8c4fc9d9a4
Add a dedicated length-prefixing method to `Hasher` This accomplishes two main goals: - Make it clear who is responsible for prefix-freedom, including how they should do it - Make it feasible for a `Hasher` that *doesn't* care about Hash-DoS resistance to get better performance by not hashing lengths This does not change rustc-hash, since that's in an external crate, but that could potentially use it in future. Fixes #94026 r? rust-lang/libs --- The core of this change is the following two new methods on `Hasher`: ```rust pub trait Hasher { /// Writes a length prefix into this hasher, as part of being prefix-free. /// /// If you're implementing [`Hash`] for a custom collection, call this before /// writing its contents to this `Hasher`. That way /// `(collection![1, 2, 3], collection![4, 5])` and /// `(collection![1, 2], collection![3, 4, 5])` will provide different /// sequences of values to the `Hasher` /// /// The `impl<T> Hash for [T]` includes a call to this method, so if you're /// hashing a slice (or array or vector) via its `Hash::hash` method, /// you should **not** call this yourself. /// /// This method is only for providing domain separation. If you want to /// hash a `usize` that represents part of the *data*, then it's important /// that you pass it to [`Hasher::write_usize`] instead of to this method. /// /// # Examples /// /// ``` /// #![feature(hasher_prefixfree_extras)] /// # // Stubs to make the `impl` below pass the compiler /// # struct MyCollection<T>(Option<T>); /// # impl<T> MyCollection<T> { /// # fn len(&self) -> usize { todo!() } /// # } /// # impl<'a, T> IntoIterator for &'a MyCollection<T> { /// # type Item = T; /// # type IntoIter = std::iter::Empty<T>; /// # fn into_iter(self) -> Self::IntoIter { todo!() } /// # } /// /// use std:#️⃣:{Hash, Hasher}; /// impl<T: Hash> Hash for MyCollection<T> { /// fn hash<H: Hasher>(&self, state: &mut H) { /// state.write_length_prefix(self.len()); /// for elt in self { /// elt.hash(state); /// } /// } /// } /// ``` /// /// # Note to Implementers /// /// If you've decided that your `Hasher` is willing to be susceptible to /// Hash-DoS attacks, then you might consider skipping hashing some or all /// of the `len` provided in the name of increased performance. #[inline] #[unstable(feature = "hasher_prefixfree_extras", issue = "88888888")] fn write_length_prefix(&mut self, len: usize) { self.write_usize(len); } /// Writes a single `str` into this hasher. /// /// If you're implementing [`Hash`], you generally do not need to call this, /// as the `impl Hash for str` does, so you can just use that. /// /// This includes the domain separator for prefix-freedom, so you should /// **not** call `Self::write_length_prefix` before calling this. /// /// # Note to Implementers /// /// The default implementation of this method includes a call to /// [`Self::write_length_prefix`], so if your implementation of `Hasher` /// doesn't care about prefix-freedom and you've thus overridden /// that method to do nothing, there's no need to override this one. /// /// This method is available to be overridden separately from the others /// as `str` being UTF-8 means that it never contains `0xFF` bytes, which /// can be used to provide prefix-freedom cheaper than hashing a length. /// /// For example, if your `Hasher` works byte-by-byte (perhaps by accumulating /// them into a buffer), then you can hash the bytes of the `str` followed /// by a single `0xFF` byte. /// /// If your `Hasher` works in chunks, you can also do this by being careful /// about how you pad partial chunks. If the chunks are padded with `0x00` /// bytes then just hashing an extra `0xFF` byte doesn't necessarily /// provide prefix-freedom, as `"ab"` and `"ab\u{0}"` would likely hash /// the same sequence of chunks. But if you pad with `0xFF` bytes instead, /// ensuring at least one padding byte, then it can often provide /// prefix-freedom cheaper than hashing the length would. #[inline] #[unstable(feature = "hasher_prefixfree_extras", issue = "88888888")] fn write_str(&mut self, s: &str) { self.write_length_prefix(s.len()); self.write(s.as_bytes()); } } ``` With updates to the `Hash` implementations for slices and containers to call `write_length_prefix` instead of `write_usize`. `write_str` defaults to using `write_length_prefix` since, as was pointed out in the issue, the `write_u8(0xFF)` approach is insufficient for hashers that work in chunks, as those would hash `"a\u{0}"` and `"a"` to the same thing. But since `SipHash` works byte-wise (there's an internal buffer to accumulate bytes until a full chunk is available) it overrides `write_str` to continue to use the add-non-UTF-8-byte approach. --- Compatibility: Because the default implementation of `write_length_prefix` calls `write_usize`, the changed hash implementation for slices will do the same thing the old one did on existing `Hasher`s.
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. You can ask for help in the #new members Zulip stream.
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 in the root of the project.
The x.py command can be run directly on most systems in the following format:
./x.py <subcommand> [flags]
This is how the documentation and examples assume you are running x.py.
Systems such as Ubuntu 20.04 LTS do not create the necessary python command by default when Python is installed that allows x.py to be run directly. In that case you can either create a symlink for python (Ubuntu provides the python-is-python3 package for this), or run x.py using Python itself:
# Python 3
python3 x.py <subcommand> [flags]
# Python 2.7
python2.7 x.py <subcommand> [flags]
More information about x.py can be found
by running it with the --help flag or reading the rustc dev guide.
Building on a Unix-like system
-
Make sure you have installed the dependencies:
g++5.1 or later orclang++3.5 or laterpython3 or 2.7- GNU
make3.81 or later cmake3.13.4 or laterninjacurlgitsslwhich comes inlibssl-devoropenssl-develpkg-configif you are compiling on Linux and targeting Linux
-
Clone the source with
git:git clone https://github.com/rust-lang/rust.git cd rust
-
Configure the build settings:
The Rust build system uses a file named
config.tomlin the root of the source tree to determine various configuration settings for the build. Copy the defaultconfig.toml.exampletoconfig.tomlto get started.cp config.toml.example config.tomlIf you plan to use
x.py installto create an installation, it is recommended that you set theprefixvalue in the[install]section to a directory.Create install directory if you are not installing in default directory
-
Build and install:
./x.py build && ./x.py installWhen complete,
./x.py installwill place several programs into$PREFIX/bin:rustc, the Rust compiler, andrustdoc, 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 cargoor set thebuild.extendedkey inconfig.tomltotrueto 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:
-
Grab the latest MSYS2 installer and go through the installer.
-
Run
mingw32_shell.batormingw64_shell.batfrom 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 runmsys2_shell.cmd -mingw32ormsys2_shell.cmd -mingw64from the command line instead) -
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 -
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-gnux86_64-pc-windows-gnu
- The MSVC ABI
i686-pc-windows-msvcx86_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 (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:
- Stack Overflow - Direct questions about using the language.
- users.rust-lang.org - General discussion and broader questions.
- /r/rust - News and general discussion.
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 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.