Dylan DPC a1c34493d4
Rollup merge of #73945 - est31:unused_externs, r=Mark-Simulacrum
Add an unstable --json=unused-externs flag to print unused externs

This adds an unstable flag to print a list of the extern names not used by cargo.

This PR will enable cargo to collect unused dependencies from all units and provide warnings.
The companion PR to cargo is: https://github.com/rust-lang/cargo/pull/8437

The goal is eventual stabilization of this flag in rustc as well as in cargo.

Discussion of this feature is mostly contained inside these threads: #57274 #72342 #72603

The feature builds upon the internal datastructures added by #72342

Externs are uniquely identified by name and the information is sufficient for cargo.
If the mode is enabled, rustc will print json messages like:

```
{"unused_extern_names":["byteorder","openssl","webpki"]}
```

For a crate that got passed byteorder, openssl and webpki dependencies but needed none of them.

### Q: Why not pass -Wunused-crate-dependencies?
A: See [ehuss's comment here](https://github.com/rust-lang/rust/issues/57274#issuecomment-624839355)
   TLDR: it's cleaner. Rust's warning system wasn't built to be filtered or edited by cargo.
   Even a basic implementation of the feature would have to change the "n warnings emitted" line that rustc prints at the end.
   Cargo ideally wants to synthesize its own warnings anyways. For example, it would be hard for rustc to emit warnings like
   "dependency foo is only used by dev targets", suggesting to make it a dev-dependency instead.

### Q: Make rustc emit used or unused externs?
A: Emitting used externs has the advantage that it simplifies cargo's collection job.
   However, emitting unused externs creates less data to be communicated between rustc and cargo.
   Often you want to paste a cargo command obtained from `cargo build -vv` for doing something
   completely unrelated. The message is emitted always, even if no warning or error is emitted.
   At that point, even this tiny difference in "noise" matters. That's why I went with emitting unused externs.

### Q: One json msg per extern or a collective json msg?
A: Same as above, the data format should be concise. Having 30 lines for the 30 crates a crate uses would be disturbing to readers.
   Also it helps the cargo implementation to know that there aren't more unused deps coming.

### Q: Why use names of externs instead of e.g. paths?
A: Names are both sufficient as well as neccessary to uniquely identify a passed `--extern` arg.
   Names are sufficient because you *must* pass a name when passing an `--extern` arg.
   Passing a path is optional on the other hand so rustc might also figure out a crate's location from the file system.
   You can also put multiple paths for the same extern name, via e.g. `--extern hello=/usr/lib/hello.rmeta --extern hello=/usr/local/lib/hello.rmeta`,
   but rustc will only ever use one of those paths.
   Also, paths don't identify a dependency uniquely as it is possible to have multiple different extern names point to the same path.
   So paths are ill-suited for identification.

### Q: What about 2015 edition crates?
A: They are fully supported.
   Even on the 2015 edition, an explicit `--extern` flag is is required to enable `extern crate foo;` to work (outside of sysroot crates, which this flag doesn't warn about anyways).
   So the lint would still fire on 2015 edition crates if you haven't included a dependency specified in Cargo.toml using `extern crate foo;` or similar.
   The lint won't fire if your sole use in the crate is through a `extern crate foo;`   statement, but that's not its job.
   For detecting unused `extern crate foo` statements, there is the `unused_extern_crates` lint
   which can be enabled by `#![warn(unused_extern_crates)]` or similar.

cc ```@jsgf``` ```@ehuss``` ```@petrochenkov``` ```@estebank```
2021-04-04 19:19:58 +02:00
2021-03-10 18:28:05 -08:00
2021-04-01 20:43:44 +09:00
2021-03-10 08:41:53 +01:00

The Rust Programming Language

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

Note: this README is for users rather than contributors. If you wish to contribute to the compiler, you should read the Getting Started section of the rustc-dev-guide instead.

Quick Start

Read "Installation" from The Book.

Installing from Source

The Rust build system uses a Python script called x.py to build the compiler, which manages the bootstrapping process. More information about it can be found by running ./x.py --help or reading the rustc dev guide.

Building on a Unix-like system

  1. Make sure you have installed the dependencies:

    • g++ 5.1 or later or clang++ 3.5 or later
    • python 3 or 2.7
    • GNU make 3.81 or later
    • cmake 3.13.4 or later
    • ninja
    • 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
    

    If you plan to use x.py install to create an installation, it is recommended that you set the prefix value in the [install] section to a directory.

    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', 'cmake' and 'ninja'
    # 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-python \
                mingw-w64-x86_64-cmake \
                mingw-w64-x86_64-gcc \
                mingw-w64-x86_64-ninja
    
  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 and the build system doesn't understand, 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

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 youd like to build the documentation, its 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 (kernel 2.6.32, glibc 2.11 or later)
macOS (10.7 Lion or later) (*)

(*): Apple dropped support for running 32-bit binaries starting from macOS 10.15 and iOS 11. Due to this decision from Apple, the targets are no longer useful to our users. Please read our blog post for more info.

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

Getting Help

The Rust community congregates in a few places:

Contributing

If you are interested in contributing to the Rust project, please take a look at the Getting Started guide in the rustc-dev-guide.

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.

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