This was one of the unresolved questions of https://github.com/rust-lang/rfcs/pull/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 https://github.com/rust-lang/rust/pull/58175
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
https://github.com/rust-lang/rfcs/pull/2480
