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update bootstrap guide (rust-lang#1583)
Signed-off-by: ozkanonur <[email protected]>
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‎src/building/bootstrapping.md

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<!-- toc -->
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[*Bootstrapping*][boot] is the process of using a compiler to compile itself.
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More accurately, it means using an older compiler to compile a newer version
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of the same compiler.
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This is exactly how `x.py` works: it downloads the current beta release of
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rustc, then uses it to compile the new compiler.
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Note that this documentation mostly covers user-facing information. See
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[bootstrap/README.md][bootstrap-internals] to read about bootstrap internals.
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[bootstrap-internals]: https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md
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## Stages of bootstrapping
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Compiling `rustc` is done in stages. Here's a diagram, adapted from Joshua Nelson's
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[intrinsics]: ../appendix/glossary.md#intrinsic
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[ocaml-compiler]: https://github.com/rust-lang/rust/tree/ef75860a0a72f79f97216f8aaa5b388d98da6480/src/boot
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## Contributing to bootstrap
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When you use the bootstrap system, you'll call it through `x.py`.
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However, most of the code lives in `src/bootstrap`.
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`bootstrap` has a difficult problem: it is written in Rust, but yet it is run
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before the Rust compiler is built! To work around this, there are two
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components of bootstrap: the main one written in rust, and `bootstrap.py`.
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`bootstrap.py` is what gets run by `x.py`. It takes care of downloading the
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`stage0` compiler, which will then build the bootstrap binary written in
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Rust.
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Because there are two separate codebases behind `x.py`, they need to
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be kept in sync. In particular, both `bootstrap.py` and the bootstrap binary
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parse `config.toml` and read the same command line arguments. `bootstrap.py`
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keeps these in sync by setting various environment variables, and the
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programs sometimes have to add arguments that are explicitly ignored, to be
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read by the other.
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### Adding a setting to config.toml
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This section is a work in progress. In the meantime, you can see an example
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contribution [here][bootstrap-build].
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[bootstrap-build]: https://github.com/rust-lang/rust/pull/71994
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## Understanding stages of bootstrap
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### Overview
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the unlikely case that you actually need to invoke rustc in such a situation,
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you can tell the bootstrap shim to print all env variables by adding `-vvv` to your `x.py` command.
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## Directories and artifacts generated by `bootstrap`
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This is an incomplete reference for the outputs generated by bootstrap:
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| Stage 0 Action | Output |
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| ------------------------------------------------------------------ | -------------------------------------------- |
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| `beta` extracted | `build/HOST/stage0` |
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| `stage0` builds `bootstrap` | `build/bootstrap` |
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| `stage0` builds `test`/`std` | `build/HOST/stage0-std/TARGET` |
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| copy `stage0-std` (HOST only) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
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| `stage0` builds `rustc` with `stage0-sysroot` | `build/HOST/stage0-rustc/HOST` |
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| copy `stage0-rustc` (except executable) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
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| build `llvm` | `build/HOST/llvm` |
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| `stage0` builds `codegen` with `stage0-sysroot` | `build/HOST/stage0-codegen/HOST` |
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| `stage0` builds `rustdoc`, `clippy`, `miri`, with `stage0-sysroot` | `build/HOST/stage0-tools/HOST` |
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`--stage=0` stops here.
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| Stage 1 Action | Output |
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|-----------------------------------------------------|---------------------------------------|
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| copy (uplift) `stage0-rustc` executable to `stage1` | `build/HOST/stage1/bin` |
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| copy (uplift) `stage0-codegen` to `stage1` | `build/HOST/stage1/lib` |
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| copy (uplift) `stage0-sysroot` to `stage1` | `build/HOST/stage1/lib` |
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| `stage1` builds `test`/`std` | `build/HOST/stage1-std/TARGET` |
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| copy `stage1-std` (HOST only) | `build/HOST/stage1/lib/rustlib/HOST` |
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| `stage1` builds `rustc` | `build/HOST/stage1-rustc/HOST` |
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| copy `stage1-rustc` (except executable) | `build/HOST/stage1/lib/rustlib/HOST` |
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| `stage1` builds `codegen` | `build/HOST/stage1-codegen/HOST` |
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`--stage=1` stops here.
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| Stage 2 Action | Output |
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|--------------------------------------------------------|-----------------------------------------------------------------|
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| copy (uplift) `stage1-rustc` executable | `build/HOST/stage2/bin` |
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| copy (uplift) `stage1-sysroot` | `build/HOST/stage2/lib and build/HOST/stage2/lib/rustlib/HOST` |
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| `stage2` builds `test`/`std` (not HOST targets) | `build/HOST/stage2-std/TARGET` |
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| copy `stage2-std` (not HOST targets) | `build/HOST/stage2/lib/rustlib/TARGET` |
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| `stage2` builds `rustdoc`, `clippy`, `miri` | `build/HOST/stage2-tools/HOST` |
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| copy `rustdoc` | `build/HOST/stage2/bin` |
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`--stage=2` stops here.
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Finally, bootstrap makes use of the [cc-rs crate] which has [its own
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method][env-vars] of configuring C compilers and C flags via environment
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variables.
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[cc-rs crate]: https://github.com/rust-lang/cc-rs
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[env-vars]: https://github.com/rust-lang/cc-rs#external-configuration-via-environment-variables
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## Clarification of build command's stdout
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