Add the alloc::prelude module

[SimonSapin]

It contains the re-exports that are in std::prelude::v1 but not in core::prelude::v1.

Calling it prelude is somewhat of a misnomer since (unlike those modules in std or core) its contents are never implicitly imported in modules. Rather it is intended to be used with an explicit glob import like use alloc::prelude::*;. However there is precedent for the same misnomer with std::io::prelude, for example.

This new module is unstable with the same feature name as the alloc care. They are proposed for stabilization together in RFC rust-lang/rfcs#2480.

[rust-highfive]

r? @shepmaster

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[rust-highfive]

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.

[01:13:46] 
[01:13:46] 
[01:13:46] running 399 tests
[01:14:15] ....................................................................................................
[01:14:38] ...................................................................i................F...............
[01:15:19] ...................................................................................................
[01:15:19] failures:
[01:15:19] 
[01:15:19] ---- prelude.rs - prelude (line 16) stdout ----
[01:15:19] ---- prelude.rs - prelude (line 16) stdout ----
[01:15:19] error[E0433]: failed to resolve. Did you mean `std::alloc`?
[01:15:19]  --> prelude.rs:18:5
[01:15:19] 5 | use alloc::prelude::*;
[01:15:19]   |     ^^^^^ Did you mean `std::alloc`?
[01:15:19] 
[01:15:19] thread 'prelude.rs - prelude (line 16)' panicked at 'couldn't compile the test', librustdoc/test.rs:327:17
---
[01:15:19] 
[01:15:19] error: test failed, to rerun pass '--doc'
[01:15:19] 
[01:15:19] 
[01:15:19] 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 jemalloc backtrace" "--manifest-path" "/checkout/src/libstd/Cargo.toml" "-p" "alloc" "--" "--quiet"
[01:15:19] 
[01:15:19] 
[01:15:19] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[01:15:19] Build completed unsuccessfully in 0:29:28
[01:15:19] Build completed unsuccessfully in 0:29:28
[01:15:19] make: *** [check] Error 1
[01:15:19] Makefile:58: recipe for target 'check' failed
2842592 ./obj
2840232 ./obj/build
2245784 ./obj/build/x86_64-unknown-linux-gnu
730396 ./src
---
144184 ./obj/build/bootstrap/debug/incremental
143672 ./obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu
143668 ./obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release
129672 ./obj/build/bootstrap/debug/incremental/bootstrap-nf9tlms0eza
129668 ./obj/build/bootstrap/debug/incremental/bootstrap-nf9tlms0eza/s-f2nxwxmvrw-820vkd-jf8cfeolaswv
129452 ./obj/build/x86linux-gnu/stage2-tools
60840 ./src/llvm-emscripten/lib
60776 ./obj/build/x86_64-unknown-linux-gnu/stage0-rustc
57604 ./obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu

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[shepmaster]

I swear I always get the "insta-stable" PRs... 😝

r? @alexcrichton

[SimonSapin]

Yeah, I may be cutting a corner here. I don’t mind changing this to unstable and proposing stabilization together with the crate in rust-lang/rfcs#2480.

[SimonSapin]

Changed to unstable with the same feature name as the crate.

[alexcrichton]

I'm still not 100% sure this is the end state we want to be in, but I think this is a good option to have at least. In any case it's an unstable module so we have time to tinker! Should this perhaps follow the standard of core/std prelude and be in a v1 submodule as well?

[SimonSapin]

It’s not injected automatically anywhere, so I rather followed the example of std::io::prelude, std::os::windows::prelude, etc.

[alexcrichton]

Er sorry, was cleaning out branches on the rust-lang repo and accidentally closed this, can the PR be resent from your own repo @SimonSapin?