[WIP] Add an Inside Rust blog post for stage 0 std redesign

content/inside-rust/stage0-redesign/index.md

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+path = "inside-rust/9999/12/31/redesigning-stage-0-std"
+title = "Redesigning stage 0 std"
+authors = ["Jieyou Xu"]
+
+[extra]
+team = "Bootstrap"
+team_url = "https://www.rust-lang.org/governance/teams/infra#team-bootstrap"
++++
+
+# Redesigning stage 0 std
+
+## Summary
+
+We are reworking how the stage 0 bootstrap sequence works (the sequence used to build a stage 1 compiler).
+
+- Before: a stage 0 (beta by default) compiler is used to build the in-tree std, which in turn is used to build the stage 1 compiler.
+- After: a stage 0 (beta by default) compiler and a precompiled stage 0 std is instead used to build the stage 1 compiler.
+
+![Difference in stage 0 bootstrap sequence](stage0-redesign-diff.svg)
+
+Notably, this means that after [redesign stage 0 std #119899](https://github.com/rust-lang/rust/pull/119899) PR lands:
+
+- `./x {build,test,check} library --stage 0` becomes no-op, as stage 0 std is no longer the built in-tree std, and the minimum supported stage to build std is now `1`.
+    - Consequently, default (test, check, bench) stage values in the library profile are no longer `0`, but instead defaults to `1`.
+- Some additional `cfg(bootstrap)` usages may be needed in the compiler sources for dogfooding unstable library features.
+
+## Motivation
+
+The previous stage 0 bootstrapping sequence was a source of endless confusion for compiler, library and bootstrap contributors alike, because std had to support being built by *both* a previous beta rustc and in-tree rustc, with `cfg(bootstrap)` in std sources necessary to distinguish between them. By adjusting the stage 0 bootstrap sequence to instead use a precompiled stage 0 std instead of building the in-tree std, we hope to (1) simplify library development workflow to no longer need `cfg(bootstrap)` and (2) enable simplifying some bootstrap logic related to building in-tree std in stage 0.
+
+This was [originally proposed by @jyn514 in the MCP rust-lang/compiler-team#619](https://github.com/rust-lang/compiler-team/issues/619).
+
+## What does this mean for a typical library workflow?
+
+- Crucially, `./x {build,test,check} library --stage 0` becomes no-op and are no longer supported. Building the in-tree std now requires a stage 1 compiler.
+    - Consequently, library contributors are *strongly* encouraged to enable `rust.download-rustc = "if-unchanged"` to avoid having to build a stage 1 compiler. Note that this is the default for `profile = "library"`, but you may need to specify it manually if you don't use a `profile`.
+- `cfg(bootstrap)` should no longer be needed for library sources.
+
+### Caveat: `libtest` changes
+
+See the *Recommended config: have `compiletest` instead depend on precompiled stage 0 libtest* section below.
+
+## What does this mean for a typical compiler workflow?
+
+- If unstable library features are being dogfooded, some additional `cfg(bootstrap)` usages may now be needed in compiler sources.
+
+### Caveat: `libtest` changes
+
+See the *Recommended config: have `compiletest` instead depend on precompiled stage 0 libtest* section below.
+
+## Recommended config: have `compiletest` instead depend on precompiled stage 0 libtest
+
+`compiletest` currently depends on in-tree libtest. For workflows that involve building the compiler, this can cause `compiletest` rebuilds if stage 1 library is rebuilt as a consequence of compiler changes. If you don't intend to change libtest, you can [specify `build.compiletest-use-stage0-libtest = true` to instead have `compiletest` depend on precompiled stage 0 libtest](https://github.com/rust-lang/rust/pull/139386). That way, compiler test iterations can avoid rebuilding `compiletest` unnecessarily. This is already the default unless the `dist` profile is used.
+
+Note that some CI jobs have `build.compiletest-use-stage0-libtest = true` set while others have `build.compiletest-use-stage0-libtest = false`, meaning that libtest programmatic API changes can require adding `cfg(bootstrap)`/`cfg(not(bootstrap))` to `compiletest`'s libtest API use sites. However, in practice we expect this to be very rare.