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Nilstrieb bfc2675362
Rollup merge of #118094 - JarvisCraft:SpecFromElem-for-empty-tuple, r=thomcc
feat: specialize `SpecFromElem` for `()`

# Description

This PR adds a specialization `SpecFromElem for ()` which allows to significantly reduce `vec![(), N]` time in debug builds (specifically, tests) turning it from observable $O(n)$ to $O(1)$.

# Observing the change

The problem this PR aims to fix explicitly is slow `vec![(), N]` on big `N`s which may appear in tests (see [Background section](#Background) for more details).

The minimal example to see the problem:

```rust
#![feature(test)]

extern crate test;

#[cfg(test)]
mod tests {
    const HUGE_SIZE: usize = i32::MAX as usize + 1;

    #[bench]
    fn bench_vec_literal(b: &mut test::Bencher) {
        b.iter(|| vec![(); HUGE_SIZE]);
    }

    #[bench]
    fn bench_vec_repeat(b: &mut test::Bencher) {
        b.iter(|| [(); 1].repeat(HUGE_SIZE));
    }
}
```
<details><summary>Output</summary>
<p>

```bash
cargo +nightly test -- --report-time -Zunstable-options
   Compiling huge-zst-vec-literal-bench v0.1.0 (/home/progrm_jarvis/RustroverProjects/huge-zst-vec-literal-bench)
    Finished test [unoptimized + debuginfo] target(s) in 0.31s
     Running unittests src/lib.rs (target/debug/deps/huge_zst_vec_literal_bench-e43b1ef287ba8b36)

running 2 tests
test tests::bench_vec_repeat  ... ok <0.000s>
test tests::bench_vec_literal ... ok <14.382s>

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 14.38s

   Doc-tests huge-zst-vec-literal-bench

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```
</p>
</details>

> [!IMPORTANT]
> This problem is only observable in Debug (unoptimized) builds, while Release (optimized) ones do not observe this problem. It still is worth fixing it, IMO, since the original scenario observes the problem in tests for which optimizations are disabled by default and it seems unreasonable to override this for the whole project while the problem is very local.

# Background

While working on a crate for a custom data format which has an `i32::MAX` limit on its list's sizes, I wrote the following test to ensure that this invariant is upheld:

```rust
#[test]
fn lists_should_have_i32_size() {
    assert!(
        RawNbtList::try_from(vec![(); i32::MAX as usize]).is_ok(),
        "lists should permit the size up to {}",
        i32::MAX
    );
    assert!(
        RawNbtList::try_from(vec![(); i32::MAX as usize + 1]).is_err(),
        "lists should have the size of at most {}",
        i32::MAX
    );
}
```

Soon I discovered that this takes $\approx 3--4s$ per assertion on my machine, almost all of which is spent on `vec![..]`.
While this would be logical for a non-ZST vector (which would require actual $O(n)$ allocation), here `()` was used intentionally considering that for ZSTs size-changing operations should anyway be $O(1)$ (at least from allocator perspective). Yet, this "overhead" is logical if we consider that in general case `clone()` (which is used by `Vec` literal) may have a non-trivial implementation and thus each element has to actually be visited (even if they occupy no space).

In my specific case, the following (contextual) equivalent solved the issue:

```rust
#[test]
fn lists_should_have_i32_size() {
    assert!(
        RawNbtList::try_from([(); 1].repeat(i32::MAX as usize)).is_ok(),
        "lists should permit the size up to {}",
        i32::MAX
    );
    assert!(
        RawNbtList::try_from([(); 1].repeat(i32::MAX as usize + 1)).is_err(),
        "lists should have the size of at most {}",
        i32::MAX
    );
}
```

which works since `repeat` explicitly uses `T: Copy` and so does not have to think about non-trivial `Clone`.

But it still may be counter-intuitive for users to observe such long time on the "canonical" vec literal thus the PR.

# Generic solution

This change is explicitly non-generic. Initially I considered it possible to implement in generically, but this would require the specialization to have the following type requirements:
-  the type must be a ZST: easily done via
  ```rust
  if core::mem::size_of::<T>() == 0 {
    todo!("specialization")
  }
  ```
  or
  ```rust
  use core::mem::SizedTypeProperties;
  if T::IS_ZST {
    todo!("specialization")
  }
  ```
- :white_check_mark`: the type must implement `Copy`: implementable non-conflictable via a separate specialization:
  ```rust
  trait IsCopyZst: Sized {
    fn is_copy_zst() -> bool;
  }
  impl<T> IsCopyZst for T {
    fn is_copy_zst() -> bool {
        false
    }
  }
  impl<T: Copy> IsCopyZst for T {
    fn is_copy_zst() -> bool {
        Self::IS_ZST
    }
  }
  ```
-  the type should have a trivial `Clone` implementation: since `vec![t; n]` is specified to use `clone()`, we can only use this "performance optimization" when we are guaranteed that `clone()` does nothing except for copying.

The latter is the real blocker for a generic fix since I am unaware of any way to get this information in a compiler-guaranteed way.

While there may be a fix for this (my friend `@CertainLach` has suggested a potential solution by an perma-unstable fn in `Clone` like `is_trivially_cloneable()` defaulting to `false` and only overridable by `rustc` on derive), this is surely out of this PRs scope.
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src Rollup merge of #118059 - Nilstrieb:unset-cargo, r=dtolnay 2023-11-21 09:06:28 +01:00
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README.md

The Rust Programming Language

Rust Community

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 CONTRIBUTING.md instead.

Table of content

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. It lives at the root of the project. It also uses a file named config.toml to determine various configuration settings for the build. You can see a full list of options in config.example.toml.

The x.py command can be run directly on most Unix systems in the following format:

./x.py <subcommand> [flags]

This is how the documentation and examples assume you are running x.py. See the rustc dev guide if this does not work on your platform.

More information about x.py can be found by running it with the --help flag or reading the rustc dev guide.

Dependencies

Make sure you have installed the dependencies:

  • python 3 or 2.7
  • git
  • A C compiler (when building for the host, cc is enough; cross-compiling may need additional compilers)
  • curl (not needed on Windows)
  • pkg-config if you are compiling on Linux and targeting Linux
  • libiconv (already included with glibc on Debian-based distros)

To build Cargo, you'll also need OpenSSL (libssl-dev or openssl-devel on most Unix distros).

If building LLVM from source, you'll need additional tools:

  • g++, clang++, or MSVC with versions listed on LLVM's documentation
  • ninja, or GNU make 3.81 or later (Ninja is recommended, especially on Windows)
  • cmake 3.13.4 or later
  • libstdc++-static may be required on some Linux distributions such as Fedora and Ubuntu

On tier 1 or tier 2 with host tools platforms, you can also choose to download LLVM by setting llvm.download-ci-llvm = true. Otherwise, you'll need LLVM installed and llvm-config in your path. See the rustc-dev-guide for more info.

Building on a Unix-like system

Build steps

  1. Clone the source with git:

    git clone https://github.com/rust-lang/rust.git
    cd rust
    
  1. Configure the build settings:

    ./configure
    

    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: ./configure --set install.prefix=<path>

  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. By default, it will also include Cargo, Rust's package manager. You can disable this behavior by passing --set build.extended=false to ./configure.

Configure and Make

This project provides a configure script and makefile (the latter of which just invokes x.py). ./configure is the recommended way to programmatically generate a config.toml. make is not recommended (we suggest using x.py directly), but it is supported and we try not to break it unnecessarily.

./configure
make && sudo make install

configure generates a config.toml which can also be used with normal x.py invocations.

Building on Windows

On Windows, we suggest using winget to install dependencies by running the following in a terminal:

winget install -e Python.Python.3
winget install -e Kitware.CMake
winget install -e Git.Git

Then edit your system's PATH variable and add: C:\Program Files\CMake\bin. See this guide on editing the system PATH from the Java documentation.

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. Use the MSVC build of Rust to interop with software produced by Visual Studio and the GNU build to interop with GNU software built using the MinGW/MSYS2 toolchain.

MinGW

MSYS2 can be used to easily build Rust on Windows:

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

  2. Run mingw32_shell.bat or mingw64_shell.bat from the MSYS2 installation directory (e.g. 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:

    python x.py setup user && python x.py build && python 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 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 setup user
python x.py build

Right now, 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 creating a config.toml file (as described in Building on a Unix-like system), and passing --set build.build=<triple> to ./configure.

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. That is, 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 an Internet connection to fetch snapshots, and an OS that can execute the available snapshot binaries.

See https://doc.rust-lang.org/nightly/rustc/platform-support.html for a list of supported platforms. Only "host tools" platforms have a pre-compiled snapshot binary available; to compile for a platform without host tools you must cross-compile.

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

Getting Help

See https://www.rust-lang.org/community for a list of chat platforms and forums.

Contributing

See CONTRIBUTING.md.

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 Foundation 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.