1 # rustbuild - Bootstrapping Rust
3 This is an in-progress README which is targeted at helping to explain how Rust
4 is bootstrapped and in general some of the technical details of the build
9 The rustbuild build system has a primary entry point, a top level `x.py` script:
15 Note that if you're on Unix you should be able to execute the script directly:
21 The script accepts commands, flags, and arguments to determine what to do:
23 * `build` - a general purpose command for compiling code. Alone `build` will
24 bootstrap the entire compiler, and otherwise arguments passed indicate what to
28 # build the whole compiler
29 ./x.py build --stage 2
31 # build the stage1 compiler
35 ./x.py build --stage 0 library/std
37 # build a particular crate in stage0
38 ./x.py build --stage 0 library/test
41 If files are dirty that would normally be rebuilt from stage 0, that can be
42 overridden using `--keep-stage 0`. Using `--keep-stage n` will skip all steps
43 that belong to stage n or earlier:
46 # build stage 1, keeping old build products for stage 0
47 ./x.py build --keep-stage 0
50 * `test` - a command for executing unit tests. Like the `build` command this
51 will execute the entire test suite by default, and otherwise it can be used to
52 select which test suite is run:
61 # execute the UI test suite
62 ./x.py test src/test/ui
64 # execute only some tests in the UI test suite
65 ./x.py test src/test/ui --test-args substring-of-test-name
67 # execute tests in the standard library in stage0
68 ./x.py test --stage 0 library/std
70 # execute tests in the core and standard library in stage0,
71 # without running doc tests (thus avoid depending on building the compiler)
72 ./x.py test --stage 0 --no-doc library/core library/std
74 # execute all doc tests
78 * `doc` - a command for building documentation. Like above can take arguments
81 ## Configuring rustbuild
83 There are currently two methods for configuring the rustbuild build system.
85 First, rustbuild offers a TOML-based configuration system with a `config.toml`
86 file. An example of this configuration can be found at `config.toml.example`,
87 and the configuration file can also be passed as `--config path/to/config.toml`
88 if the build system is being invoked manually (via the python script).
90 Next, the `./configure` options serialized in `config.mk` will be
91 parsed and read. That is, if any `./configure` options are passed, they'll be
92 handled naturally. `./configure` should almost never be used for local
93 installations, and is primarily useful for CI. Prefer to customize behavior
96 Finally, rustbuild makes use of the [cc-rs crate] which has [its own
97 method][env-vars] of configuring C compilers and C flags via environment
100 [cc-rs crate]: https://github.com/alexcrichton/cc-rs
101 [env-vars]: https://github.com/alexcrichton/cc-rs#external-configuration-via-environment-variables
105 The rustbuild build system goes through a few phases to actually build the
106 compiler. What actually happens when you invoke rustbuild is:
108 1. The entry point script, `x.py` is run. This script is
109 responsible for downloading the stage0 compiler/Cargo binaries, and it then
110 compiles the build system itself (this folder). Finally, it then invokes the
111 actual `bootstrap` binary build system.
112 2. In Rust, `bootstrap` will slurp up all configuration, perform a number of
113 sanity checks (compilers exist for example), and then start building the
115 3. The stage0 `cargo` downloaded earlier is used to build the standard library
116 and the compiler, and then these binaries are then copied to the `stage1`
117 directory. That compiler is then used to generate the stage1 artifacts which
118 are then copied to the stage2 directory, and then finally the stage2
119 artifacts are generated using that compiler.
121 The goal of each stage is to (a) leverage Cargo as much as possible and failing
122 that (b) leverage Rust as much as possible!
124 ## Incremental builds
126 You can configure rustbuild to use incremental compilation with the
127 `--incremental` flag:
130 $ ./x.py build --incremental
133 The `--incremental` flag will store incremental compilation artifacts
134 in `build/<host>/stage0-incremental`. Note that we only use incremental
135 compilation for the stage0 -> stage1 compilation -- this is because
136 the stage1 compiler is changing, and we don't try to cache and reuse
137 incremental artifacts across different versions of the compiler.
139 You can always drop the `--incremental` to build as normal (but you
140 will still be using the local nightly as your bootstrap).
144 This build system houses all output under the `build` directory, which looks
148 # Root folder of all output. Everything is scoped underneath here
151 # Location where the stage0 compiler downloads are all cached. This directory
152 # only contains the tarballs themselves as they're extracted elsewhere.
159 # Output directory for building this build system itself. The stage0
160 # cargo/rustc are used to build the build system into this location.
165 # Output of the dist-related steps like dist-std, dist-rustc, and dist-docs
168 # Temporary directory used for various input/output as part of various stages
171 # Each remaining directory is scoped by the "host" triple of compilation at
173 x86_64-unknown-linux-gnu/
175 # The build artifacts for the `compiler-rt` library for the target this
176 # folder is under. The exact layout here will likely depend on the platform,
177 # and this is also built with CMake so the build system is also likely
182 # Output folder for LLVM if it is compiled for this target
185 # build folder (e.g. the platform-specific build system). Like with
186 # compiler-rt this is compiled with CMake
189 # Installation of LLVM. Note that we run the equivalent of 'make install'
190 # for LLVM to setup these folders.
197 # Output folder for all documentation of this target. This is what's filled
198 # in whenever the `doc` step is run.
201 # Output for all compiletest-based test suites
208 # Location where the stage0 Cargo and Rust compiler are unpacked. This
209 # directory is purely an extracted and overlaid tarball of these two (done
210 # by the bootstrapy python script). In theory the build system does not
211 # modify anything under this directory afterwards.
214 # These to build directories are the cargo output directories for builds of
215 # the standard library and compiler, respectively. Internally these may also
216 # have other target directories, which represent artifacts being compiled
217 # from the host to the specified target.
219 # Essentially, each of these directories is filled in by one `cargo`
220 # invocation. The build system instruments calling Cargo in the right order
221 # with the right variables to ensure these are filled in correctly.
227 # This is a special case of the above directories, **not** filled in via
228 # Cargo but rather the build system itself. The stage0 compiler already has
229 # a set of target libraries for its own host triple (in its own sysroot)
230 # inside of stage0/. When we run the stage0 compiler to bootstrap more
231 # things, however, we don't want to use any of these libraries (as those are
232 # the ones that we're building). So essentially, when the stage1 compiler is
233 # being compiled (e.g. after libstd has been built), *this* is used as the
234 # sysroot for the stage0 compiler being run.
236 # Basically this directory is just a temporary artifact use to configure the
237 # stage0 compiler to ensure that the libstd we just built is used to
238 # compile the stage1 compiler.
241 # These output directories are intended to be standalone working
242 # implementations of the compiler (corresponding to each stage). The build
243 # system will link (using hard links) output from stageN-{std,rustc} into
244 # each of these directories.
246 # In theory there is no extra build output in these directories.
254 The current build is unfortunately not quite as simple as `cargo build` in a
255 directory, but rather the compiler is split into three different Cargo projects:
257 * `library/std` - the standard library
258 * `library/test` - testing support, depends on libstd
259 * `compiler/rustc` - the actual compiler itself
261 Each "project" has a corresponding Cargo.lock file with all dependencies, and
262 this means that building the compiler involves running Cargo three times. The
263 structure here serves two goals:
265 1. Facilitating dependencies coming from crates.io. These dependencies don't
266 depend on `std`, so libstd is a separate project compiled ahead of time
267 before the actual compiler builds.
268 2. Splitting "host artifacts" from "target artifacts". That is, when building
269 code for an arbitrary target you don't need the entire compiler, but you'll
270 end up needing libraries like libtest that depend on std but also want to use
271 crates.io dependencies. Hence, libtest is split out as its own project that
272 is sequenced after `std` but before `rustc`. This project is built for all
275 There is some loss in build parallelism here because libtest can be compiled in
276 parallel with a number of rustc artifacts, but in theory the loss isn't too bad!
280 We've actually got quite a few tools that we use in the compiler's build system
281 and for testing. To organize these, each tool is a project in `src/tools` with a
282 corresponding `Cargo.toml`. All tools are compiled with Cargo (currently having
283 independent `Cargo.lock` files) and do not currently explicitly depend on the
284 compiler or standard library. Compiling each tool is sequenced after the
285 appropriate libstd/libtest/librustc compile above.
287 ## Extending rustbuild
289 So you'd like to add a feature to the rustbuild build system or just fix a bug.
290 Great! One of the major motivational factors for moving away from `make` is that
291 Rust is in theory much easier to read, modify, and write. If you find anything
292 excessively confusing, please open an issue on this and we'll try to get it
293 documented or simplified pronto.
295 First up, you'll probably want to read over the documentation above as that'll
296 give you a high level overview of what rustbuild is doing. You also probably
297 want to play around a bit yourself by just getting it up and running before you
298 dive too much into the actual build system itself.
300 After that, each module in rustbuild should have enough documentation to keep
301 you up and running. Some general areas that you may be interested in modifying
304 * Adding a new build tool? Take a look at `bootstrap/tool.rs` for examples of
306 * Adding a new compiler crate? Look no further! Adding crates can be done by
307 adding a new directory with `Cargo.toml` followed by configuring all
308 `Cargo.toml` files accordingly.
309 * Adding a new dependency from crates.io? This should just work inside the
310 compiler artifacts stage (everything other than libtest and libstd).
311 * Adding a new configuration option? You'll want to modify `bootstrap/flags.rs`
312 for command line flags and then `bootstrap/config.rs` to copy the flags to the
314 * Adding a sanity check? Take a look at `bootstrap/sanity.rs`.
316 If you make a major change, please remember to:
318 + Update `VERSION` in `src/bootstrap/main.rs`.
319 * Update `changelog-seen = N` in `config.toml.example`.
320 * Add an entry in `src/bootstrap/CHANGELOG.md`.
322 A 'major change' includes
325 * A change in the default options.
327 Changes that do not affect contributors to the compiler or users
328 building rustc from source don't need an update to `VERSION`.
330 If you have any questions feel free to reach out on the `#t-infra` channel in
331 the [Rust Zulip server][rust-zulip] or ask on internals.rust-lang.org. When
332 you encounter bugs, please file issues on the rust-lang/rust issue tracker.
334 [rust-zulip]: https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra