[WIP] Address review feedback (round 3)

Author: jieyouxu

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

@@ -25,32 +25,42 @@ Notably, this means that after [redesign stage 0 std #119899](https://github.com
     - 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
+## Comparison of common invocations
 
-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.
+For `profile = "library"`:
 
-This was [originally proposed by @jyn514 in the MCP rust-lang/compiler-team#619](https://github.com/rust-lang/compiler-team/issues/619).
+| Invocation                                 | Before stage 0 std redesign                                 | After stage 0 std redesign                               |
+|--------------------------------------------|-------------------------------------------------------------|----------------------------------------------------------|
+| `./x {check,build,test} library`           | Checks/builds/tests in-tree library                         | Check/build/tests stage 1 library                        |
+| `./x {check,build,test} library --stage 0` | Checks/builds/tests in-tree library                         | No-op in case of build, checks/tests precompiled library |
+| `./x {check,build,test} library --stage 1` | Builds in-tree library, checks/builds/tests stage 1 library | Check/build/tests stage 1 library                        |
+
+For `profile = "compiler"`:
+
+| Invocation                                 | Before stage 0 std redesign                                 | After stage 0 std redesign                               |
+|--------------------------------------------|-------------------------------------------------------------|----------------------------------------------------------|
+| `./x check library`                        | Checks in-tree library                                      | Check/build/tests stage 1 library                        |
+| `./x {build,test} library`                 | Builds in-tree library, builds/tests stage 1 library        | Check/build/tests stage 1 library                        |
+| `./x {check,build,test} library --stage 0` | Checks/builds/tests in-tree library                         | No-op in case of build, checks/tests precompiled library |
+| `./x {check,build,test} library --stage 1` | Builds in-tree library, checks/builds/tests stage 1 library | Check/build/tests stage 1 library                        |
+
+For `profile = "tools"`, by default not affected if `download-rustc` is enabled.
 
 ## 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
+## Why are we making this change?
 
-See the *Recommended config: have `compiletest` instead depend on precompiled stage 0 libtest* section below.
+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:
 
-## Recommended config: have `compiletest` instead depend on precompiled stage 0 libtest
+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.
 
-`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.
+This was [originally proposed by @jyn514 in the MCP rust-lang/compiler-team#619](https://github.com/rust-lang/compiler-team/issues/619).