Move alloc::prelude::* to alloc::prelude::v1, make alloc a subset of std

[SimonSapin]

This was one of the unresolved questions of rust-lang/rfcs#2480. As the RFC says this is maybe not useful in the sense that we are unlikely to ever have a second version, but making the crate a true subset makes one less issue to think about if we stabilize it and later want to merge standard library crates and have Cargo feature flags to enable or disable parts of the std crate.

See also discussion in #58175.

Also rename the feature gate and point to a dedicated tracking issue: #58935

[rust-highfive]

r? @joshtriplett

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[SimonSapin]

This is a breaking change to unstable APIs. CC @Nemo157, @dhardy, @alexcrichton, and @tarcieri who were involved in related discussions in rust-lang/rfcs#2480.

[SimonSapin]

I previously argued the other way around:

this not using a v1 submodule in order to follow the precedent of std::io::prelude rather than that of std::prelude::v1, since this import is never inserted automatically.

But I’ve changed my mind, I think making the crate actually a subset of std is more valuable.

[Nemo157]

Big 👍 from me. This makes a very useful pattern possible while alloc is unstable, allowing easier opt-in experimentation on nightly:

#[cfg(all(feature = "alloc", not(feature = "std")))]
extern crate alloc;
#[cfg(feature = "std")]
extern crate std as alloc;

[rust-highfive]

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:0b3a6bf4:start=1551782993320238380,finish=1551782994396548827,duration=1076310447
$ 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
Setting environment variables from .travis.yml
---
travis_time:start:test_debuginfo
Check compiletest suite=debuginfo mode=debuginfo-both (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[01:21:46] 
[01:21:46] running 119 tests
[01:22:15] .iiiii...i.....i..i...i..i.i..i.ii...i.....i..i....i..........iiii..........i...ii...i.......ii.i.i. 100/119
[01:22:20] i......iii.i.....ii
[01:22:20] 
[01:22:20]  finished in 34.396
[01:22:20] travis_fold:end:test_debuginfo

---
[01:31:03] 
[01:31:04] 
[01:31:04] running 419 tests
[01:31:24] .................................................................................................... 100/419
[01:31:40] .............................................................................i..............F....... 200/419
[01:32:08] .................................................................................................... 400/419
[01:32:11] ...................
[01:32:11] failures:
[01:32:11] 
[01:32:11] 
[01:32:11] ---- prelude/mod.rs - prelude (line 6) stdout ----
[01:32:11] error[E0658]: use of unstable library feature 'alloc_prelude' (see issue #58935)
[01:32:11]  --> prelude/mod.rs:10:5
[01:32:11]   |
[01:32:11] 7 | use alloc::prelude::v1::*;
[01:32:11]   |
[01:32:11]   = help: add #![feature(alloc_prelude)] to the crate attributes to enable
[01:32:11] 
[01:32:11] thread 'prelude/mod.rs - prelude (line 6)' panicked at 'couldn't compile the test', src/librustdoc/test.rs:352:13
---
[01:32:11] 
[01:32:11] error: test failed, to rerun pass '--doc'
[01:32:11] 
[01:32:11] 
[01:32:11] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--" "--quiet"
[01:32:11] 
[01:32:11] 
[01:32:11] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:32:11] Build completed unsuccessfully in 0:23:11
[01:32:11] Build completed unsuccessfully in 0:23:11
[01:32:11] Makefile:48: recipe for target 'check' failed
[01:32:11] make: *** [check] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0107cf04
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Tue Mar  5 12:22:16 UTC 2019
---
travis_time:end:0890a207:start=1551788538949106424,finish=1551788538955073105,duration=5966681
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:22c2b7ec
$ 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:0384bae4
travis_time:start:0384bae4
$ 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:1fd297e4
$ 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)

[tarcieri]

👍 from me as well. I wound up using this feature a bunch, and have this in several crates:

#[cfg(all(feature = "alloc", not(feature = "std")))]
pub use alloc::prelude::*;

I guess this change will break all those crates, but hey, that's what I signed up for so I'm not complaining 😉 and if it helps simplify the story around alloc stabilization, I'm all for it.

[dhardy]

I don't think this affects me directly, but definitely support making alloc a subset of std. 👍

[Amanieu]

@bors r+

[bors]

📌 Commit 5d1022a has been approved by Amanieu