RFC: Stabilize the alloc crate

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+- Feature Name: `liballoc`
+- Start Date: 2018-06-14
+- RFC PR:
+- Rust Issue: [rust-lang/rust#27783](https://github.com/rust-lang/rust/issues/27783)
+
+# Summary
+[summary]: #summary
+
+Stabilize the `alloc` crate, with a module structure matching `std`.
+
+This crate provides the subset of the standard library’s functionality that requires
+a global allocator (unlike the `core` crate) but not other operating system
+capabilities (unlike the `std` crate).
+
+
+# Motivation
+[motivation]: #motivation
+
+## Background: `no_std`
+
+In some environments the `std` crate is not available:
+micro-controllers that don’t have an operating system at all, kernel-space code, etc.
+The `#![no_std]` attribute allows a crate to not link to `std` implicitly,
+using `core` instead with only the subset of functionality that doesn’t have a runtime dependency.
+
+## Use case 1: pushing `no_std` programs toward stable Rust
+
+Programs (or `staticlib`s) that do not link `std` at all may still want to use `Vec<T>`
+or other functionality that requires a memory allocator.
+Blockers for doing so on stable Rust are diminishing:
+
+* [The `#[global_allocator]` attribute][global_allocator] to specify an allocator
+  and remove the need for an operating-system-provided one is stable since Rust 1.28.
+* [PR #51607] adds a fallback handling for OOM (allocation failure) conditions
+  for when `std` is not available,
+  removing the need for programs to define an unstable `oom` lang item themselves.
+
+With this, the only allocation-related blocker is being able to import `Vec` in the first place.
+This RFC proposes stabilizing the current unstable way to do it: `extern crate alloc;`
+
+[global_allocator]: https://doc.rust-lang.org/nightly/std/alloc/#the-global_allocator-attribute
+[PR #51607]: https://github.com/rust-lang/rust/pull/51607
+
+## Use case 2: making stable libraries `no_std`
+
+Even if a `no_std` program might still require other features that are still unstable,
+it is very common to use libraries from crates.io that have other users.
+Such a library might support stable Rust and use `Vec<T>`
+(or something else that requires a memory allocator)
+but not other operating-sytem functionality.
+
+Today, making such a library possible to use without `std` without breaking stable users
+requires a compile-time flag:
+
+```rust
+#![no_std]
+
+#[cfg(feature = "no_std")] extern crate alloc;
+#[cfg(not(feature = "no_std"))] extern crate std as alloc;
+
+use alloc::vec::Vec;
+```
+
+With this RFC, this can be simplified to:
+
+```rust
+#![no_std]
+
+extern crate alloc;
+
+use alloc::vec::Vec;
+```
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+When using `#![no_std]` in a crate, that crate does not implicitly depend on `std`
+but depends on `core` instead. For example:
+
+```diff
+-use std::cell::RefCell;
++use core::cell::RefCell;
+```
+
+APIs that require a memory allocator are not available in `core`.
+In order to use them, `no_std` Rust code must explicitly depend on the `alloc` crate:
+
+```rust
+extern crate alloc;
+
+use core::cell::RefCell;
+use alloc::rc::Rc;
+```
+
+Like `std` and `core`, this dependency does not need to be declared in `Cargo.toml`
+since `alloc` is part of the standard library and distributed with Rust.
+
+The `alloc` crate does not have a prelude (items that are implicitly in scope).
+So its items that are in the `std` prelude must be imported explicitly
+to be used in `no_std` crates:
+
+```rust
+use alloc::vec::Vec;
+```
+
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `alloc` crate already exists (marked unstable),
+and every public API in it is already available in `std`.
+
+[PR #51569] moves them around so that the module structure matches that of `std`,
+and the public APIs become a subset:
+any path that starts with `alloc::` should still be valid and point to the same item
+after replacing that prefix with `std::` (assuming both crates are available).
+
+All that remains is stabilizing the `alloc` crate itself and tweaking its doc-comment.
+(In particular, removing the “not intended for general usage” sentence
+and mention `no_std` instead.)
+Since it is the only remaining unstable crate tracked by [tracking issue #27783],
+that issue can be closed after this RFC is implemented.
+
+The structure of the standard library is therefore:
+
+* `core`: has (almost) no runtime dependency, every Rust crate is expected to depend on this.
+* `alloc`: requires a global memory allocator,
+  either specified through the `#[global_allocator]` attribute
+  or provided by the `std` crate.
+* `std`: re-exports the contents of `core` and `alloc`
+  so that non-`no_std` crate do not need care about what’s in what crate between these three.
+  Depends on various operating system features such as files, threads, etc.
+* `proc-macro`: depends on parts of the compiler, typically only used at build-time
+  (in procedural macro crates or Cargo build scripts).
+
+[PR #51569]: https://github.com/rust-lang/rust/pull/51569
+[tracking issue #27783]: https://github.com/rust-lang/rust/issues/27783
+
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+[Tracking issue #27783] is the tracking issue for the `alloc` crate and, historically, some other crates.
+Although I could not find much discussion of that, I believe it has been kept unstable so far
+because of uncertainty of uncertainty of what is the eventual desired crate structure
+for the standard library, given infinite time and resources.
+
+In particular, could we have a single crate with some mechanism for selectively disabling
+or enabling some of the crate’s components, depending on which runtime dependencies
+are available in targetted environments?
+In that world, the `no_std` attribute and standard library crates other than `std`
+would be unecessary.
+
+By stabilizing the `alloc` crate, we commit to having it − and its public API − exist “forever”.
+
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+The `core` and the `no_std` attribute are already stable,
+so in a sense it’s already too late for the “pure” version of the vision described above
+where `std` really is the only standard library crate that exists.
+
+It may still be [desirable] to regroup the standard library into one crate,
+and it is proably still possible.
+The `core` crate could be replaced with a set of `pub use` reexport
+to maintained compatibility with existing users.
+Whatever the eventual status is for `core` is,
+we can do the same for `alloc`.
+[PR #51569] mentioned above also hopes to make this easier.
+
+While we want to leave the possibility open for it,
+at the time of this writing there are no concrete plans
+for implementing such a standard library crates unification any time soon.
+So the only alternative to this RFC seems to be
+leaving heap allocation for `no_std` in unstable limbo for the forseeable future.
+
+[desirable]: https://aturon.github.io/2018/02/06/portability-vision/#the-vision
+
+
+# Prior art
+[prior-art]: #prior-art
+
+I am not aware of a mechanism similar to `no_std` in another programming language.
+
+[Newlib] is a C library for “embedded” systems that typically don’t have an operating system.
+It does provide a memory allocator through `malloc` and related functions, unconditionally.
+
+[Newlib]: https://sourceware.org/newlib/
+
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Did I miss something in [PR #51569] that makes `alloc` not a subset of `std`?
+A double-check from someone else would be appreciated.