.0.0 (/checkout/src/libstd)
[00:27:56] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>`:
[00:27:56]     |
[00:27:56]     |
[00:27:56] 106 | impl<T, const N: usize> TryFrom<&[T]> for [T; N] where T: Copy {
[00:27:56]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation
[00:27:56]    ::: src/libcore/convert.rs:565:1
[00:27:56]     |
[00:27:56]     |
[00:27:56] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:56]     | -------------------------------------------- first implementation here
[00:27:56] 
[00:27:56] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>` for type `&[type error]`:
[00:27:56]     |
[00:27:56]     |
[00:27:56] 115 | impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
[00:27:56]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[type error]`
[00:27:56]    ::: src/libcore/convert.rs:565:1
[00:27:56]     |
[00:27:56]     |
[00:27:56] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:56]     | -------------------------------------------- first implementation here
[00:27:56] 
[00:27:56] error[E0119]: conflicting implementations of trait `convert::TryFrom<&mut [_]>` for type `&mut [type error]`:
[00:27:56]     |
[00:27:56]     |
[00:27:56] 129 | impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
[00:27:56]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut [type error]`
[00:27:56]    ::: src/libcore/convert.rs:565:1
[00:27:56]     |
[00:27:56]     |
[00:27:56] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:27:56]     | -------------------------------------------- first implementation here
[00:27:56] 
[00:27:56] error[E0119]: conflicting implementations of trait `iter::traits::collect::IntoIterator` for type `&[_; _]`:
[00:27:56]     |
[00:27:56]     |
[00:27:56] 241 | impl<I: Iterator> IntoIterator for I {
[00:27:56]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[_; _]`
[00:27:56]    ::: src/libcore/array.rs:157:1
[00:27:56]     |
[00:27:56]     |
[00:27:56] 157 | impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
[00:27:56]     | ------------------------------------------------------- first implementation here
[00:27:56]    Compiling rustc_lsan v0.0.0 (/checkout/src/librustc_lsan)
[00:27:56]    Compiling rustc_msan v0.0.0 (/checkout/src/librustc_msan)
[00:27:56] error: aborting due to 4 previous errors
[00:27:56] 
---
19252 ./src/llvm-project/lldb/www/cpp_reference/html
18164 ./obj/build/x86_64-unknown-linux-gnu/stage0-codegen
17940 ./src/llvm-project/lldb/www/python_reference
travis_time:end:177a9f8c:start=1557209842834305831,finish=1557209843411631209,duration=57t ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:04e03f4f
$ dmesg | grep -i kill

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crlf0710 · 2019-06-03T15:32:19Z

minimal repro of the ICE:

#![feature(const_generics)]

fn f<const LEN: usize>() {
    let a = [true; LEN];
}

play link

~~Filed #61490 .~~ See #61336 .

craterbot · 2019-06-03T20:05:25Z

🎉 Experiment pr-60466 is completed!
📊 0 regressed and 2 fixed (60951 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

varkor · 2019-06-03T20:16:55Z

The crater analysis looks good. Performance is also improved with this change.

Const generics as an entire feature still have a number of issues, but there don't seem to be any issues with replacing the array implementation. Now it's up to the teams to decide what to do here: #61415.

bors · 2019-06-09T02:13:02Z

☔ The latest upstream changes (presumably #61672) made this pull request unmergeable. Please resolve the merge conflicts.

rust-highfive · 2019-06-12T08:03:25Z

The job x86_64-gnu-llvm-6.0 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:end:0d7f522b:start=1560326291052938313,finish=1560326291844622880,duration=791684567
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
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$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:04:53] 
[00:04:53]    Compiling build_helper v0.1.0 (/checkout/src/build_helper)
[00:04:54]    Compiling autocfg v0.1.4
[00:04:56]    Compiling backtrace v0.3.29
[00:04:58] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>`:
[00:04:58]     |
[00:04:58]     |
[00:04:58] 115 | impl<T, const N: usize> TryFrom<&[T]> for [T; N] where T: Copy {
[00:04:58]     | 
[00:04:58]    ::: src/libcore/convert.rs:565:1
[00:04:58]     |
[00:04:58]     |
[00:04:58] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:04:58]     | -------------------------------------------- first implementation here
[00:04:58] 
[00:04:58] error[E0119]: conflicting implementations of trait `convert::TryFrom<&[_]>` for type `&[type error]`:
[00:04:58]     |
[00:04:58]     |
[00:04:58] 124 | impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
[00:04:58]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[type error]`
[00:04:58]    ::: src/libcore/convert.rs:565:1
[00:04:58]     |
[00:04:58]     |
[00:04:58] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:04:58]     | -------------------------------------------- first implementation here
[00:04:58] 
[00:04:58] error[E0119]: conflicting implementations of trait `convert::TryFrom<&mut [_]>` for type `&mut [type error]`:
[00:04:58]     |
[00:04:58]     |
[00:04:58] 138 | impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
[00:04:58]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut [type error]`
[00:04:58]    ::: src/libcore/convert.rs:565:1
[00:04:58]     |
[00:04:58]     |
[00:04:58] 565 | impl<T, U> TryFrom<U> for T where U: Into<T> {
[00:04:58]     | -------------------------------------------- first implementation here
[00:04:58] 
[00:04:58] error[E0119]: conflicting implementations of trait `iter::traits::collect::IntoIterator` for type `&[_; _]`:
[00:04:58]     |
[00:04:58]     |
[00:04:58] 241 | impl<I: Iterator> IntoIterator for I {
[00:04:58]     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&[_; _]`
[00:04:58]    ::: src/libcore/array.rs:166:1
[00:04:58]     |
[00:04:58]     |
[00:04:58] 166 | impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
[00:04:58]     | ------------------------------------------------------- first implementation here
[00:04:58] error: aborting due to 4 previous errors
[00:04:58] 
[00:04:58] For more information about this error, try `rustc --explain E0119`.
[00:04:58] error: Could not compile `core`.

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)

crlf0710 · 2019-06-12T09:38:20Z

Rebased the commits.

This error disappeared for a while, but come out again on newest master.

But the diagnostics might imply something wierd is going on: it says
conflicting implementation for &mut [type error]

I can repro similar error with similar code. but i'm not sure whether array is actually defined this way, or is something special:

#![feature(fundamental)]
#![feature(const_generics)]
use std::marker::PhantomData;

#[fundamental] //after commenting out this line the compilation will pass.
struct MyArr<T, const L: usize>(PhantomData<T>);

trait MyInto<T> {
    fn my_into(self) -> T;
}

trait MyTryFrom<T> {
    fn my_try_from(v: T) -> Self;
}

impl<T, U> MyTryFrom<U> for T where U: MyInto<T> {
    fn my_try_from(v: U) -> Self {
        v.my_into()
    }
}

impl<T, const N: usize> MyTryFrom<&[T]> for MyArr<T, {N}> where T: Copy {
    fn my_try_from(v: &[T]) -> Self {
        MyArr(PhantomData)
    }
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=99b624ec1f0cd6c219abf2c8915820da

Centril · 2019-06-22T00:39:10Z

r? @Centril

Per T-Libs and T-Lang decisions in #61415 (comment) and #61415 (comment) respectively, we would be inclined to accept @petrochenkov idea in #61415 (comment) provided that there are:

UI tests to show that implementations above 32 elements do not work.
A perf run to demonstrate this is still beneficial.

@crlf0710 Feel free to either a) turn this PR into that, b) close this PR and file a new one for that ^--.

crlf0710 · 2019-06-22T03:19:49Z

Sure, i will turn this PR into that in the next few days.

scottmcm · 2019-07-06T08:45:51Z

Oops, I made a PR for this in #62435 without noticing this one.

crlf0710 · 2019-07-07T03:40:06Z

Oops, I made a PR for this in #62435 without noticing this one.

That's fine. I'll just close this in favor of #62435 .

rust-highfive assigned shepmaster May 2, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 2, 2019

crlf0710 force-pushed the array_const_generics branch 3 times, most recently from 05d9148 to 6067107 Compare May 2, 2019 09:21

jonas-schievink added the relnotes Marks issues that should be documented in the release notes of the next release. label May 4, 2019

jonas-schievink added this to the 1.36 milestone May 4, 2019

ollie27 reviewed May 4, 2019

View reviewed changes

crlf0710 mentioned this pull request May 5, 2019

Tracking issue for const generics (RFC 2000) #44580

Closed

16 tasks

crlf0710 force-pushed the array_const_generics branch from 0024746 to 6871fe2 Compare May 7, 2019 05:46

crlf0710 force-pushed the array_const_generics branch from 52a3741 to 3e8c156 Compare June 3, 2019 13:36

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Jun 3, 2019

crlf0710 added 5 commits June 12, 2019 15:37

Switch array implementation to const generics.

3651470

Add array_stage0.rs for bootstrapping.

e9cbd50

Temporarily revert Default implementation back to old macro-based one.

1975a3d

Change expected output of one test case.

6485b6b

Add two tests for const generics array impls.

0138ad4

crlf0710 force-pushed the array_const_generics branch from 3e8c156 to 0138ad4 Compare June 12, 2019 07:55

Centril removed needs-fcp This change is insta-stable, so needs a completed FCP to proceed. relnotes Marks issues that should be documented in the release notes of the next release. labels Jun 22, 2019

Centril removed this from the 1.37 milestone Jun 22, 2019

Centril removed T-lang Relevant to the language team, which will review and decide on the PR/issue. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Jun 22, 2019

rust-highfive assigned Centril and unassigned shepmaster Jun 22, 2019

Centril added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 3, 2019

crlf0710 closed this Jul 7, 2019

mriise mentioned this pull request Mar 31, 2021

const generics multiformats/rust-multihash#116

Closed

Switch libcore array implementations to const generics. #60466

Switch libcore array implementations to const generics. #60466

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