# How to Build and Run the Compiler The compiler is built using a tool called `x.py`. You will need to have Python installed to run it. But before we get to that, if you're going to be hacking on `rustc`, you'll want to tweak the configuration of the compiler. The default configuration is oriented towards running the compiler as a user, not a developer. ### Create a config.toml To start, copy [`config.toml.example`] to `config.toml`: [`config.toml.example`]: https://github.com/rust-lang/rust/blob/master/config.toml.example ```bash > cd $RUST_CHECKOUT > cp config.toml.example config.toml ``` Then you will want to open up the file and change the following settings (and possibly others, such as `llvm.ccache`): ```toml [llvm] # Enables LLVM assertions, which will check that the LLVM bitcode generated # by the compiler is internally consistent. These are particularly helpful # if you edit `codegen`. assertions = true [rust] # This will make your build more parallel; it costs a bit of runtime # performance perhaps (less inlining) but it's worth it. codegen-units = 0 # This enables full debuginfo and debug assertions. The line debuginfo is also # enabled by `debuginfo-level = 1`. Full debuginfo is also enabled by # `debuginfo-level = 2`. Debug assertions can also be enabled with # `debug-assertions = true`. Note that `debug = true` will make your build # slower, so you may want to try individually enabling debuginfo and assertions # or enable only line debuginfo which is basically free. debug = true ``` If you have already built `rustc`, then you may have to execute `rm -rf build` for subsequent configuration changes to take effect. Note that `./x.py clean` will not cause a rebuild of LLVM, so if your configuration change affects LLVM, you will need to manually `rm -rf build/` before rebuilding. ### What is `x.py`? `x.py` is the script used to orchestrate the tooling in the `rustc` repository. It is the script that can build docs, run tests, and compile `rustc`. It is the now preferred way to build `rustc` and it replaces the old makefiles from before. Below are the different ways to utilize `x.py` in order to effectively deal with the repo for various common tasks. ### Running `x.py` and building a stage1 compiler One thing to keep in mind is that `rustc` is a _bootstrapping_ compiler. That is, since `rustc` is written in Rust, we need to use an older version of the compiler to compile the newer version. In particular, the newer version of the compiler and some of the artifacts needed to build it, such as `libstd` and other tooling, may use some unstable features internally, requiring a specific version which understands these unstable features. The result is that compiling `rustc` is done in stages: - **Stage 0:** the stage0 compiler is usually (you can configure `x.py` to use something else) the current _beta_ `rustc` compiler and its associated dynamic libraries (which `x.py` will download for you). This stage0 compiler is then used only to compile `rustbuild`, `std`, `test`, and `rustc`. When compiling `test` and `rustc`, this stage0 compiler uses the freshly compiled `std`. There are two concepts at play here: a compiler (with its set of dependencies) and its 'target' or 'object' libraries (`std`, `test`, and `rustc`). Both are staged, but in a staggered manner. - **Stage 1:** the code in your clone (for new version) is then compiled with the stage0 compiler to produce the stage1 compiler. However, it was built with an older compiler (stage0), so to optimize the stage1 compiler we go to next the stage. - In theory, the stage1 compiler is functionally identical to the stage2 compiler, but in practice there are subtle differences. In particular, the stage1 compiler itself was built by stage0 and hence not by the source in your working directory: this means that the symbol names used in the compiler source may not match the symbol names that would have been made by the stage1 compiler. This can be important when using dynamic linking (e.g., with derives. Sometimes this means that some tests don't work when run with stage1. - **Stage 2:** we rebuild our stage1 compiler with itself to produce the stage2 compiler (i.e. it builds itself) to have all the _latest optimizations_. (By default, we copy the stage1 libraries for use by the stage2 compiler, since they ought to be identical.) - _(Optional)_ **Stage 3**: to sanity check our new compiler, we can build the libraries with the stage2 compiler. The result ought to be identical to before, unless something has broken. #### A note on stage meanings When running `x.py` you will see output such as: ```txt Building stage0 std artifacts Copying stage0 std from stage0 Building stage0 test artifacts Copying stage0 test from stage0 Building stage0 compiler artifacts Copying stage0 rustc from stage0 Building LLVM for x86_64-apple-darwin Building stage0 codegen artifacts Assembling stage1 compiler Building stage1 std artifacts Copying stage1 std from stage1 Building stage1 test artifacts Copying stage1 test from stage1 Building stage1 compiler artifacts Copying stage1 rustc from stage1 Building stage1 codegen artifacts Assembling stage2 compiler Uplifting stage1 std Copying stage2 std from stage1 Generating unstable book md files Building stage0 tool unstable-book-gen Building stage0 tool rustbook Documenting standalone Building rustdoc for stage2 Documenting book redirect pages Documenting stage2 std Building rustdoc for stage1 Documenting stage2 test Documenting stage2 whitelisted compiler Documenting stage2 compiler Documenting stage2 rustdoc Documenting error index Uplifting stage1 test Copying stage2 test from stage1 Uplifting stage1 rustc Copying stage2 rustc from stage1 Building stage2 tool error_index_generator ``` A deeper look into `x.py`'s phases can be seen here: A diagram of the rustc compilation phases Keep in mind this diagram is a simplification, i.e. `rustdoc` can be built at different stages, the process is a bit different when passing flags such as `--keep-stage`, or if there are non-host targets. The following tables indicate the outputs of various stage actions: | Stage 0 Action | Output | |-----------------------------------------------------------|----------------------------------------------| | `beta` extracted | `build/HOST/stage0` | | `stage0` builds `bootstrap` | `build/bootstrap` | | `stage0` builds `libstd` | `build/HOST/stage0-std/TARGET` | | copy `stage0-std` (HOST only) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` | | `stage0` builds `libtest` with `stage0-sysroot` | `build/HOST/stage0-test/TARGET` | | copy `stage0-test` (HOST only) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` | | `stage0` builds `rustc` with `stage0-sysroot` | `build/HOST/stage0-rustc/HOST` | | copy `stage0-rustc (except executable)` | `build/HOST/stage0-sysroot/lib/rustlib/HOST` | | build `llvm` | `build/HOST/llvm` | | `stage0` builds `codegen` with `stage0-sysroot` | `build/HOST/stage0-codegen/HOST` | | `stage0` builds `rustdoc` with `stage0-sysroot` | `build/HOST/stage0-tools/HOST` | `--stage=0` stops here. | Stage 1 Action | Output | |-----------------------------------------------------|---------------------------------------| | copy (uplift) `stage0-rustc` executable to `stage1` | `build/HOST/stage1/bin` | | copy (uplift) `stage0-codegen` to `stage1` | `build/HOST/stage1/lib` | | copy (uplift) `stage0-sysroot` to `stage1` | `build/HOST/stage1/lib` | | `stage1` builds `libstd` | `build/HOST/stage1-std/TARGET` | | copy `stage1-std` (HOST only) | `build/HOST/stage1/lib/rustlib/HOST` | | `stage1` builds `libtest` | `build/HOST/stage1-test/TARGET` | | copy `stage1-test` (HOST only) | `build/HOST/stage1/lib/rustlib/HOST` | | `stage1` builds `rustc` | `build/HOST/stage1-rustc/HOST` | | copy `stage1-rustc` (except executable) | `build/HOST/stage1/lib/rustlib/HOST` | | `stage1` builds `codegen` | `build/HOST/stage1-codegen/HOST` | `--stage=1` stops here. | Stage 2 Action | Output | |-------------------------------------------|-----------------------------------------------------------------| | copy (uplift) `stage1-rustc` executable | `build/HOST/stage2/bin` | | copy (uplift) `stage1-sysroot` | `build/HOST/stage2/lib and build/HOST/stage2/lib/rustlib/HOST` | | `stage2` builds `libstd` (except HOST?) | `build/HOST/stage2-std/TARGET` | | copy `stage2-std` (not HOST targets) | `build/HOST/stage2/lib/rustlib/TARGET` | | `stage2` builds `libtest` (except HOST?) | `build/HOST/stage2-test/TARGET` | | copy `stage2-test` (not HOST targets) | `build/HOST/stage2/lib/rustlib/TARGET` | | `stage2` builds `rustdoc` | `build/HOST/stage2-tools/HOST` | | copy `rustdoc` | `build/HOST/stage2/bin` | `--stage=2` stops here. Note that the convention `x.py` uses is that: - A "stage N artifact" is an artifact that is _produced_ by the stage N compiler. - The "stage (N+1) compiler" is assembled from "stage N artifacts". - A `--stage N` flag means build _with_ stage N. In short, _stage 0 uses the stage0 compiler to create stage0 artifacts which will later be uplifted to stage1_. Every time any of the main artifacts (`std`, `test`, `rustc`) are compiled, two steps are performed. When `std` is compiled by a stage N compiler, that `std` will be linked to programs built by the stage N compiler (including test and `rustc` built later on). It will also be used by the stage (N+1) compiler to link against itself. This is somewhat intuitive if one thinks of the stage (N+1) compiler as "just" another program we are building with the stage N compiler. In some ways, `rustc` (the binary, not the `rustbuild` step) could be thought of as one of the few `no_core` binaries out there. So "stage0 std artifacts" are in fact the output of the downloaded stage0 compiler, and are going to be used for anything built by the stage0 compiler: e.g. `rustc`, `test` artifacts. When it announces that it is "building stage1 std artifacts" it has moved on to the next bootstrapping phase. This pattern continues in latter stages. Also note that building host `std` and target `std` are different based on the stage (e.g. see in the table how stage2 only builds non-host `std` targets. This is because during stage2, the host `std` is uplifted from the "stage 1" `std` -- specifically, when "Building stage 1 artifacts" is announced, it is later copied into stage2 as well (both the compiler's `libdir` and the `sysroot`). This `std` is pretty much necessary for any useful work with the compiler. Specifically, it's used as the `std` for programs compiled by the newly compiled compiler (so when you compile `fn main() { }` it is linked to the last `std` compiled with `x.py build --stage 1 src/libstd`). The `rustc` generated by the stage0 compiler is linked to the freshly-built `libstd`, which means that for the most part only `std` needs to be cfg-gated, so that `rustc` can use featured added to std immediately after their addition, without need for them to get into the downloaded beta. The `libstd` built by the `stage1/bin/rustc` compiler, also known as "stage1 std artifacts", is not necessarily ABI-compatible with that compiler. That is, the `rustc` binary most likely could not use this `std` itself. It is however ABI-compatible with any programs that the `stage1/bin/rustc` binary builds (including itself), so in that sense they're paired. This is also where `--keep-stage 1 src/libstd` comes into play. Since most changes to the compiler don't actually change the ABI, once you've produced a `libstd` in stage 1, you can probably just reuse it with a different compiler. If the ABI hasn't changed, you're good to go, no need to spend the time recompiling that `std`. `--keep-stage` simply assumes the previous compile is fine and copies those artifacts into the appropriate place, skipping the cargo invocation. The reason we first build `std`, then `test`, then `rustc`, is largely just because we want to minimize `cfg(stage0)` in the code for `rustc`. Currently `rustc` is always linked against a "new" `std`/`test` so it doesn't ever need to be concerned with differences in std; it can assume that the std is as fresh as possible. The reason we need to build it twice is because of ABI compatibility. The beta compiler has it's own ABI, and then the `stage1/bin/rustc` compiler will produce programs/libraries with the new ABI. We used to build three times, but because we assume that the ABI is constant within a codebase, we presume that the libraries produced by the "stage2" compiler (produced by the `stage1/bin/rustc` compiler) is ABI-compatible with the `stage1/bin/rustc` compiler's produced libraries. What this means is that we can skip that final compilation -- and simply use the same libraries as the `stage2/bin/rustc` compiler uses itself for programs it links against. This `stage2/bin/rustc` compiler is shipped to end-users, along with the `stage 1 {std,test,rustc}` artifacts. If you want to learn more about `x.py`, read its README.md [here](https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md). #### Build Flags There are other flags you can pass to the build command of `x.py` that can be beneficial to cutting down compile times or fitting other things you might need to change. They are: ```txt Options: -v, --verbose use verbose output (-vv for very verbose) -i, --incremental use incremental compilation --config FILE TOML configuration file for build --build BUILD build target of the stage0 compiler --host HOST host targets to build --target TARGET target targets to build --on-fail CMD command to run on failure --stage N stage to build --keep-stage N stage to keep without recompiling --src DIR path to the root of the rust checkout -j, --jobs JOBS number of jobs to run in parallel -h, --help print this help message ``` For hacking, often building the stage 1 compiler is enough, but for final testing and release, the stage 2 compiler is used. `./x.py check` is really fast to build the rust compiler. It is, in particular, very useful when you're doing some kind of "type-based refactoring", like renaming a method, or changing the signature of some function. Once you've created a config.toml, you are now ready to run `x.py`. There are a lot of options here, but let's start with what is probably the best "go to" command for building a local rust: ```bash > ./x.py build -i --stage 1 src/libstd ``` This may *look* like it only builds libstd, but that is not the case. What this command does is the following: - Build `libstd` using the stage0 compiler (using incremental) - Build `librustc` using the stage0 compiler (using incremental) - This produces the stage1 compiler - Build libstd using the stage1 compiler (cannot use incremental) This final product (stage1 compiler + libs built using that compiler) is what you need to build other rust programs (unless you use `#![no_std]` or `#![no_core]`). The command includes the `-i` switch which enables incremental compilation. This will be used to speed up the first two steps of the process: in particular, if you make a small change, we ought to be able to use your old results to make producing the stage1 **compiler** faster. Unfortunately, incremental cannot be used to speed up making the stage1 libraries. This is because incremental only works when you run the *same compiler* twice in a row. In this case, we are building a *new stage1 compiler* every time. Therefore, the old incremental results may not apply. **As a result, you will probably find that building the stage1 `libstd` is a bottleneck for you** -- but fear not, there is a (hacky) workaround. See [the section on "recommended workflows"](#workflow) below. Note that this whole command just gives you a subset of the full `rustc` build. The **full** `rustc` build (what you get if you just say `./x.py build`) has quite a few more steps: - Build `librustc` and `rustc` with the stage1 compiler. - The resulting compiler here is called the "stage2" compiler. - Build libstd with stage2 compiler. - Build librustdoc and a bunch of other things with the stage2 compiler. ### Build specific components Build only the libcore library ```bash > ./x.py build src/libcore ``` Build the libcore and libproc_macro library only ```bash > ./x.py build src/libcore src/libproc_macro ``` Build only libcore up to Stage 1 ```bash > ./x.py build src/libcore --stage 1 ``` Sometimes you might just want to test if the part you’re working on can compile. Using these commands you can test that it compiles before doing a bigger build to make sure it works with the compiler. As shown before you can also pass flags at the end such as --stage. ### Creating a rustup toolchain Once you have successfully built `rustc`, you will have created a bunch of files in your `build` directory. In order to actually run the resulting `rustc`, we recommend creating rustup toolchains. The first one will run the stage1 compiler (which we built above). The second will execute the stage2 compiler (which we did not build, but which you will likely need to build at some point; for example, if you want to run the entire test suite). ```bash > rustup toolchain link stage1 build//stage1 > rustup toolchain link stage2 build//stage2 ``` The `` would typically be one of the following: - Linux: `x86_64-unknown-linux-gnu` - Mac: `x86_64-apple-darwin` - Windows: `x86_64-pc-windows-msvc` Now you can run the `rustc` you built with. If you run with `-vV`, you should see a version number ending in `-dev`, indicating a build from your local environment: ```bash > rustc +stage1 -vV rustc 1.25.0-dev binary: rustc commit-hash: unknown commit-date: unknown host: x86_64-unknown-linux-gnu release: 1.25.0-dev LLVM version: 4.0 ``` ### Suggested workflows for faster builds of the compiler There are two workflows that are useful for faster builds of the compiler. **Check, check, and check again.** The first workflow, which is useful when doing simple refactorings, is to run `./x.py check` continuously. Here you are just checking that the compiler can **build**, but often that is all you need (e.g., when renaming a method). You can then run `./x.py build` when you actually need to run tests. In fact, it is sometimes useful to put off tests even when you are not 100% sure the code will work. You can then keep building up refactoring commits and only run the tests at some later time. You can then use `git bisect` to track down **precisely** which commit caused the problem. A nice side-effect of this style is that you are left with a fairly fine-grained set of commits at the end, all of which build and pass tests. This often helps reviewing. **Incremental builds with `--keep-stage`.** Sometimes just checking whether the compiler builds is not enough. A common example is that you need to add a `debug!` statement to inspect the value of some state or better understand the problem. In that case, you really need a full build. By leveraging incremental, though, you can often get these builds to complete very fast (e.g., around 30 seconds). The only catch is this requires a bit of fudging and may produce compilers that don't work (but that is easily detected and fixed). The sequence of commands you want is as follows: - Initial build: `./x.py build -i --stage 1 src/libstd` - As [documented above](#command), this will build a functional stage1 compiler as part of running all stage0 commands (which include building a `libstd` compatible with the stage1 compiler) as well as the first few steps of the "stage 1 actions" up to "stage1 (sysroot stage1) builds libstd". - Subsequent builds: `./x.py build -i --stage 1 src/libstd --keep-stage 1` - Note that we added the `--keep-stage 1` flag here As mentioned, the effect of `--keep-stage 1` is that we just *assume* that the old standard library can be re-used. If you are editing the compiler, this is almost always true: you haven't changed the standard library, after all. But sometimes, it's not true: for example, if you are editing the "metadata" part of the compiler, which controls how the compiler encodes types and other states into the `rlib` files, or if you are editing things that wind up in the metadata (such as the definition of the MIR). **The TL;DR is that you might get weird behavior from a compile when using `--keep-stage 1`** -- for example, strange [ICEs](appendix/glossary.html) or other panics. In that case, you should simply remove the `--keep-stage 1` from the command and rebuild. That ought to fix the problem. You can also use `--keep-stage 1` when running tests. Something like this: - Initial test run: `./x.py test -i --stage 1 src/test/ui` - Subsequent test run: `./x.py test -i --stage 1 src/test/ui --keep-stage 1` ### Building with system LLVM By default, LLVM is built from source, and that can take significant amount of time. An alternative is to use LLVM already installed on your computer. This is specified in the `target` section of `config.toml`: ```toml [target.x86_64-unknown-linux-gnu] llvm-config = "/path/to/llvm/llvm-7.0.1/bin/llvm-config" ``` ### Other `x.py` commands Here are a few other useful `x.py` commands. We'll cover some of them in detail in other sections: - Building things: - `./x.py clean` – clean up the build directory (`rm -rf build` works too, but then you have to rebuild LLVM) - `./x.py build --stage 1` – builds everything using the stage 1 compiler, not just up to libstd - `./x.py build` – builds the stage2 compiler - Running tests (see the [section on running tests](./tests/running.html) for more details): - `./x.py test --stage 1 src/libstd` – runs the `#[test]` tests from libstd - `./x.py test --stage 1 src/test/ui` – runs the `ui` test suite - `./x.py test --stage 1 src/test/ui/const-generics` - runs all the tests in the `const-generics/` subdirectory of the `ui` test suite - `./x.py test --stage 1 src/test/ui/const-generics/const-types.rs` - runs the single test `const-types.rs` from the `ui` test suite ### ctags One of the challenges with rustc is that the RLS can't handle it, since it's a bootstrapping compiler. This makes code navigation difficult. One solution is to use `ctags`. `ctags` has a long history and several variants. Exuberant Ctags seems to be quite commonly distributed but it does not have out-of-box Rust support. Some distributions seem to use [Universal Ctags][utags], which is a maintained fork and does have built-in Rust support. The following script can be used to set up Exuberant Ctags: [https://github.com/nikomatsakis/rust-etags][etags]. `ctags` integrates into emacs and vim quite easily. The following can then be used to build and generate tags: ```console $ rust-ctags src/lib* && ./x.py build ``` This allows you to do "jump-to-def" with whatever functions were around when you last built, which is ridiculously useful. [etags]: https://github.com/nikomatsakis/rust-etags [utags]: https://github.com/universal-ctags/ctags ### Cleaning out build directories Sometimes you need to start fresh, but this is normally not the case. If you need to run this then rustbuild is most likely not acting right and you should file a bug as to what is going wrong. If you do need to clean everything up then you only need to run one command! ```bash > ./x.py clean ``` ### Compiler Documentation The documentation for the rust components are found at [rustc doc]. [rustc doc]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/