alloc not added to extern prelude

[sanmai-NL]

After #50605, core and/or std are added to the extern prelude when the feature extern_prelude is enabled.

It seems to me that if, additionally, the feature alloc is enabled, alloc should be added to the prelude too.

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rustc 1.30.0-nightly (20dc0c507 2018-09-19)

[cramertj]

This was discussed on another thread (trying to track it down, but a quick look didn't turn it up) and the plan was to allow adding alloc to the extern list by providing a flag in Cargo.toml that made cargo pass a --extern alloc (with no value for x in --extern alloc=<x>). This would cause the named crates to be included in the extern prelude (if found in the link path).

[Nemo157]

I believe the change @cramertj mentions is slated to be a post-2018 feature (since there are still design considerations for the cargo side of it). Until then use of extern crate {alloc,test,proc_macro}; are all still needed (only the last is possible on stable, the first two are unstable crates).

Edit: see #54116 (comment) and I think
#53130 (comment) is the most fleshed out comment on how this may look in the future.

[petrochenkov]

alloc still requires using extern crate alloc as now documented in https://rust-lang-nursery.github.io/edition-guide/rust-2018/module-system/path-clarity.html