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Aug 30, 2020
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Add post about changes to x.py defaults #666

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e8f47da
Add x.py post
jyn514 Aug 12, 2020
bbe45ab
rustc is a program, so it should be rendered as code
jyn514 Aug 12, 2020
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Improve wording
jyn514 Aug 13, 2020
9b770da
Mention what `build` does, not just what it doesn't do
jyn514 Aug 13, 2020
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Add a short rationale for the changes
jyn514 Aug 13, 2020
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Improve wording
jyn514 Aug 26, 2020
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Highlight `x.py` as code
jyn514 Aug 26, 2020
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Improve wording
jyn514 Aug 26, 2020
29eef78
libstd was built three times, not rustc
jyn514 Aug 30, 2020
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Update the filename with the current date
jyn514 Aug 30, 2020
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'stage 2 libstd' is just a copy of stage 1
jyn514 Aug 30, 2020
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47 changes: 47 additions & 0 deletions posts/inside-rust/2020-08-12-changes-to-x-py-defaults.md
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---
layout: post
title: "Changes to x.py defaults"
author: Joshua Nelson
team: the compiler team <https://www.rust-lang.org/governance/teams/compiler>
---

Recently, the defaults for [`x.py`], the tool used to [bootstrap] the Rust compiler from source, changed. If you regularly contribute to Rust, this might affect your workflow.

## What changes were made?

- The default stage is now dependent on the subcommand:
+ `dist`: stage 2
+ `build`: stage 1
+ `test`: stage 1
+ `doc`: stage 0

- stage1 `rustc` artifacts are no longer built by `x.py build --stage 1`. To get the old behavior back, use `x.py build --stage 1 src/rustc`. The new behavior for `build --stage 1` builds everything except `rustc`, which includes the standard library, `rustdoc`, and various other tools (if the tools are enabled).

- `debuginfo` now defaults to `1` when `debug = true`. Previously, the default was 2.

## Why were the changes made?

Previously, `x.py build` would build `rustc` three separate times:

1. `build/stage0-rustc`
2. `build/stage1-rustc`
3. `build/stage2-rustc`
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This seems false - we only built 2 times, and uplifted stage 1 to stage 2 without full bootstrap being on, IIRC.

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Ah hmm I'm confused then - maybe libstd was being built three times? --stage 2 means that something was built 3 times, because libstd is built during all of stage 0, 1, and 2.

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@RalfJung RalfJung Aug 30, 2020 •
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As far as I know, the stage 2 libstd "build" just creates a copy of the one built for stage 1. No compilation happens.

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Well my changes seem much less impressive when you put it that way 😆 Updated to only say stage1 rustc/libstd


Normally, contributors only want to build the compiler once, which lets them test their changes quickly. After this change, that's now the case:

1. `build/stage0-rustc`

`debuginfo = 2` generates several gigabytes of debug information,
making the previous default settings for `debug = true` very painful.

For a detailed rationale of the changes, as well as more information about the alternatives considered, see

- [the MCP]
- the [implementation PR]
- the [Zulip stream]

[`x.py`]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html#what-is-xpy
[bootstrap]: https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html
[the MCP]: https://github.com/rust-lang/compiler-team/issues/326
[implementation PR]: https://github.com/rust-lang/rust/pull/73964
[Zulip stream]: https://rust-lang.zulipchat.com/#narrow/stream/233931-xxx/topic/Use.20sane.20defaults.20in.20x.py.20compiler-team.23326