+
+Keep in mind this diagram is a simplification, i.e. `rustdoc` can be built at
+different stages, the process is a bit different when passing flags such as
+`--keep-stage`, or if there are non-host targets.
+
+The following tables indicate the outputs of various stage actions:
+
+| Stage 0 Action | Output |
+|-----------------------------------------------------------|----------------------------------------------|
+| `beta` extracted | `build/HOST/stage0` |
+| `stage0` builds `bootstrap` | `build/bootstrap` |
+| `stage0` builds `libstd` | `build/HOST/stage0-std/TARGET` |
+| copy `stage0-std` (HOST only) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
+| `stage0` builds `libtest` with `stage0-sysroot` | `build/HOST/stage0-test/TARGET` |
+| copy `stage0-test` (HOST only) | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
+| `stage0` builds `rustc` with `stage0-sysroot` | `build/HOST/stage0-rustc/HOST` |
+| copy `stage0-rustc (except executable)` | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
+| build `llvm` | `build/HOST/llvm` |
+| `stage0` builds `codegen` with `stage0-sysroot` | `build/HOST/stage0-codgen/HOST` |
+| `stage0` builds `rustdoc` with `stage0-sysroot` | `build/HOST/stage0-tools/HOST` |
+
+`--stage=0` stops here.
+
+| Stage 1 Action | Output |
+|-----------------------------------------------------|---------------------------------------|
+| copy (uplift) `stage0-rustc` executable to `stage1` | `build/HOST/stage1/bin` |
+| copy (uplift) `stage0-codegen` to `stage1` | `build/HOST/stage1/lib` |
+| copy (uplift) `stage0-sysroot` to `stage1` | `build/HOST/stage1/lib` |
+| `stage1` builds `libstd` | `build/HOST/stage1-std/TARGET` |
+| copy `stage1-std` (HOST only) | `build/HOST/stage1/lib/rustlib/HOST` |
+| `stage1` builds `libtest` | `build/HOST/stage1-test/TARGET` |
+| copy `stage1-test` (HOST only) | `build/HOST/stage1/lib/rustlib/HOST` |
+| `stage1` builds `rustc` | `build/HOST/stage1-rustc/HOST` |
+| copy `stage1-rustc` (except executable) | `build/HOST/stage1/lib/rustlib/HOST` |
+| `stage1` builds `codegen` | `build/HOST/stage1-codegen/HOST` |
+
+`--stage=1` stops here.
+
+| Stage 2 Action | Output |
+|-------------------------------------------|-----------------------------------------------------------------|
+| copy (uplift) `stage1-rustc` executable | `build/HOST/stage2/bin` |
+| copy (uplift) `stage1-sysroot` | `build/HOST/stage2/lib and build/HOST/stage2/lib/rustlib/HOST` |
+| `stage2` builds `libstd` (except HOST?) | `build/HOST/stage2-std/TARGET` |
+| copy `stage2-std` (not HOST targets) | `build/HOST/stage2/lib/rustlib/TARGET` |
+| `stage2` builds `libtest` (except HOST?) | `build/HOST/stage2-test/TARGET` |
+| copy `stage2-test` (not HOST targets) | `build/HOST/stage2/lib/rustlib/TARGET` |
+| `stage2` builds `rustdoc` | `build/HOST/stage2-tools/HOST` |
+| copy `rustdoc` | `build/HOST/stage2/bin` |
+
+`--stage=2` stops here.
+
+Note that the convention `x.py` uses is that:
+- A "stage N artifact" is an artifact that is _produced_ by the stage N compiler.
+- The "stage (N+1) compiler" is assembled from "stage N artifacts".
+- A `--stage N` flag means build _with_ stage N.
+
+In short, _stage 0 uses the stage0 compiler to create stage0 artifacts which
+will later be uplifted to stage1_.
+
+Every time any of the main artifacts (`std`, `test`, `rustc`) are compiled, two
+steps are performed.
+When `std` is compiled by a stage N compiler, that `std` will be linked to
+programs built by the stage N compiler (including test and `rustc` built later
+on). It will also be used by the stage (N+1) compiler to link against itself.
+This is somewhat intuitive if one thinks of the stage (N+1) compiler as "just"
+another program we are building with the stage N compiler. In some ways, `rustc`
+(the binary, not the `rustbuild` step) could be thought of as one of the few
+`no_core` binaries out there.
+
+So "stage0 std artifacts" are in fact the output of the downloaded stage0
+compiler, and are going to be used for anything built by the stage0 compiler:
+e.g. `rustc`, `test` artifacts. When it announces that it is "building stage1
+std artifacts" it has moved on to the next bootstrapping phase. This pattern
+continues in latter stages.
+
+Also note that building host `std` and target `std` are different based on the
+stage (e.g. see in the table how stage2 only builds non-host `std` targets.
+This is because during stage2, the host `std` is uplifted from the "stage 1"
+`std` -- specifically, when "Building stage 1 artifacts" is announced, it is
+later copied into stage2 as well (both the compiler's `libdir` and the
+`sysroot`).
+
+This `std` is pretty much necessary for any useful work with the compiler.
+Specifically, it's used as the `std` for programs compiled by the newly compiled
+compiler (so when you compile `fn main() { }` it is linked to the last `std`
+compiled with `x.py build --stage 1 src/libstd`).
+
+The `rustc` generated by the stage0 compiler is linked to the freshly-built
+`libstd`, which means that for the most part only `std` needs to be cfg-gated,
+so that `rustc` can use featured added to std immediately after their addition,
+without need for them to get into the downloaded beta. The `libstd` built by the
+`stage1/bin/rustc` compiler, also known as "stage1 std artifacts", is not
+necessarily ABI-compatible with that compiler.
+That is, the `rustc` binary most likely could not use this `std` itself.
+It is however ABI-compatible with any programs that the `stage1/bin/rustc`
+binary builds (including itself), so in that sense they're paired.
+
+This is also where `--keep-stage 1 src/libstd` comes into play. Since most
+changes to the compiler don't actually change the ABI, once you've produced a
+`libstd` in stage 1, you can probably just reuse it with a different compiler.
+If the ABI hasn't changed, you're good to go, no need to spend the time
+recompiling that `std`.
+`--keep-stage` simply assumes the previous compile is fine and copies those
+artifacts into the appropriate place, skipping the cargo invocation.
+
+The reason we first build `std`, then `test`, then `rustc`, is largely just
+because we want to minimize `cfg(stage0)` in the code for `rustc`.
+Currently `rustc` is always linked against a "new" `std`/`test` so it doesn't
+ever need to be concerned with differences in std; it can assume that the std is
+as fresh as possible.
+
+The reason we need to build it twice is because of ABI compatibility.
+The beta compiler has it's own ABI, and then the `stage1/bin/rustc` compiler
+will produce programs/libraries with the new ABI.
+We used to build three times, but because we assume that the ABI is constant
+within a codebase, we presume that the libraries produced by the "stage2"
+compiler (produced by the `stage1/bin/rustc` compiler) is ABI-compatible with
+the `stage1/bin/rustc` compiler's produced libraries.
+What this means is that we can skip that final compilation -- and simply use the
+same libraries as the `stage2/bin/rustc` compiler uses itself for programs it
+links against.
+
+This `stage2/bin/rustc` compiler is shipped to end-users, along with the
+`stage 1 {std,test,rustc}` artifacts.
+
+If you want to learn more about `x.py`, read its README.md
+[here](https://github.com/rust-lang/rust/blob/master/src/bootstrap/README.md).
+
#### Build Flags
-There are other flags you can pass to the build portion of x.py that can be
+There are other flags you can pass to the build command of `x.py` that can be
beneficial to cutting down compile times or fitting other things you might
need to change. They are:
-```bash
+```txt
Options:
-v, --verbose use verbose output (-vv for very verbose)
-i, --incremental use incremental compilation
@@ -127,16 +310,17 @@ probably the best "go to" command for building a local rust:
This may *look* like it only builds libstd, but that is not the case.
What this command does is the following:
-- Build libstd using the stage0 compiler (using incremental)
-- Build librustc using the stage0 compiler (using incremental)
+- Build `libstd` using the stage0 compiler (using incremental)
+- Build `librustc` using the stage0 compiler (using incremental)
- This produces the stage1 compiler
- Build libstd using the stage1 compiler (cannot use incremental)
This final product (stage1 compiler + libs built using that compiler)
-is what you need to build other rust programs.
+is what you need to build other rust programs (unless you use `#![no_std]` or
+`#![no_core]`).
-Note that the command includes the `-i` switch. This enables incremental
-compilation. This will be used to speed up the first two steps of the process:
+The command includes the `-i` switch which enables incremental compilation.
+This will be used to speed up the first two steps of the process:
in particular, if you make a small change, we ought to be able to use your old
results to make producing the stage1 **compiler** faster.
@@ -145,15 +329,15 @@ stage1 libraries. This is because incremental only works when you run
the *same compiler* twice in a row. In this case, we are building a
*new stage1 compiler* every time. Therefore, the old incremental
results may not apply. **As a result, you will probably find that
-building the stage1 libstd is a bottleneck for you** -- but fear not,
+building the stage1 `libstd` is a bottleneck for you** -- but fear not,
there is a (hacky) workaround. See [the section on "recommended
workflows"](#workflow) below.
-Note that this whole command just gives you a subset of the full rustc
-build. The **full** rustc build (what you get if you just say `./x.py
+Note that this whole command just gives you a subset of the full `rustc`
+build. The **full** `rustc` build (what you get if you just say `./x.py
build`) has quite a few more steps:
-- Build librustc and rustc with the stage1 compiler.
+- Build `librustc` and `rustc` with the stage1 compiler.
- The resulting compiler here is called the "stage2" compiler.
- Build libstd with stage2 compiler.
- Build librustdoc and a bunch of other things with the stage2 compiler.
@@ -185,12 +369,11 @@ compile. Using these commands you can test that it compiles before doing
a bigger build to make sure it works with the compiler. As shown before
you can also pass flags at the end such as --stage.
-
### Creating a rustup toolchain
-Once you have successfully built rustc, you will have created a bunch
+Once you have successfully built `rustc`, you will have created a bunch
of files in your `build` directory. In order to actually run the
-resulting rustc, we recommend creating rustup toolchains. The first
+resulting `rustc`, we recommend creating rustup toolchains. The first
one will run the stage1 compiler (which we built above). The second
will execute the stage2 compiler (which we did not build, but which
you will likely need to build at some point; for example, if you want
@@ -207,7 +390,7 @@ The `