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bors c3681d62ee Auto merge of #68122 - Centril:stabilize-transparent-enums, r=petrochenkov 
Stabilize `#[repr(transparent)]` on `enum`s in Rust 1.42.0

# Stabilization report

The following is the stabilization report for `#![feature(transparent_enums)]`.

Tracking issue: https://github.com/rust-lang/rust/issues/60405
[Version target](https://forge.rust-lang.org/#current-release-versions): 1.42 (2020-01-30 => beta, 2020-03-12 => stable).

## User guide

A `struct` with only a single non-ZST field (let's call it `foo`) can be marked as `#[repr(transparent)]`. Such a `struct` has the same layout and ABI as `foo`. Here, we also extend this ability to `enum`s with only one variant, subject to the same restrictions as for the equivalent `struct`. That is, you can now write:

```rust
#[repr(transparent)]
enum Foo { Bar(u8) }
```

which, in terms of layout and ABI, is equivalent to:

```rust
#[repr(transparent)]
struct Foo(u8);
```

## Motivation

This is not a major feature that will unlock new and important use-cases. The utility of `repr(transparent)` `enum`s is indeed limited. However, there is still some value in it:

1. It provides conceptual simplification of the language in terms of treating univariant `enum`s and `struct`s the same, as both are product types. Indeed, languages like Haskell only have `data` as the only way to construct user-defined ADTs in the language.

2. In rare occasions, it might be that the user started out with a univariant `enum` for whatever reason (e.g. they thought they might extend it later). Now they want to make this `enum` `transparent` without breaking users by turning it into a `struct`. By lifting the restriction here, now they can.

## Technical specification

The reference specifies [`repr(transparent)` on a `struct`](https://doc.rust-lang.org/nightly/reference/type-layout.html#the-transparent-representation) as:

> ### The transparent Representation
>
>  The `transparent` representation can only be used on `struct`s that have:
>  - a single field with non-zero size, and
>  - any number of fields with size 0 and alignment 1 (e.g. `PhantomData<T>`).
>
> Structs with this representation have the same layout and ABI as the single non-zero sized field.
>
> This is different than the `C` representation because a struct with the `C` representation will always have the ABI of a `C` `struct` while, for example, a struct with the `transparent` representation with a primitive field will have the ABI of the primitive field.
>
> Because this representation delegates type layout to another type, it cannot be used with any other representation.

Here, we amend this to include univariant `enum`s as well with the same static restrictions and the same effects on dynamic semantics.

## Tests

All the relevant tests are adjusted in the PR diff but are recounted here:

- `src/test/ui/repr/repr-transparent.rs` checks that `repr(transparent)` on an `enum` must be univariant, rather than having zero or more than one variant. Restrictions on the fields inside the only variants, like for those on `struct`s, are also checked here.

- A number of codegen tests are provided as well:
    - `src/test/codegen/repr-transparent.rs` (the canonical test)
    - `src/test/codegen/repr-transparent-aggregates-1.rs`
    - `src/test/codegen/repr-transparent-aggregates-2.rs`
    - `src/test/codegen/repr-transparent-aggregates-3.rs`

- `src/test/ui/lint/lint-ctypes-enum.rs` tests the interactions with the `improper_ctypes` lint.

## History

- 2019-04-30, RFC https://github.com/rust-lang/rfcs/pull/2645
  Author: @mjbshaw
  Reviewers: The Language Team

  This is the RFC that proposes allowing `#[repr(transparent)]` on `enum`s and `union`.

- 2019-06-11, PR https://github.com/rust-lang/rust/pull/60463
  Author: @mjbshaw
  Reviewers: @varkor and @rkruppe

  The PR implements the RFC aforementioned in full.

- 2019, PR https://github.com/rust-lang/rust/pull/67323
  Author: @Centril
  Reviewers: @davidtwco

  The PR reorganizes the static checks taking advantage of the fact that `struct`s and `union`s are internally represented as ADTs with a single variant.

- This PR stabilizes `transparent_enums`.

## Related / possible future work

The remaining work here is to figure out the semantics of `#[repr(transparent)]` on `union`s and stabilize those. This work continues to be tracked in https://github.com/rust-lang/rust/issues/60405.
2020-01-27 00:05:57 +00:00
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README.md

The Rust Programming Language

This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.

Quick Start

Read "Installation" from The Book.

Installing from Source

Note: If you wish to contribute to the compiler, you should read this chapter of the rustc-guide instead of this section.

The Rust build system has a Python script called x.py to bootstrap building the compiler. More information about it may be found by running ./x.py --help or reading the rustc guide.

Building on *nix

  1. Make sure you have installed the dependencies:

    • g++ 5.1 or later or clang++ 3.5 or later
    • python 2.7 (but not 3.x)
    • GNU make 3.81 or later
    • cmake 3.4.3 or later
    • curl
    • git
    • ssl which comes in libssl-dev or openssl-devel
    • pkg-config if you are compiling on Linux and targeting Linux

  2. Clone the source with git:

    $ git clone https://github.com/rust-lang/rust.git
$ cd rust

  1. Configure the build settings:

    The Rust build system uses a file named config.toml in the root of the source tree to determine various configuration settings for the build. Copy the default config.toml.example to config.toml to get started.

    $ cp config.toml.example config.toml

    It is recommended that if you plan to use the Rust build system to create an installation (using ./x.py install) that you set the prefix value in the [install] section to a directory that you have write permissions.

    Create install directory if you are not installing in default directory

  2. Build and install:

    $ ./x.py build && ./x.py install

    When complete, ./x.py install will place several programs into $PREFIX/bin: rustc, the Rust compiler, and rustdoc, the API-documentation tool. This install does not include Cargo, Rust's package manager. To build and install Cargo, you may run ./x.py install cargo or set the build.extended key in config.toml to true to build and install all tools.

Building on Windows

There are two prominent ABIs in use on Windows: the native (MSVC) ABI used by Visual Studio, and the GNU ABI used by the GCC toolchain. Which version of Rust you need depends largely on what C/C++ libraries you want to interoperate with: for interop with software produced by Visual Studio use the MSVC build of Rust; for interop with GNU software built using the MinGW/MSYS2 toolchain use the GNU build.

MinGW

MSYS2 can be used to easily build Rust on Windows:

  1. Grab the latest MSYS2 installer and go through the installer.

  2. Run mingw32_shell.bat or mingw64_shell.bat from wherever you installed MSYS2 (i.e. C:\msys64), depending on whether you want 32-bit or 64-bit Rust. (As of the latest version of MSYS2 you have to run msys2_shell.cmd -mingw32 or msys2_shell.cmd -mingw64 from the command line instead)

  3. From this terminal, install the required tools:

    # Update package mirrors (may be needed if you have a fresh install of MSYS2)
$ pacman -Sy pacman-mirrors

# Install build tools needed for Rust. If you're building a 32-bit compiler,
# then replace "x86_64" below with "i686". If you've already got git, python,
# or CMake installed and in PATH you can remove them from this list. Note
# that it is important that you do **not** use the 'python2' and 'cmake'
# packages from the 'msys2' subsystem. The build has historically been known
# to fail with these packages.
$ pacman -S git \
            make \
            diffutils \
            tar \
            mingw-w64-x86_64-python2 \
            mingw-w64-x86_64-cmake \
            mingw-w64-x86_64-gcc

  4. Navigate to Rust's source code (or clone it), then build it:

    $ ./x.py build && ./x.py install

MSVC

MSVC builds of Rust additionally require an installation of Visual Studio 2017 (or later) so rustc can use its linker. The simplest way is to get the Visual Studio, check the “C++ build tools” and “Windows 10 SDK” workload.

(If you're installing cmake yourself, be careful that “C++ CMake tools for Windows” doesn't get included under “Individual components”.)

With these dependencies installed, you can build the compiler in a cmd.exe shell with:

> python x.py build

Currently, building Rust only works with some known versions of Visual Studio. If you have a more recent version installed the build system doesn't understand then you may need to force rustbuild to use an older version. This can be done by manually calling the appropriate vcvars file before running the bootstrap.

> CALL "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"
> python x.py build

Building rustc with older host toolchains

It is still possible to build Rust with the older toolchain versions listed below, but only if the LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN option is set to true in the config.toml file.

  • Clang 3.1
  • Apple Clang 3.1
  • GCC 4.8
  • Visual Studio 2015 (Update 3)

Toolchain versions older than what is listed above cannot be used to build rustc.

Specifying an ABI

Each specific ABI can also be used from either environment (for example, using the GNU ABI in PowerShell) by using an explicit build triple. The available Windows build triples are:

  • GNU ABI (using GCC)
    • i686-pc-windows-gnu
    • x86_64-pc-windows-gnu
  • The MSVC ABI
    • i686-pc-windows-msvc
    • x86_64-pc-windows-msvc

The build triple can be specified by either specifying --build=<triple> when invoking x.py commands, or by copying the config.toml file (as described in Installing From Source), and modifying the build option under the [build] section.

Configure and Make

While it's not the recommended build system, this project also provides a configure script and makefile (the latter of which just invokes x.py).

$ ./configure
$ make && sudo make install

When using the configure script, the generated config.mk file may override the config.toml file. To go back to the config.toml file, delete the generated config.mk file.

Building Documentation

If you’d like to build the documentation, it’s almost the same:

$ ./x.py doc

The generated documentation will appear under doc in the build directory for the ABI used. I.e., if the ABI was x86_64-pc-windows-msvc, the directory will be build\x86_64-pc-windows-msvc\doc.

Notes

Since the Rust compiler is written in Rust, it must be built by a precompiled "snapshot" version of itself (made in an earlier stage of development). As such, source builds require a connection to the Internet, to fetch snapshots, and an OS that can execute the available snapshot binaries.

Snapshot binaries are currently built and tested on several platforms:

Platform / Architecture x86 x86_64
Windows (7, 8, 10, ...) ✓ ✓
Linux (2.6.18 or later) ✓ ✓
macOS (10.7 Lion or later) ✓ ✓

You may find that other platforms work, but these are our officially supported build environments that are most likely to work.

There is more advice about hacking on Rust in CONTRIBUTING.md.

Getting Help

The Rust community congregates in a few places:

Contributing

To contribute to Rust, please see CONTRIBUTING.

Most real-time collaboration happens in a variety of channels on the Rust Discord server, with channels dedicated for getting help, community, documentation, and all major contribution areas in the Rust ecosystem. A good place to ask for help would be the #help channel.

The rustc guide might be a good place to start if you want to find out how various parts of the compiler work.

Also, you may find the rustdocs for the compiler itself useful.

License

Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.

Trademark

The Rust programming language is an open source, community project governed by a core team. It is also sponsored by the Mozilla Foundation (“Mozilla”), which owns and protects the Rust and Cargo trademarks and logos (the “Rust Trademarks”).

If you want to use these names or brands, please read the media guide.

Third-party logos may be subject to third-party copyrights and trademarks. See Licenses for details.