Fix and clean up the terminology for stages

- Use 'stage0 rustc' to mean `stage0-rustc`
- Use 'bootstrap' to mean `build/$target/stage0` (i.e. the beta compiler)
- Mention that 'ignore-stage1' means 'ignore programs linked to stage1',
not built by it.

Author: jyn514

src/building/bootstrapping.md

@@ -78,6 +78,47 @@ contribution [here][bootstrap-build].
 
 ## Stages of bootstrap
 
+Like most other bootstrapping compilers, `rustc` is compiled in stages.
+_Unlike_ most other compilers, where `stage0` refers to the bootstrap compiler,
+`stage0` refers to the first compiler built by bootstrap. So the following command:
+
+```sh
+x.py build --stage 0 src/rustc
+```
+
+will actually perform a full build of rustc. Confusingly, the `build/$target/stageN` directories are named after the compiler they are *used to build*, not the commands you need to build them.
+
+- **Stage 0:** The stage0 compiler is built by the bootstrap compiler.
+  The  bootstrap compiler is usually (you can configure `x.py` to use
+  something else) the current _beta_ `rustc` compiler and its associated dynamic
+  libraries (which `x.py` will download for you). This bootstrap compiler is then
+  used to compile `rustbuild`, `std`, and `rustc` (plus a few other tools, like `tidy`). When compiling
+  `rustc`, this bootstrap compiler uses the freshly compiled `std`.
+  There are two concepts at play here: a compiler (with its set of dependencies)
+  and its 'target' or 'object' libraries (`std` and `rustc`).
+  Both are staged, but in a staggered manner.
+  The `stage0` standard library is the one _linked_ to `stage0` rustc
+  (allowing you to `use std::vec::Vec` from within the compiler itself),
+  while `stage1 libstd` is the library _built_ by stage1. `libstd` also include `libcore`,
+  so without it there are very few programs that rustc can compile.
+
+- **Stage 1:** In theory, the stage0 compiler is functionally identical to the
+    stage1 compiler, but in practice there are subtle differences. In
+    particular, the stage0 compiler was built by bootstrap and
+    hence not by the source in your working directory: this means that
+    the symbol names used in the compiler source may not match the
+    symbol names that would have been made by the stage1 compiler. This is
+    important when using dynamic linking because Rust does not have ABI compatibility
+    between versions. This primarily manifests when tests try to link with any
+    of the `rustc_*` crates or use the (now deprecated) plugin infrastructure.
+    These tests are marked with `ignore-stage1`. The `stage1` is because
+    these plugins *link* to the stage1 compiler, even though they are being
+    *built* by stage0. Rebuilding again also gives us the benefit of the latest optimizations (i.e. those added since the beta fork).
+
+- _(Optional)_ **Stage 2**: to sanity check our new compiler, we
+  can build the libraries with the stage1 compiler. The result ought
+  to be identical to before, unless something has broken.
+
 This is a detailed look into the separate bootstrap stages. When running
 `x.py` you will see output such as:
 

src/building/how-to-build-and-run.md

@@ -109,38 +109,6 @@ to build it, such as `libstd` and other tooling, may use some unstable features
 internally, requiring a specific version which understands these unstable
 features.
 
-The result is that compiling `rustc` is done in stages:
-
-- **Stage 0:** the stage0 compiler is usually (you can configure `x.py` to use
-  something else) the current _beta_ `rustc` compiler and its associated dynamic
-  libraries (which `x.py` will download for you). This stage0 compiler is then
-  used only to compile `rustbuild`, `std`, and `rustc`. When compiling
-  `rustc`, this stage0 compiler uses the freshly compiled `std`.
-  There are two concepts at play here: a compiler (with its set of dependencies)
-  and its 'target' or 'object' libraries (`std` and `rustc`).
-  Both are staged, but in a staggered manner.
-- **Stage 1:** the code in your clone (for new version) is then
-  compiled with the stage0 compiler to produce the stage1 compiler.
-  However, it was built with an older compiler (stage0), so to
-  optimize the stage1 compiler we go to next the stage.
-  - In theory, the stage1 compiler is functionally identical to the
-    stage2 compiler, but in practice there are subtle differences. In
-    particular, the stage1 compiler itself was built by stage0 and
-    hence not by the source in your working directory: this means that
-    the symbol names used in the compiler source may not match the
-    symbol names that would have been made by the stage1 compiler. This is
-    important when using dynamic linking and the lack of ABI compatibility
-    between versions. This primarily manifests when tests try to link with any
-    of the `rustc_*` crates or use the (now deprecated) plugin infrastructure.
-    These tests are marked with `ignore-stage1`.
-- **Stage 2:** we rebuild our stage1 compiler with itself to produce
-  the stage2 compiler (i.e. it builds itself) to have all the _latest
-  optimizations_. (By default, we copy the stage1 libraries for use by
-  the stage2 compiler, since they ought to be identical.)
-- _(Optional)_ **Stage 3**: to sanity check our new compiler, we
-  can build the libraries with the stage2 compiler. The result ought
-  to be identical to before, unless something has broken.
-
 To read more about the bootstrap process, [read this chapter][bootstrap].
 
 [bootstrap]: ./bootstrapping.md