Add post about changes to x.py defaults

posts/inside-rust/2020-08-30-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
+
+- stage 1 `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` twice:
+
+1. `build/stage0-std`
+2. `build/stage0-rustc`
+3. `build/stage1-std`
+4. `build/stage1-rustc`
+
+Normally, contributors only want to build the compiler once, which lets them test their changes quickly. After this change, that's now the default:
+
+1. `build/stage0-std`
+2. `build/stage0-rustc`
+3. `build/stage1-std`
+
+`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